precambrian basement geology of the permian basin region of west texas and eastern new mexico a geophysical perspective because most of the permian basin region of west texas and southern new mexico is covered by phanerozoic rocks other means must be found to examine the precambrian upper crustal geology of the region we have combined geologic information on the precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region geophysical anomalies can be related to six precambrian events formation of the early proterozoic outer tectonic belt igneous activity in the southern granite rhyolite province an episode of pre grenville extension the grenville orogeny rifting to form the delaware aulacogen and eocambrian rifting to form the early paleozoic continental mar in two geophysical features were studied in detail the abilene gravity minimum and the central basin platform gravity high the abilene gravity minimum is shown to extend from the delaware basin across north central texas and is interpreted to be caused by a granitic batholith similar in size to the sierra nevada batholith in california and nevada this batholith appears to be related to formation of the southern granite rhyolite province possibly as a continental margin arc batholith because of this interpretation we have located the grenville tectonic front southward from its commonly quoted position closer to the llano uplift middle proterozoic mafic intrusions are found to core the central basin platform and the roosevelt uplift these intrusions formed at about 1 1 ga and are related in time to both the mid continent rift system and the grenville orogeny in texas because these features are likely to be rift related they suggest that the concept of a delaware aulacogen needs to be revised only to the extent that the rifting is proterozoic in age not eocambrian precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying permian basin and thus have potential exploration significance interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre ellenburger basins which may be extensive and of potential exploration interest

regional geologic and petrologic framework for iron oxide apatite rare earth element and iron oxide copper gold deposits of the mesoproterozoic st francois mountains terrane southeast missouri usa this paper provides an overview on the genesis of mesoproterozoic igneous rocks and associated iron oxide apatite ioa rare earth element iron oxide copper gold iocg and iron rich sedimentary deposits in the st francois mountains terrane of southeast missouri usa the st francois mountains terrane lies along the southeastern margin of laurentia as part of the eastern granite rhyolite province the province formed during two major pulses of igneous activity 1 an older early mesoproterozoic ca 1 50 1 44 ga episode of volcanism and granite plutonism and 2 a younger middle mesoproterozoic ca 1 33 1 30 ga episode of bimodal gabbro and granite plutonism the volcanic rocks are predominantly high silica rhyolite pyroclastic flows volcanogenic breccias and associated volcanogenic sediments with lesser amounts of basaltic to andesitic volcanic and associated subvolcanic intrusive rocks the iron oxide deposits are all hosted in the early mesoproterozoic volcanic and volcaniclastic sequences previous studies have characterized the st francois mountains terrane as a classic a type within plate granitic terrane however our new whole rock geochemical data indicate that the felsic volcanic rocks are effusive derivatives from multicomponent source types having compositional similarities to a type within plate granites as well as to s and i type granites generated in an arc setting in addition the volcanic hosted ioa and iocg deposits occur within bimodal volcanic sequences some of which have volcanic arc geochemical affinities suggesting an extensional tectonic setting during volcanism prior to emplacement of the ore forming systems the missouri iron orebodies are magmatic related hydrothermal deposits that when considered in aggregate display a vertical zonation from high temperature magmatic hydrothermal ioa deposits emplaced at moderate depths 1 2 km to magnetite dominant ioa veins and iocg deposits emplaced at shallow subvolcanic depths the shallowest parts of these systems include near surface iron oxide only replacement deposits surficial epithermal sediment hosted replacement deposits synsedimentary ironstone deposits and mn rich exhalite deposits alteration associated with the ioa and iocg mineralizing systems of the host volcanic rocks dominantly produced potassic with lesser amounts of calcic and sodic rich mineral assemblages no deposits are known to be hosted in granite implying that the mineralizing systems were operative during a relatively short postvolcanic period yet prior to intrusion of the granitoids companion studies in this special issue on mineral chemistry stable isotopes and iron isotopes suggest that the magnetite within the ioa deposits formed from high temperature fluids of magmatic or magmatic hydrothermal origin however the data do not discriminate between a magmatic hydrothermal source fluid exsolved from an fe rich immiscible liquid or an fe rich silicate magma mineral chemical fluid inclusion and stable isotope data from these new studies record the effects of metasomatic fluids that interacted with crustal reservoirs such as volcanic rocks or seawater

isotopic connections between basement rocks exposed in the st francois mountains and the arbuckle mountains southern mid continent north america two distinct crustal provinces have been identified in the southern mid continent based on u pb crystallization ages both contain large volumes of undeformed granite and rhyolite with minor amounts of metamorphic rock and mafic intrusions the eastern granite rhyolite province is characterized by felsic rocks with crystallization ages of 1 470 30 ma and exposures are restricted to the st francois mountains in southeastern missouri similarly the southern granite rhyolite sgr province is characterized by felsic units with ages of 1 370 30 ma with primary exposures in the eastern arbuckle mountains of southern oklahoma within the sgr province three magmatic pulses can be identified starting at 1 400 1 370 and 1 340 ma although the crystallization ages are different the sm nd isotopic signatures are similar for the units exposed in these areas as well as the buried basement in between depleted mantle model ages for rocks within the arbuckle mountains range from 1 530 to 1 430 ma with nd t values of 3 2 to 4 1 while units of the st francois mountains range from 1 550 to 1 430 ma and 4 5 to 4 7 comparison of sm nd isotopic data also indicate similarities between the 147sm 144nd and 143nd 144nd ratios for the rock units in these areas suggesting a common source

mars definition and characterization of global surface units with emphasis on composition units of similar optical properties have been defined for a large part of the surface of mars using primarily viking 2 approach maps of red 0 59 m and violet 0 45 m normal albedo of available global data sets spectral albedo most closely relates to surface mineralogy the resultant unit map represents mars at the time the viking images were acquired ls 103 105 morning northern hemisphere summer in 1976 computer processing of these data reveals clusters of albedo values which relate to specific contiguous regions on the planet final unit definitions were determined by an interactive iterative process which included scientific judgment on the part of the researchers the results therefore are not the product of an unsupervised statistical classification some unit boundaries are of a gradational nature while others are more discrete certain of the units are temporal and relate to the presence of condensates at the time of observation other units by virtue of their redness and low albedo are almost certainly not affected by condensates a study of the albedo properties of analog materials measured in the laboratory and of telescopic spectrophotometry of mars indicates that both hydroxylated low temperature or secondary and hematitic high temperature or primary oxidized materials exist on the surface the most likely surface expression of global dust and or high albedo soils is in the classical bright region arabia this material is probably a low temperature hydrated oxidation product of mafic volcanic glass such as a palagonite or amorphous iron silica gel most low albedo features in the region studied have red and violet albedos consistent with a dark substrate such as basalt coated by or mixed with the fine grained alteration product mentioned above there may be no genetic relationship between these two components in this majority of low albedo surface materials certain of the other low albedo regions are also the most red highest r v areas observed on the planet these areas are consistent with a nonhydroxylated more hematitic material such as that produced by high temperature hydrothermal alteration of basalt some of these dark very red regions correlate with exposures of very ancient terrain at this spatial resolution 10 20 km pixel there is no evidence of surface exposures of uncoated or unweathered basaltic rock or glass comparisons of the unit map with other data sets available to us revealed no striking detailed correlations larger scale correlations certainly do exist however among the various albedo data sets and with thermal inertia data

regional residual gravity anomaly separation using the minimum curvature technique one of the most difficult problems in gravity interpretation is the separation of regional and residual gravity anomalies from the bouguer gravity anomaly this study discusses the application of the minimum curvature method to determine the regional and residual gravity anomalies

mineralogy chemistry and fluid aided evolution of the pea ridge fe oxide y ree deposit southeast missouri usa the kiruna type pea ridge iron oxide apatite ioa deposit is hosted by a sequence of 1 47 ga rhyolite tuffs of the st francois mountains southeast missouri usa it consists of a series of altered zones composed mainly of amphibole magnetite hematite and quartz together with the presence of several rare earth element y ree rich breccia pipes in many cases the fluorapatite within these zones is rich in inclusions of monazite iron oxide and quartz inclusions plus minor xenotime monazite and minor xenotime are also found intergrown as inclusions in the fluorapatite as well as in surrounding recrystallized magnetite and hematite in the magnetite ore monazite and xenotime typically occur as inclusions within both oxides monazite ce and xenotime y are both relatively poor 2 wt in tho2 and uo2 no significant compositional differences exist in the y ree chemistry between monazite and xenotime inclusions in fluorapatite compared to grains intergrown with magnetite and hematite suggesting that these two ree rich minerals are cogenetic monazite xenotime geothermometry and geochronology of monazite inclusions in fluorapatite provide evidence that formation remobilization of the y ree phosphates took place at ca 50 to 400 c approximately 5 to 10 m y after emplacement of the main iron oxide phosphate orebody evidence from field relationships and fluid inclusion chemistry together with the massive recrystallization and remobilization of fluorapatite monazite xenotime and iron oxides at pea ridge suggest a subvolcanic origin coupled with a strong metasomatic reworking of the ioa deposit

mars near infrared comparative spectroscopy during the 1986 opposition abstract near infrared 0 7 2 5 m spectral observations of mars during the 1986 opposition were carried out at the mauna kea observatory utilizing the university of hawaii 2 2 m telescope spectra were obtained of several martian locations using a continuously variable filter cvf spectrometer with a resolution of 1 25 during two separate runs in june and august approximately 60 distinct spots between 354 w and 163 w and 32 n and 53 s were observed at an angular resolution of 0 5 to 1 5 arcsec corresponding to a spatial resolution on mars of 200 to 460 km varying with nightly seeing conditions these different spots fall roughly into a set of eight distinct geologic regions volcanic regions ridged plains ridged volcanic plains scoured plains impact basins channels and canyons densely cratered regions and layered terrain and ice the spectra exhibit typical noise related or weather induced errors of less than 4 of the full scale signal to analyze these spectral data spot to spot ratios or relative reflectance spectra were produced between spectra taken in different geologic regions spectral features observed in these ratios can act as indicators of mineralogic differences between areas under consideration perhaps the most striking result obtained from the many ratios taken close in time and under similar viewing geometries was the consistent lack of noticeable differences between spectra taken of areas which in geologic maps and viking orbiter images appear to have very different morphologies this observation leads to several possible conclusions a the spectral feature differences are below the detection limit of these measurements b the spatial resolution of spots observed on mars was not high enough to merit comparison with medium to high resolution viking orbiter images and geologic maps i e the local morphology varies significantly within the aperture region and a mixing or averaging effect operates to reduce the spectral feature differences c all of these regions are the same spectrally in the near infrared it is likely that all three of these effects operate to some degree to prevent spectral features from being observed among the areas measured possibility c may best explain these results as it is consistent with several previous studies indicating a grossly uniform mantle of global dust over much of the observed surface the message to mars orbiting spectroscopy experiments such as mars observer vims is that the spectral differences are small only a few percent of the continuum or nonexistent among many areas on mars on the scale of several hundred kilometers

recrystallization or cooling ages in situ uv laser 40ar 39ar geochronology of muscovite in mylonitic rocks the intra grain 40 ar 39 ar age distributions of muscovite from greenschist facies mylonite zones are shown to be sensors of metamorphic history textures compositions and in situ uv laser ablation 40 ar 39 ar data from two extensional shear zones demonstrate that cooling ages can be distinguished from neo or recrystallization ages deformed muscovite porphyroclasts and recrystallized shear band muscovite from the pogallo shear zone ivrea zone southern alps reveal variable intra grain 40 ar 39 ar ages with internal age variations of more than 65 ma the wide range of 40 ar 39 ar ages within compositionally homogeneous grains is consistent with diffusion dominated argon loss controlled by observable intra grain microstructures in contrast intra grain 40 ar 39 ar ages of muscovite fish from proterozoic mylonite of the porsgrunn kristiansand shear zone southern norway display only minor age dispersion and age variations are correlated with changes in muscovite composition the sympathetic age composition correlations in different muscovite microstructures directly relate to sequential neo and recrystallization of muscovite and ultimately provide insight into the protracted recrystallization history during extensional deformation along the porsgrunn kristiansand shear zone

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

hydrothermally altered impact melt rock and breccia contributions to the soil of mars samples of melt rock and breccia from 12 terrestrial impact craters were examined to identify alteration minerals and their conditions of formation the dominant assemblage in most cases is clay silica k feldspar zeolite indicating hydrothermal alteration at low pressures and temperatures of 100 300 c the clays are predominantly fe chlorites and smectites in most cases depleted in al and enriched in fe and mg relative to their source rocks alteration of impact glass is often complete while alteration of crystalling melt rock is limited to a few percent of the rock volume impact breccia is altered to only a small extent compared to the alteration of glass impact induced hydrothermal alteration is to be expected at martian impact sites if significant quantities of ground ice and or water are present the alteration products are likely to resemble the clays found at terrestrial craters al fe fractionation similar to that observed on earth is strongly suggested by the viking soil analyses the quantitative contribution of hydrothermal clays to the soil of mars is estimated based on a model which predicts the generation of the equivalent of a 61 m thick blanket of impact melt planet wide the inefficiency of melt production in small craters and the inefficiency of alteration in large craters limit the expected production of clay to a few percent of this figure clays produced by impact induced hydrothermal alteration may be expected to contribute to the fines on mars but this contribution is probably not dominant relative to other suggested sources

strength characteristics of the r f and c slip systems in calcite abstract numerical modelling of crystallographic preferred orientation or texture development in plastically deformed materials requires specification of all potential slip and twinning systems and their relative strengths or critical resolved shear stresses crrs it is widely believed that the most important intracrystalline deformation systems in calcite are twinning on e 1 018 40 4 1 slip on r 10 1 4 2 021 and slip on f 1 012 2 2 01 the superscripted signs indicating slip sense it is these systems which have been used in modelling texture development in calcite polycrystals to date however modelling has been limited by insufficient and partly conflicting data on the relative strengths of the various slip and twinning systems in calcite deformation experiments on calcite single crystals performed by us in recent years taken together provide new data for a more complete and consistent quantification of the strength properties of the calcite glide systems in our experiments single crystals of calcite were compressed in both the 4041 and 2243 orientations at temperatures and constant strain rates in the range 300 800 c and 3 10 4 3 10 8 s 1 respectively the stress strain curves obtained mostly show two yield points slip line analysis shows that these yield points are related to the onset of macroscopically observable slip first on an r 2 021 system and then on an f 10 1 1 or c a system the latter two systems dominate at higher temperature 600 c at strain rate 3 10 5 s 1 and have not been taken into account in any texture modelling so far with increasing absolute temperature tabs calculated crss values c for the r f and c systems decrease in a manner which can be described by empirical power law functions of the type c a tabs b where b r 2 021 and f 10 1 1 systems the yield stresses for r 2 021 and f 10 1 1 slip were found to be rather insensitive to strain rate with conventional power law stress exponents 12 comparing the present results with previous data we conclude that two regimes of slip system activity exist namely a low temperature regime involving e twinning slip on r 2 021 and on f 2 2 01 positive direction not reported and a high temperature regime with r 2 021 f 10 1 1 and c a slip this slip system transition occurs at t 400 c at strain rate 10 4 10 5 s 1 and is expected to be associated with one or more texture transitions the results point to a clear need for renewed texture modelling of calcite polycrystals

new insights into thermal history from single grain 40ar 39ar analysis of biotite single grains of biotite extracted from two hand samples of two granitoid rocks from the red lake greenstone belt of northwestern ontario were examined using a high sensitivity 40ar 39ar laser dating system for variations in age for small grains a striking correlation is seen between the ages and the grain sizes as measured by the cylindrical radii with smaller samples yielding younger ages this effect saturates at a radius of about 225 m above which the integrated age is independent of grain size by combining this grain size effect with biotite diffusion parameters taken from the literature continuous cooling curves were derived for the region these curves suggest the region cooled at a rate of about 0 3 c myr between 2635 and 2570 ma with its ability to define a grain size effect the laser probe mass spectrometer system provides new insight into thermal history

the geounri shear zone in the paleozoic taebaeksan basin of korea tectonic implications abstract the songrim and daebo orogenies represent two major phanerozoic tectonic events that are well preserved across much of the present day korean peninsula the songrim orogeny corresponds to the late permian triassic collision of the north and south china cratons whereas the jurassic daebo tectonic event represents a thin skinned contractional deformation in a continental arc setting it is well established that the songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the korean peninsula e g the pyeongnam basin and the imjingang belt with only a minor impact on the geology of southern korea e g the okcheon and taebaeksan basins it was the daebo tectonic event however which generated most of the deformational structures observed within the okcheon and taebaeksan basins the deokpori thrust in the taebaeksan basin is a significant fault structure that formed during the daebo tectonic event no other regional structures related to the songrim orogeny have been found in the taebaeksan basin in the vicinity of the deokpori thrust we have identified in this study a previously undocumented reverse slip shear zone which is named the geounri shear zone microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at 400 c in contrast with the nearby deokpori thrust which formed in a brittle deformation regime a geochronological analysis of muscovite isolated from phyllonite samples of the geounri shear zone yielded two 40 ar 39 ar age spectra with a combined weighted mean age of 209 5 2 ma thermal modeling of the ar data combined with structural interpretations suggests that this muscovite 40 ar 39 ar age represents a robust lower age limit for the timing of the geounri shearing event therefore linking formation of the shear zone with deformational events caused by the songrim orogeny these results imply that structural features caused by songrim deformation were developed at higher temperatures than structures caused by daebo deformation as found within the taebaeksan basin

on the spectral reflectance properties of materials exposed at the viking landing sites bidirectional reflectances calibrated to within 10 uncertainties for blue 0 40 0 52 m green 0 50 0 59 m and red 0 60 0 74 m channels were determined for 31 block and soil exposures imaged by the viking lander cameras reflectances were corrected for atmospheric attenuation by using optical depth measurements obtained by the lander cameras and for skylight illumination by subtracting the brightness values of adjacent shadowed areas reflectances for two facets on each of two large blocks were computed for a common geometry by determining facet orientations from high resolution lander stereo images and by fitting the hapke irvine photometric function to reflectances estimated for several geometries results indicate that individual facets on the same block have different optical properties the darkest grayest block facets have properties similar to laboratory reflectance spectra of mafic rocks that are thinly coated with ferric iron rich palagonite most block surfaces have reflectances consistent with thin to optically thick covers of palagonitic material limited exposures of soil have spectra that are also similar to palagonitic material the majority of the soils sampled and a few block surfaces have spectra with steeper slopes and greater spectral curvatures than the palagonite analog consistent with greater degree of ferric iron crystallinity the soils could be an intimate mixture of palagonitic material and other materials not seen in end member form but if so these other materials would need an even greater slope and curvature than observed soils the differences in reflectance properties for common block surfaces compared to common soils imply that local weathering of blocks has not contributed significantly to the soil exposed at the landing sites

neocrystallization or cooling 40ar 39ar ages of white micas from low grade mylonites abstract 40 ar 39 ar data for white micas from quartzite mylonites from a thrust system are evaluated with respect to whether the minerals record cooling ages or alternatively ages of deformation induced neocrystallization to address these interpretations three types of information are needed 1 the effective diffusion length scale controlling volume diffusion of argon in white micas 2 an estimate of the diffusion parameters for volume diffusion of argon and 3 the thermal history completely recrystallized quartzites exhibit smooth grain size distributions for white micas with good exponential or power law fits suggesting that the grains grew under similar conditions the microstructures indicate that the grains grew over a narrow range in time near the end of the deformation history measured grain size distributions provide a maximum limit for the diffusion length scale thermal histories are estimated by 40 ar 39 ar diffusion experiments on k feldspars using the multi diffusion domain method of lovera lovera o m richter f m harrison t m 1989 the 40 ar 39 ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes j geophys res 94 17917 17935 these results are consistent with the expected temperature time path for the thrusting environment and predict maximum temperatures of 250 350 c during the time span recorded by the white micas experimentally and empirically calibrated diffusion parameters for white micas are applied to a cylindrical diffusion geometry to explore the question of cooling ages versus neocrystallization ages forward modeling of the passage of the white micas through the closure window for argon diffusion using the thermal histories derived from the k feldspars indicates that the white micas record neocrystallization rather than cooling and supports the idea that the white micas record the timing of when ductile deformation ceased

structural style of the okcheon fold thrust belt in the taebaeksan zone korea abstract structural interpretation of the distinct map pattern defined by highly connected thrust traces in the map view of the taebaeksan zone provides insight into the structural style of the northeastern okcheon belt the map pattern can generally be explained by either a folded imbricate fan or a hinterland dipping duplex the same geometry could also be formed by a complex combination of major imbricate thrusts and their connecting splays having the structural architecture of a typical fold thrust belt however a folded imbricate fan model is not adequate to explain the absence of late paleozoic to early mesozoic strata i e the pyeongan supergroup between two major thrusts viz the pyeongchang and machari thrusts in this area this result further suggests that the yeongwol area the western part of the taebaeksan zone is a duplex that corresponds to a more internal and deeper hinterland part of the fold thrust belt while the imbricate thrusts with low connectivity in the taebaek area the eastern part of the taebaeksan zone indicate a more external and shallower foreland portion of the belt in addition cross cutting relations and newly obtained sensitive high resolution ion microprobe shrimp u pb zircon ages from a cross cutting dike and syntectonic sedimentary rocks suggest limited thrust reactivation after the early jurassic and early tertiary respectively in spite of this the original geometry of the fold thrust wedge in the taebaeksan zone remained well preserved

the southeast missouri usa proterozoic iron metallogenic province types of deposits and genetic relationships to magnetite apatite and iron oxide copper gold deposits abstract the southeast missouri usa iron metallogenic province within the mesoproterozoic st francois mountains terrane includes eight major iron deposits and approximately 30 minor deposits three of the major deposits have been mined pilot knob magnetite iron mountain and pea ridge these deposits have similarities to kiruna type iron deposits and likely formed either by crystallization of iron oxide magma and or by hydrothermal replacement of volcanic rocks iron oxides within all district deposits are dominantly magnetite and hematite but the gangue minerals are diverse at pilot knob magnetite the dominant gangue mineral assemblages are albitic plagioclase k feldspar quartz chlorite at iron mountain andradite actinolite apatite quartz and at pea ridge apatite quartz in the main part of the deposit plus ree bearing breccia pipes discussed below pilot knob magnetite and pea ridge have associated carbonates which have been suggested to indicate carbonatitic affinities for these deposits the boss bixby deposit has been described as an iron oxide copper gold iocg deposit and contains the only significant copper resource known in the district 40 mt of 0 8 cu as yet undeveloped the district also has steeply dipping iron oxide vein deposits that represent part of the plumbing system where iron bearing hydrothermal fluids moved upward towards the surface from the iron orebodies developing at depth some of these hydrothermal fluids were exhaled into caldera lakes and formed small deposits composed of laminated oolitic sedimentary hematite

tectonic and sedimentary evolution of the korean peninsula a review and new view abstract this review focuses on the tectonics and sedimentation of major sedimentary basins and orogenic belts late proterozoic neogene in the korean peninsula the korean peninsula is part of the amuria plate and represents an important link between continental blocks of north and south china and the island arcs of japan the basement rocks exposed in the kyonggi and yongnam massifs consist of 2 7 to 1 1 ga high grade gneiss and schist these massifs are separated by the okchon fold belt which comprises metasedimentary rocks and bimodal meta volcanic rocks the stratigraphy of the okchon group is unclear at present the okchon basin was probably initiated as an intraplate rift prior to the late proterozoic the hwanggangri formation clast bearing phyllite most likely represents deposition by subaqueous debris flows in slope environments of an enclosed basin the stratigraphic relationship between the okchon group and the choson supergroup cambro ordovician of the taebaeksan basin is poorly constrained the choson supergroup unconformably overlies the yongnam massif and consists mainly of carbonate sequence that formed mostly in shallow marine and tidal environments reflecting numerous sea level fluctuations the sequence is disconformably overlain by siliciclastic sequence of pyongan supergroup carboniferous triassic which formed most likely in shallow marine deltaic and fluvial environments the imjingang belt is an east trending fold and thrust zone and consists of metasedimentary rocks and volcaniclastics devonian carboniferous underlain unconformably by proterozoic basement rocks late proterozoic amphibolites of oceanic affinity were metamorphosed at about 8 13 kbar and 630 740 c during the late permian to the early triassic the south vergent contraction and top down to the north normal faulting are suggestive of a suture belt between the north china block sino korea craton and the south china block yangtze craton an extension of sulu belt across the yellow sea entire peninsula experienced strong deformation and metamorphism during the suturing event namely songrim orogeny during this orogenic event the kyonggi massif and the okchon basin accreted to the yongnam massif and taebaeksan basin along the south korean tectonic line running northeast southwest a series of northward trending thrust formed along the boundary zone to the east kaktong and kongsuwon thrusts and others piggyback basins locally developed along the thrust faults forming the taedong group the crustal deformation resumed in the early to late jurassic daebo event under contractional setting dextral ductile shearing associated with thrusting and folding continued in the mid southern part of the peninsula it was due to orthogonal northwestward subduction of the izanagi plate under the asian continent the yongnam massif experienced continuous dextral offset along the honam shear zone in the early cretaceous the izanagi plate began to subduct northward and caused formation of strike slip basins in retroarc setting i e kyongsang basin in the southeastern part and a number of small scale basins in the mid southwestern part of the peninsula small scale alluvial fans and fluvial channel networks formed in the basin margin and were transitional to ephemeral lacustrine systems under semiarid to arid conditions extensive intrusion of granitoids occurred from triassic to early tertiary with a gap between 160 and 100 ma representing continental magmatic arc in the tertiary the southeastern margin of the korean peninsula experienced back arc opening pull apart basins formed in the miocene bounded by the yangsan and hupo faults the yonil group sedimentary fill of the pohang basin comprises more than 1 km thick siliciclastic sequence which represents deposition in fan delta systems on the hanging wall of the yangsan fault thick more than 10 km thick sediments in the ulleung basin margin were deformed in the late miocene due to the northward movement of kyushu block quarternary volcanic events in cheju island represent intraplate hot spots

pretectonic and posttectonic emplacements of the granitoids in the south central okchon belt south korea implications for the timing of strike slip shearing and thrusting structural analyses of three granitic plutons in the south central okchon belt of the korean peninsula reveal that the baegnok and cheongsan plutons are pretectonic with respect to right lateral strike slip ductile shearing along the cheongsan shear zone and later top to the ese thrusting whereas the boeun pluton is posttectonic with respect to these two deformation events u pb sphene age data from the three plutons indicate that the intrusion ages of the baegnok cheongsan and boeun plutons are 222 7 2 1 ma 216 9 2 2 ma and 171 7 1 4 ma respectively microstructural evidence preserved in deformed rocks from the ne striking cheongsan shear zone and adjacent nne striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ese directed thrusting since these thrusts crosscut the late triassic early jurassic taedong group we estimate that the thrusts developed at 180 ma during the daebo tectonic event just prior to the intrusion of the boeun pluton we also constrain the age of the cheongsan shear zone at middle late triassic time corresponding to the songrim orogeny so that the shear zone is not related to deformation along the middle jurassic honam shear zone 176 ma the honam shear zone is not a simple tectonic boundary between the okchon belt and the yongnam massif but an anastomosing ductile shear zone not only affecting the okchon belt and the yongnam massif but also crosscutting the boundary between them at a low angle we suggest that the right lateral cheongsan shear zone is a major boundary between the okchon zone and the yongnam massif with paleozoic cover rocks the kyonggi massif and the okchon zone south china block were juxtaposed against the yongnam massif and taebaeksan basin north china block along the cheongsan shear zone during a late stage of the middle triassic songrim orogeny probably related to the late paleozoic early mesozoic collision between the north and south china blocks

spectral evidence for the mineralogy of high albedo soils and dust on mars because of the lack of direct mineralogic data available for mars spectral remote sensing techniques and in particular earth based reflectance spectroscopy remain the primary source of this information presented here are laboratory observations which further constrain the mineralogy and origin of the high albedo soils and dust earth based observations show the reflectance spectra of classical bright regions to be fairly uniform with a strong fe3 o2 charge transfer absorption edge extending from the near uv through the visible this absorption is relatively smooth unlike those observed for crystalline ferric oxides which have superimposed fe3 crystal field bands dilution of ferric oxide with a spectrally more neutral medium montmorillonite weakens the charge transfer and crystal field absorptions together and does not serve to make the ferric oxide spectrum more mars like smoother nontronite ferric iron bearing smectite clay was also investigated for spectral agreement with telescopic observations pure nontronite has fe3 absorptions analogous to but different from ferric oxides as in the previous case the absorptions are very distinct and inappropriate for mars admixture of neither montmorillonite nor ferric oxide serves to improve the nontronite spectrum the conclusion is that nontronite is not a major component of martian soils although the presence of other iron poor clays cannot be totally excluded based on currently available observational data the best spectral analogs known for high albedo soil and dust are a specific type of palagonite from hawaii x ray amorphous weathering products of mafic volcanic glass the indication is therefore that ferric iron on mars is likely to occur in poorly defined crystallo graphic sites such as found in these amorphous materials these materials form slowly under semiarid conditions at ambient temperature even low temperature hydrothermal alteration of glass might provide enough ion mobility to favor some formation of crystalline clays over amorphous gels and may therefore not be the primary mechanism responsible for soil formation on mars the amorphous hawaiian soils exist metastably for thousands of years on earth similar materials on mars would be expected to survive considerably longer under the present cold and dry climatic conditions

extensional deformation along the southern boundary of the gyeonggi massif south korea structural characteristics age constraints and tectonic implications the permo triassic collision of the north and south china blocks caused the development of the dabie sulu orogen in china and songrim orogen in the korean peninsula extension after this collision is known from the dabie sulu orogen but post orogenic extension is not well defined in the korean peninsula extensional deformation along the southern boundary of the gyeonggi massif in korea is characterized by top down to the south ductile shearing and subsequent brittle normal faulting and was predated by regional metamorphism and north vergent contractional deformation extension occurred between 220 and 185 ma based on the ages of pre extensional regional metamorphism and post extensional pluton emplacement 40ar 39ar dating of syn extensional muscovite in quartz mica mylonite yields an age of 187 8 5 6 2 ma in agreement with constraints from structural relationships together with the extensional deformation identified along the northern boundary of the gyeonggi massif 226 ma the extension along the southern boundary is probably related to the exhumation of the massif during late orogenic or post orogenic extension associated with the songrim orogeny of the korean peninsula and forms an important event in the phanerozoic crustal evolution of east asia

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

ages of deformation from k ar and 40ar 39ar dating of white micas a structural and isotopic dating study of white micas from mylonites in the ruby gap duplex off central australia shows that mylonitic deformation occurred during the paleozoic alice springs orogeny deformation of the white micas took place under greenschist facies conditions in the approximate temperature range 250 350 c two distinct populations of white mica have been found 1 porphyroclasts of original muscovite and 2 neocrystallized phengites that define the fabric associated with deformation 40ar 39ar age spectra of neocrystallized phengites yield plateau like spectra of paleozoic age in contrast 40ar 39ar age spectra of muscovite porphyroclasts exhibit proterozoic apparent ages indicating that white micas were not open to significant argon diffusion during formation of the ruby gap duplex we conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system

on the loss of 40ar from muscovite during polymetamorphism abstract recent laser studies have documented 40ar concentration gradients in micas that correspond to age variations of tens to hundreds of millions of years a fundamental problem to interpretation of such variations is to evaluate whether they represent loss via lattice diffusion or alternatively loss during chemical reactions and deformation new data of this study and comparisons with previous work suggest that muscovite is far less prone to lose accumulated 40ar via lattice diffusion during overprinting events than has been inferred previously kyanite muscovite assemblages in proterozoic schist along the eastern flank of the green mountain massif central vermont appalachians formed during proterozoic metamorphism at ca 1 1 ga and were remetamorphosed in the greenschist facies during both taconian ca 465 ma and acadian ca 390 ma events the overprinting events resulted in deformation of earlier kyanite and muscovite growth of chemically and isotopically distinct rims on earlier muscovite and growth of neoblastic muscovite and chloritoid with each event at inferred minimum temperatures of ca 425 c laser spot analyses of the grenville muscovite range from 988 11 ma in the centers of porphyroclasts to 397 7 ma along rims ages younger than 750 ma occur only on the deformed edges of porphyroclasts and in highly deformed areas within crystals parts of early formed muscovite that escaped late deformation and chemical reaction seem remarkably retentive of accumulated 40ar laser spot analyses along the outer 100 m of an undeformed growth face yield ages close to the timing of proterozoic metamorphism and cooling the present study suggests episodic heating events are ineffective for producing measurable diffusive argon loss along the edges of large muscovite crystals ca 1 mm in diameter particularly in comparison with deformation and reaction mechanisms and are unlikely to result in complete 40 ar 39 ar age resetting of such crystals the results of this study further emphasize that the argon diffusivity in muscovite is lower than commonly considered in previous work

shrimp u pb zircon ages of pyroclastic rocks in the bansong group taebaeksan basin south korea and their implication for the mesozoic tectonics abstract the bansong group daedong supergroup in the korean peninsula has long been considered to be an important time marker for two well known orogenies in that it was deposited after the songnim orogeny permian triassic collision of the north and south china blocks but was deformed during the early to middle jurassic daebo tectonic event here we present a new interpretation on the origin of the bansong group and associated faults on the basis of structural and geochronological data shrimp sensitive high resolution ion microprobe u pb zircon age determination of two felsic pyroclastic rocks from the bansong group formed in the foreland basin of the gongsuweon thrust in the taebaeksan basin yielded ages of 186 3 1 5 and 187 2 1 5 ma respectively indicating the deposition of the bansong group during the late early jurassic inherited zircon component indicates ca 1 9 ga source material for the volcanic rocks agreeing with known basement ages the bansong group represents syntectonic sedimentation during the late early jurassic in a compressional regime during the daebo tectonic event the northeast trending regional folds and thrusts including the deokpori gakdong and gongsuweon thrusts with a southeast vergence developed in the taebaeksan basin this is ascribed to deformation in a continental arc setting due to the northwesterly orthogonal convergence of the izanagi plate on the asiatic margin which occurred immediately after the juxtaposition of the taebaeksan basin against the okcheon basin in the late stage of the songnim orogeny thus the deokpori thrust is not a continental transform fault between the north and south china blocks but an intracontinental thrust that developed after their juxtaposition

occurrence of changshania trilobita cambrian in the taebaeksan basin korea and its stratigraphic and paleogeographic significance in the cambrian the taebaeksan basin was a shallow marine siliciclastic carbonate system in which two contrasting types of lithofacies and biofacies occur in juxtaposition the shallow marine facies taebaek group comprises diverse trilobite taxa endemic to the sino korean block whereas the deep water oceanic facies yeongwol group is characterized by predominance of cosmopolitan and pelagic trilobites changshania is a representative furongian trilobite genus endemic to the sino korean block and has been employed as a zonal taxon for middle furongian in north china this article reports the occurrence of changshania from the deep water facies machari formation yeongwol group of the taebaeksan basin for the first time in korea this observation enables to correlate the eochuangia hana zone of korea with the changshania irvingella zone of north china with confidence the dominance of laminated dark gray to black shale and pandemic trilobites in the machari formation has been interpreted to indicate deposition in a dysaerobic deep water environment the occurrence of changshania along with some endemic polymerid trilobites though uncommon in the deeper water oceanic facies suggests that the machari formation was formed in the offshore region fringing the sino korean block during the cambrian

microfabric study on the deformational and thermal history of the alpi apuane marbles carrara marbles italy marbles from different geometrical and structural positions within the alpi apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure c axis orientation and temperature analysis statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80 100 m to 250 300 m this is correlated with an equilibration calcite dolomite temperature from 360 380 c to 420 430 c two kinds of dynamically recrystallized microstructures have been investigated a first one exhibiting coarse grains 150 200 m with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size 40 50 m and predominantly straight grain boundaries these microstructural types associated with localized post thermal peak shear zones and meter to kilometer scale folds are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms dislocation creep associated with predominant grain boundary migration type b1 or subgrain rotation recrystallization type b2 these differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type b1 390 c than in type b2 370 340 c we have been able to relate the development of the different microfabric types to the successive stages of deformation of the alpi apuane metamorphic complex

strain dependent variation of microstructure and texture in naturally deformed carrara marble this study investigates the microstructure and texture variations across a mm scale shear zone in carrara marble of the alpi apuane italy the microstructures have been investigated for grain size texture and shape fabrics textures have been measured with computer integrated polarization microscopy cip and electron back scattered diffraction ebsd separating porphyroclast and recrystallized grains the deformation which post dates an earlier deformation phase and subsequent annealing is strongly localized the microstructures and textures change across the shear strain gradient and are interpreted to preserve a time sequence of progressive stages of deformation the bulk shear strain rate is estimated to be about 10 11 sec 1 at deformation temperatures of approximately 325 c 30 c the protomylonite is characterized by a core mantle structure with a bimodal grain size distribution which changes gradually to a completely dynamically recrystallized microstructure with a unimodal grain size distribution in the mylonitic center of the shear zone core mantlestructures are produced by dominant rotation recrystallization accompanied by some grain boundary migration the microstructural transition from protomylonite to mylonite coincides with a change in texture with increasing strain the single c axis maximum of an earlier inherited texture in the protomylonite is replaced by a similar texture in a different orientation maximum normal to the shear plane which is consistent with dominant basal hai and r h 2201i slip the microstructural and textural variations depend on the proportion of recrystallized grains as dynamic recrystallization progresses with finite strain the texture development is finite strain dependent the comparison of the microstructures and textures to other natural and to experimental examples explains the progressive change of the texture and demonstrates the texture evolution produced by dynamic recrystallization 2006 elsevier ltd all rights reserved

oxygen hydrogen sulfur and carbon isotopes in the pea ridge magnetite apatite deposit southeast missouri and sulfur isotope comparisons to other iron deposits in the region oxygen hydrogen sulfur and carbon isotopes have been analyzed in the pea ridge magnetite apatite deposit the largest historic producer among the known iron deposits in the southeast missouri portion of the 1 5 to 1 3 ga eastern granite rhyolite province the data were collected to investigate the sources of ore fluids conditions of ore formation and provenance of sulfur and to improve the general understanding of the copper gold and rare earth element potential of iron deposits regionally the 18 o values of pea ridge magnetite are 1 9 to 4 0 consistent with a model in which some magnetite crystallized from a melt and other magnetite perhaps the majority precipitated from an aqueous fluid of magmatic origin the 18 o values of quartz apatite actinolite k feldspar sulfates and calcite are significantly higher enough so as to indicate growth or equilibration under cooler conditions than magnetite and or in the presence of a fluid that was not entirely magmatic a variety of observations including stable isotope observations implicate a second fluid that may ultimately have been meteoric in origin and may have been modified by isotopic exchange with rocks or by evaporation during storage in lakes sulfur isotope analyses of sulfides from pea ridge and seven other mineral deposits in the region reveal two distinct populations that average 3 and 13 two sulfur sources are implied one was probably igneous melts or rocks belonging to the mafic to intermediate composition volcanic suite that is present at or near most of the iron deposits the other was either melts or volcanic rocks that had degassed very extensively or else volcanic lakes that had trapped rising magmatic gases the higher 34 s values correspond to deposits or prospects where copper is noteworthy the central dome portion of the boss deposit the bourbon deposit and the vilander prospective area the correspondence suggests that 1 sulfur either limited the deposition of copper or was cotransported with copper and 2 sulfur isotope analysis may be useful in evaluating southeast missouri iron deposits for copper and possibly for gold

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

thermal inertia mapping of mars from 60 s to 60 n twenty micrometer brightness temperatures are used to derive the thermal inertia for 81 of the martian surface between latitudes 60 these data were acquired by the two viking infrared thermal mappers in 1977 and 1978 following the two global dust storms of 1977 the spatial resolution used is 2 in latitude by 2 in longitude and the total range in derived inertia is 1 to 15 10 3 cal cm 2 sec 12 k 1 the distribution of thermal inertia is strongly bimodal with all values of thermal inertia less than 4 10 3 cal cm 2 sec 12 k 1 being associated with three disjoint bright regions mostly in the northern hemisphere sufficient dust is raised in global storms to provide fine material adequate to produce these low inertia areas but the specific deposition mechanism has not been defined at the low resolution used no complete exposures of clean rock were found there is some tendency for darker material to be associated with higher thermal inertia although the trend is far from one to one the distribution of high and low inertia areas is sufficiently nonrandom to produce a variation in whole disk brightness temperature with central meridian longitude this variation and the change in surface kinetic temperature associated with dust storms are factors in establishing the whole disk brightness temperature at radio and infrared wavelengths and will be important for those who use mars as a calibration source

mesoproterozoic trans laurentian magmatism a synthesis of continent wide age distributions new sims u pb ages zircon saturation temperatures and hf and nd isotopic compositions abstract the origin and tectonic setting of the mesoproterozoic 1 50 1 34 ga granite rhyolite provinces of the midcontinent region of the united states and the coeval a type granite plutons of the southwestern united states have been an enigma for decades in this paper we review the history of this problem examine the significance of a wealth of previously published u pb zircon and whole rock sm nd data and present new u pb zircon age and lu hf isotopic data from selected zircon samples our data show that the initiation of this magmatism spans the interval 1 50 1 40 ga across the continent but there is a general younging westward younger 1 39 1 34 ga magmatism also occurred across the continent but forms a major magmatic event only in the south central mid continent there is generally a good correlation between sm nd and lu hf isotopic compositions and inferred mantle derivation model ages except possibly in southeastern oklahoma the absence of older cores in zircons is compatible with calculated zircon saturation temperatures of ca 850 c and supports mantle driven basaltic underplating and resulting crustal partial melting as a petrogenetic model for the a type magmatism our new data compilation refines the existence of the nd line that extends from southwestern texas into southeastern michigan and the western grenville province in canada it marks an isotopic boundary between granites and rhyolites of cratonal origin to the northwest 1 8 1 6 ga model ages and those of more juvenile origin to the southeast 1 5 1 4 ga model ages our suggested tectonic model for the mesoproterozoic a type magmatism across eastern and southern laurentia involves the development of an active laurentian margin extending from eastern canada across what is now the midcontinent region of the united states and at least into the current southwestern united states magmatism along this margin was likely related to asthenosphere lithosphere interactions associated with the accretion of outboard arcs and possible exotic terranes that caused complex deformational and magmatic responses in the continental interior this activity evidently spread from northeast to southwest over a period of ca 70 million years the occurrence of the younger ca 1 39 1 34 ga event indicates that similar processes occurred some 130 million years later suggesting a long lived convergent plate margin and recurring basaltic underplating in some locations perhaps due to inboard lithospheric delamination and or mantle upwelling

impact melts and glasses on mars 1 dark mobile materials on mars are widely considered to be volcanically derived windblown particulates such a conclusion is consistent with the large volcanic edifices on mars and the assumption that a volatile rich crust would not favor significant impact melt generation several post viking discoveries however provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the pathfinder and viking landing sites first certain dark materials on mars do not exhibit the expected mafic absorption features in their reflectance spectra second the pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites rather than basalts third recent discoveries of holocene to miocene impacts in argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments fourth global dispersal of impact melt products are now recognized in the terrestrial record throughout the cenozoic consequently the generation and preservation of impact glass on mars should be reconsidered possible occurrences include possible tektite like strewnfields downrange from recent oblique impacts dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits global dispersal of glassy ejecta from hesperian and amazonian craters of chicxulub scale and dark mobile veneers emerging from noachian craters we propose that the mobile dark materials comprising many of the named features of mars as well as some blocks at the pathfinder viking landing sites may represent impact melt products

aeromagnetic images fresh insight to the buried basement rolla quadrangle southeast missouri aeromagnetic data from the rolla 1 2 quadrangle missouri displayed as a small scale grey tone image provide a lucid picture of basement features beneath sedimentary rock cover late stage tin granite plutons of the proterozoic st francois magmatic terrane are clearly delineated the plutons average about 11 km in diameter and occupy a significant fraction 12 percent of the area surveyed gravity data require that certain plutons are at least 3 km thick most of the plutons are fat ovals in plan view and show no flattening trend or structural alignment the ensemble indicates significant extension in an orthorhombic stress field with the minimum compressional stress being vertical exposed st francois terrane volcanic rocks show a characteristic intricate pattern in the magnetic signature this pattern is more widespread than the subsurface distribution of volcanic rocks indicated by drilling and likely also incorporates areas of st francois batholith granite the horseshoe shaped trend of

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

possible eastward extension of chinese collision belt in south korea the imjingang belt structural petrological and geochronological data from the middle korean peninsula indicate that the qinling dabie sulu collisional belt of east central china crosses the yellow sea and extends into the imjingang belt the yeoncheon complex first identified as the western imjingang belt comprises primarily north dipping metamorphic sequences 1 the northern jingok unit consisting of barrovian type metapelites and 2 the southern samgot unit consisting of calc silicate and amphibolitic rocks south vergent structures with reverse sense shearing are dominant in the jingok unit whereas late normal sense shearing is pervasive in the samgot unit and the deformed granitoid to the south these structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt pressure temperature p t estimates from the amphibolites suggest a high p amphibolite facies metamorphism 8 13 kbar and 630 790 c possibly evolving from eclogite facies conditions along a clockwise p t path sm nd and rb sr geochronological data suggest that the amphibolites emplaced in late proterozoic time were metamorphosed during permian triassic time

diffusion in solid earth systems recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid earth crystals melts and fluids although far from complete the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length and time scales of many natural transport phenomena in which diffusion plays a role conversely observations of diffusion progress in specific natural samples can be used to extract time temperature information for a variety of geologic and geochemical processes ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention despite this undeniable progress several major challenges remain that largely define the frontiers of research in solid earth diffusion perhaps foremost among these is the need to address and understand the multi scale multi path aspects of diffusion in many systems a complication that is not limited to polyphase materials individual mineral grains can exhibit clear indications of multi path behavior even when visible evidence of such paths is lacking many other diffusion frontiers are linked in one way or another to this multi scale issue they include diffusion of molecular h2o and the effect of h species on diffusion in minerals and rocks diffusive fractionation of multiple isotopes of a single element diffusion at the extreme conditions of the deep earth reconciliation of observations from natural samples and laboratory studies and development of theoretical approaches to predict

observational evidence of crystalline iron oxides on mars visible to near ir 0 4 1 0 mu m spectral reflectance observations of mars during the 1988 opposition were performed at mauna kea observatory using a circular variable filter spectrometer at a spectral resolution r lambda delta lambda approx 80 on august 13 and 14 1988 ut 41 regions 500 600 km in diameter were observed on mars the data have been reduced both to reflectance relative to solar analog mars 16 cyg b and to relative reflectance spot spot the spectra show the strong near uv reflectance dropoff characteristic of mars as well as absorptions at 0 62 0 72 mu m and 0 81 0 94 mu m both seen here clearly for the first time these absorption features are interpreted as fe sup 3 electronic transition bands that indicate the presence of crystalline ferric oxide or hydroxide minerals on the martian surface comparison of these data with laboratory spectra obtained by other workers supports the conclusion that a single iron oxide phase most likely hematite could account for all of the observed spectral behavior of the martian surface soils and airborne dust in the 0 4 1 0 mu m region this possibility must be reconciled with data from other possible spectral analogs and other wavelength regions as well as geochemical and mineral stability considerations to arrive at amore more complete understanding of the role of ferric minerals in martian surface mineralogy and weathering less

magnetic and gravity gradiometry framework for mesoproterozoic iron oxide apatite and iron oxide copper gold deposits southeast missouri high resolution airborne magnetic and gravity gradiometry data provide the geophysical framework for evaluating the exploration potential of hidden iron oxide deposits in mesoproterozoic basement rocks of southeast missouri the data are used to calculate mineral prospectivity for iron oxide apatite ioa rare earth element ree and iron oxide copper gold iocg deposits results delineate the geophysical footprints of all known iron oxide deposits and reveal several previously unrecognized prospective areas the airborne data are also inverted to three dimensional density and magnetic susceptibility models over four concealed deposits at pea ridge ioa ree boss iocg kratz spring ioa and bourbon iocg the pea ridge susceptibility model shows a magnetic source that is vertically extensive and traceable to a depth of greater than 2 km a smaller density source located within the shallow precambrian basement is partly coincident with the magnetic source at pea ridge in contrast the boss models show a large 625 m wide vertically extensive and coincident dense and magnetic stock with shallower adjacent lobes that extend more than 2 600 m across the shallow precambrian paleosurface the kratz spring deposit appears to be a smaller volume of iron oxides and is characterized by lower density and less magnetic rock compared to the other iron deposits a prospective area identified south of the kratz spring deposit shows the largest volume of coincident dense and nonmagnetic rock in the subsurface and is interpreted as prospective for a hematite dominant lithology that extends from the top of the precambrian to depths exceeding 2 km the bourbon deposit displays a large bowl shaped volume of coincident high density and high magnetic susceptibility rock and a geometry that suggests the iron mineralization is vertically restricted to the upper parts of the precambrian basement in order to underpin the evaluation of the prospectivity and three dimensional models an extensive statistical summary of density and apparent magnetic susceptibility measurements is presented that includes data on several hundred samples taken from the deposits altered wall rocks and unaltered country rocks

receiver function and gravity constraints on crustal structure and vertical movements of the upper mississippi embayment and ozark uplift the upper mississippi embayment ume where the seismically active new madrid seismic zone resides experienced two phases of subsidence commencing in the late precambrian and cretaceous respectively to provide new constraints on models proposed for the mechanisms responsible for the subsidence we computed and stacked p to s receiver functions recorded by 49 usarray and other seismic stations located in the ume and the adjacent ozark uplift and modeled bouguer gravity anomaly data the inferred thickness density and vp vs of the upper and lower crustal layers suggest that the ume is characterized by a mafic and high density upper crustal layer of 30 km thickness which is underlain by a higher density lower crustal layer of up to 15 km those measurements in the background of previously published geological observations on the subsidence and uplift history of the ume are in agreement with the model that the cretaceous subsidence which was suggested to be preceded by an approximately 2 km uplift was the consequence of the passage of a previously proposed thermal plume the thermo elastic effects of the plume would have induced wide spread intrusion of mafic mantle material into the weak ume crust fractured by precambrian rifting and increased its density resulting in renewed subsidence after the thermal source was removed in contrast the ozark uplift has crustal density thickness and vp vs measurements that are comparable to those observed on cratonic areas suggesting an overall normal crust without significant modification by the proposed plume probably owing to the relatively strong and thick lithosphere

laser 40ar 39ar evaluation of slow cooling and episodic loss of 40ar from a sample of polymetamorphic muscovite volume diffusion models predict that crystals with large diffusion dimensions can record a wide range of thermal conditions in the earth s crust direct measurements of the zoning of radiogenic argon 40 in single muscovite porphyroblasts from a complex terrain in the vermont appalachians record multiple crustal events that span 150 million years the crystal radius was the effective dimension for argon diffusion approximately 1000 micrometers late deformation features inside the crystals locally decreased the diffusion dimension and promoted loss of argon 40 zoning patterns of radiogenic isotopes as observed in this study are an increasingly important diagnostic tool for studying the thermal record of tectonic processes

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

ferric sulfates on mars evidence is presented for the possible formation and existence of ferric sulfato complexes and hydroxo ferric sulfate minerals in the permafrost on mars acidic groundwater derived from atmospheric oxidation of volcanogenic h2s to h2so4 aerosols promoted chemical weathering of fayalitic olivines iron rich pyroxenes plagioclase feldspar and pyrrhotite pentlandite mineral assemblages in crustal ultramafic and basic igneous rocks the acidic groundwater entered into electrochemical reactions with the iron sulfides yielding dissolved feso4 fe so4 2 and feoh2 complex ions and the precipitation of basic ferric sulfate minerals such as those belonging to the roemerite copiapite botryogen and jarosite alunite groups these phases are stabilized at low temperatures and ph in martian permafrost the occurrence of jarosites in terrestrial arid regions suggests that they could also survive on the surface of mars melting of the permafrost and raising of the ph may have initiated the hydrolysis of dissolved ferric sulfato complex ions and led to the precipitation of feooh which reacted with precipitated silica to form phyllosilicates alternatively degradation of the hydrolysate feooh to fe2o3 during sublimation of permafrost exposed on mars surface may account for the presence of eolian maghemite suspected to be the magnetic mineral observed on the viking landers

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

crust and upper mantle structure of the new madrid seismic zone insight into intraplate earthquakes we determine a 3 d p wave velocity model of the crust and upper mantle down to 400 km depth to investigate structural heterogeneity and its influences on the generation of intraplate earthquakes in the new madrid seismic zone we used 4871 high quality arrival times from 187 local earthquakes and 30 846 precise travel time residuals from 1041 teleseismic events recorded by the earthscope usarray transportable array our results show that beneath the reelfoot rift a significant low velocity low v zone exists in the upper mantle down to 200 km depth with a large volume of 200 x 200 x 150 km 3 the origin of the low v zone may be related to the passage of the bermuda hotspot and the stalled ancient farallon slab materials foundering in the mantle transition zone this low v zone may have relatively low shear strength and act as a viscously weak zone embedded in the lithosphere being apt to concentrate tectonic stress and transfer stress to the seismogenic faults in the upper crust leading to the large intraplate earthquakes in the new madrid seismic zone c 2014 elsevier b v all rights reserved

mineralogy of martian atmospheric dust inferred from thermal infrared spectra of aerosols 1 we have utilized optical constants derived from thermal infrared spectra acquired by the mariner 9 infrared interferometer spectrometer m9 iris and mars global surveyor thermal emission spectrometer mgs tes during periods of relatively high dust low water ice opacity to generate synthetic transmission and emission spectra of the atmospheric dust using libraries of transmission and emission spectra of common rock forming minerals and their alteration products we applied a linear deconvolution algorithm to the spectra to model the dust mineralogy models of the m9 iris transmission data generally are of poor quality and the mineralogical results of these fits are unlikely to be valid fits to the tes transmission spectrum and tes and iris emission spectra are of better quality and suggest that the dust mineralogy is dominated by framework silicates feldspar and or zeolite with carbonate sulfate pyroxene and olivine identified near but below generalized detection limits evidence for phyllosilicates is not robust our results combined with those from in situ measurements and other visible to infrared spectral studies suggest that framework silicates probably feldspar dominate the dust mineralogy with lesser amounts of olivine pyroxene amorphous material hematite and magnetite and that the dust largely is the product of mechanical weathering of basaltic rocks with minor chemical alteration

mining and exploration history of the southeast missouri iron metallogenic province mining has been a driving force in the development of missouri for hundreds of years a history of feast and famine success and ruin in southeast missouri the mining history spans times from early crude furnaces and small mines to large modern operations working at depths of more than 730 m below the surface eight major iron orebodies and myriad minor deposits are known in the southeast missouri iron metallogenic province which is hosted in mesoproterozoic igneous rocks of the st francois mountains terrane the first iron furnace west of the mississippi river was opened in missouri in the early part of the 19 th century marking the beginning of a prosperous industry that shaped the fortunes of citizens and the state the industry persevered through the united states civil war including the dramatic battle in 1864 that caused extensive damage to the pilot knob ironworks earlier discovery in 1844 of banded iron formation ores in michigan and depletion of easily accessible surface iron deposits in southeast missouri led to eventual downturn in the local mining industry magnetic surveys in the 1950s resulted in discovery of several large subsurface deposits including the pea ridge iron oxide apatite deposit and the boss iron oxide copper gold deposit ca 52 km to the south iron production in missouri ceased in 2001 when the pea ridge mine closed great potential nevertheless exists for renewed production of iron and other metals in the province

the pilot knob magnetite deposit in the proterozoic st francois mountains terrane southeast missouri usa a magmatic and hydrothermal replacement iron deposit abstract the pilot knob magnetite deposit is located in southeast missouri within the 1380 1480 ma st francois mountains terrane rhyolitic trachytic volcanic rocks the deposit is tabular dips about 45 and is sill like in nature being approximately parallel to the bedding in the host tuffs the deposit was uncovered by erosion and exposed to weathering in the late proterozoic and is overlain in angular unconformity by the cambrian lamotte sandstone this proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin the magnetite rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron rich magma and a surrounding envelope of lower to moderate grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin after the development of the higher grade magnetite ores and the envelope of lower to moderate grade ores late hydrothermal minerals were deposited as cross cutting veins and breccia fill the two most abundant minerals in the higher grade portions of the deposit are magnetite and albitic plagioclase and petrologically the higher grade ores could be described as a magnetite sodic syenite the most abundant gangue mineral within the lower grade impregnated envelope of ores around the higher grade ores is k feldspar apparently relict from the rhyolites trachytes thin lenses within the higher grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the ree bearing mineral ferriallanite has been identified minor portions of the deposit below the proterozoic weathering cap consist of high grade hematite ores having equilibrium textures at depth the deposit is intruded by the shepherd mountain gabbro a 120 m thick near horizontal dike which resulted in minor contact metamorphism of the ore

using gravity and magnetic data for insights into the mesoproterozoic st francois terrane southeast missouri implications for iron oxide deposits gravity and magnetic data in conjunction with available drill hole data were used to decipher the nature of the volcanic lithologies and their relationship to iron oxide ore deposits within the northwest portion of the mesoproterozoic st francois terrane in southeast missouri analyses of the data included creating a deep seated gravity anomaly map by removing the gravitational effect of the paleozoic sedimentary units residual gravity and magnetic anomaly maps and two and one half dimensional gravity and magnetic models the residual gravity and magnetic anomaly maps highlighted a series of short wavelength maxima and minima anomalies that could be interpreted to be granitic within collapsed caldera systems surrounded later by ring intrusions the gravity and magnetic models illustrate that the sources of the anomalies are situated mainly within the upper 5 km of the crust and help to constrain the location of suspected calderas the iron oxide deposits some of which contain significant quantities of ree minerals are rarely associated with both residual gravity and magnetic maxima but may be on the edge of these maxima and or occur over gravity minima the ree rich pea ridge deposit is located over a gravity minimum where modeling shows the main ore body extending down to 1 5 km with a low density region extending to 5 km below the ore body the cause of this low density body is unknown but may be associated with the formation of the ore deposit potential iron oxide deposits may occur to the south and southwest of pea ridge based on our analysis while to the northeast of the pea ridge deposit an anomaly pattern similar to that over pea ridge was found

pressure temperature time paths the past decade has been a time of unprecedented improvement in our ability to reconstruct the thermal evolution of orogenic belts like many instances of rapid advancement in science this achievement can be traced to the convergence of disparate research directions the new field of thermochronology the study of the time temperature evolution of rocks derived from the realization by geochronologists that many mineral iso topic systems could be used to determine the time of cooling of a terrain through a series of predictable temperatures e g armstrong 1966 wagner et ai 1977 harrison mcdougali 1980 berger york 1981 petrologists have made great progress in their ability to discern the pressure tem perature history of metamorphic rocks through the improvement of con ventional thermobarometric techniques see bohlen lindsley 1987 and essene 1989 for reviews and the development of important new approaches to relating changes in mineral chemistry to changes in environ mental parameters spear selvcrstonc 1983 st onge 1987 from a theoretical standpoint several geophysical papers in the midto late 1970s and early 1980s explored heat transfer processes in orogenic belts and the role of these in the development and preservation of metamorphic mineral assemblages oxburgh england 1974 bickle et a1 1975 bird et a1 1975 england richardson 1977 toksoz bird 1977 england thompson 1984 among others the integration of geochronologic and petrologic techniques aliows us

mafic and ultramafic plutons associated with the new madrid rift complex a possible major source of the copper cobalt nickel mineralization of southeast missouri the main rift structure of the new madrid rift complex the reelfoot rift strikes northeast ward extending from northeastern arkansas through the missouri boot heel and western kentucky into southern illinois the recently discovered bloomfield lineament zone transects the reelfoot rift in the boot heel area and strikes north northwest passing through the mine la motte fredericktown subdistrict of the southeast missouri lead zinc copper mining district studies suggest a strong relationship between the proximity of the new madrid rift complex to the southeast missouri district and the anomalously high amounts of cu co and ni in the region particularly in the mine la motte fredericktown subdistrict supporting evidence comes from three sources 1 mineral zoning and paragenetic sequence 2 geotectonics of the new madrid rift complex and 3 regional variations of 87 sr 86 sr and 18 o 16 o isotope ratios a detailed microscopic and sem eds study of the pb zn cu ores from the viburnum no 28 brushy creek and goose creek mines in the southeast missouri district indicates a complicated paragenetic sequence characterized by repeated periods of mineral deposition the sequence is composed of 19 minerals including abundant galena sphalerite chalcopyritc pyrite marcasite dolomite calcite and quartz as well as minor siegenite bornite bravoite blaubleibender covellite covellite gersdorffite digenite enargite tennantite dickite and malachite in addition to the mineralogy and paragenetic sequence a number of significant findings resulted from a comprehensive microscopic study of ore textures from the three mines the paragenetic sequence of the main colloform stage of deposition indicates that the ore solutions evolved in the following general order 1 cu rich 2 co ni rich 3 pb zn rich and 4 fe rich the general depositional sequence corresponds with mineral zoning and is similar throughout the entire lead district most importantly the paragenetically early cu co ni sulfides may represent a regional early development of the cu co ni rich mineralization similar to that in the mine la motte fredericktown subdistrict and the southern extension of the viburnum trend this early cu co ni rich zone always lies at the lowest stratigraphic level and in the mine la motte fredericktown subdistrict indicates a southerly source for the ore fluids it is proposed that the source of at least part of the cu co ni and some siderophile elements of the southeast missouri lead deposits may be the result of basinal brine leaching of a series of probably multiaged mafic and ultramafic intrusions or their sediments controlled by the new madrid rift complex one such branch of the complex observed on landsat 2 imagery is expressed as a fracture zone that extends north northwest from the reelfoot rift toward fredericktown missouri namely the bloomfield lineament zone strong 87 sr 86 sr and 18 o 16 o isotope anomalies show respectively a consistent decrease and a consistent increase from south to north in southeast missouri with the contour lines strikingly tracking the bloomfield fracture zone this suggests that the lineament zone was a channel for ore fluids entering southeast missouri via the mississippi embayment the fracture zone may represent a series of transform faults or a portion of a failed continental rifting event the zone is an epicenter trend and a distinct first order lineament which appears to form part of the northeast boundary of the northwest trending missouri gravity low

calderas in the precambrian terrane of the st francois mountains southeastern missouri precambrian igneous rocks in the st francois mountains of southeastern missouri consist of silicic volcanic rocks mostly ash flow tuff which crop out mostly in the western and southwestern part of the region and epizonal granitic plutons which are mostly exposed in the northeastern part of the area these relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west southwesterly tilting and beveling by erosion in the eastern st francois mountains the occurrence of a thick rhyolitic ash flow tuff grassy mountain ignimbrite and its distribution relative to a large high silica pluton butler hill granite suggest that the ash flow is the principal eruptive material of a caldera the butler hill caldera and that butler hill granite is the frozen magma chamber that was originally beneath it structural relations indicate that butler hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming four small low silica plutons silvermine stono knoblick and slabtown granites are distributed peripherally to the butler hill granite and are interpreted as ring plutons emplaced along the ring fracture of the caldera or along arcuate intracaldera faults after resurgent doming and emplacement of butler hill granite more than 3 km of silicic volcanic rocks occupy a basinal structure in the western st francois mountains these rocks lie with angular unconformity upon up domed older rocks of the butler hill caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera this feature is interpreted as the topographic wall of a second caldera which lies to the west the occurrence of the thick taum sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the st francois mountains

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

effect of mica on the grain size of dynamically recrystallized quartz in a quartz muscovite mylonite abstract quartz ribbons in a quartz muscovite mylonite from the sunchang shear zone of south korea show a variation in the average size 25 107 m of dynamically recrystallized quartz depending on the fraction and dispersion of muscovite within the ribbons micas within the quartz ribbons occur both inside quartz grains and along their boundaries having a strong preferred orientation with the basal planes parallel to the mylonitic foliation micas or mica aggregates inside the quartz show a size range of 1 25 m whereas those along the quartz grain boundaries range from 2 to 98 m the average quartz grain size d calculated from three dimensional grain size distribution decreases drastically with increasing mica volume fraction f m up to f m 3 in the ribbons and further increases in the mica fractions above 3 result in only a little decrease with some fluctuation in the quartz grain size generating the equation of d 20 11 ln f m 9 64 with almost the same fraction of mica however the quartz ribbons with dispersed mica show a smaller quartz grain size than those with clustered mica indicating the importance of dispersion as well as fraction of second phase particles to the grain size of dynamically recrystallized quartz quartz c axes tend to have a random orientation with an f m 3 presumably due to the greater contribution to the total deformation of grain boundary sliding for quartz with a smaller grain size d

quantitative investigations of the missouri gravity low a possible expression of a large late precambrian batholith intersecting the new madrid seismic zone analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the missouri gravity low an important cratonic feature of substantial width about 125 km and length 600 km filtered anomaly maps show that this prominent feature extends nw from the reelfoot rift to the midcontinent rift system geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal 11 km thick batholith with either near vertical or outward dipping boundaries considering the modeled characteristics of the batholith structural fabric of missouri and relations of the batholith with plutons and regions of alteration a tectonic model for the formation of the batholith is proposed the model includes a mantle plume that heated the crust during late precambrian and melted portions of lower and middle crust from which the low density granitic rocks forming the batholith were partly derived the batholith called the missouri batholith may be currently related to the release of seismic energy in the new madrid seismic zone earthquake concentrations occur at the intersection of the missouri batholith and the new madrid seismic zone three qualitative mechanical models are suggested to explain this relationship with seismicity

upper mantle seismic anisotropy resulting from pressure induced slip transition in olivine seismic anisotropy in the oceanic lithosphere results from flow induced crystallographic preferred orientation of dry olivine during lithosphere creation recent experiments however showed that high water activity changes the flow mechanisms of olivine and hence the crystallographic preferred orientation better explaining the seismic anisotropy in the mantle wedge above subduction zones whether changes in the crystallographic preferred orientation of olivine are unique to the effects of water has become controversial and is critical to resolve here we report low stress high strain experiments on typical dry mantle rock at high pressures and temperatures showing that at 3 gpa pressure induces the same profound transition in olivine crystallographic preferred orientation that is produced by high water activity at lower pressure one important consequence for global tectonics is that alignment of fast seismic waves parallel to trenches beneath subducting slabs probably reflects entrainment of sub lithospheric mantle in the direction of subduction rather than trench parallel flow as interpreted at present from the large variety of crystallographic preferred orientations in olivine in both experiments and natural rocks we infer that in addition to the pressure induced changes in olivine slip systems implied here there are probably further changes in slip systems at higher pressure and temperature seismic anisotropy in earth s oceanic lithosphere and in the mantle wedge above subduction zones is associated with crystallographic preferred orientations of olivine experiments at high pressure and temperature suggest that a pressure of 3 gpa can induce the same changes in the crystal structure of olivine as high water activity at lower pressures

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

digital image processing applied to analysis of geophysical and geochemical data for southern missouri linear gravity low across southern missouri underlain by low amplitude low gradient aeromagnetic anomalies due to variations in basement structures of rock types smoothed estimates of u th and k derived from aerial gamma ray data covering the rolla quadrangle were color coded and digitally overlaid onto a landsat image a shaded relief image of topography and the geologic map of missouri cambrian lamotte sandstone and bonneterre dolostone which crop out adjacent to the precambrian exposures have higher values than the younger cambrian and ordovician carbonates sandstones and shales enrichment within the interfluvial plains probably due to presence of a clay rich residuum modified journal abstract

thermally anomalous assemblages revisited patterns in the extraprovincial latitudinal range shifts of pleistocene marine mollusks pleistocene faunas of the eastern pacific shelf are characterized by thermally anomalous species assemblages i e coexisting species that inhabit different climatic regimes today we used data on the latitudinal ranges of 2887 extant molluscan species to determine the biological basis of the pleistocene faunal migrations overall the species exhibiting the most extensive range shifts termedextraprovincialspecies werenotdrawnrandomlyfromthe availablespeciespool andthepatternisclimaticallyasymmetrical the latitudinal ranges of southern extraprovincial species are significantly wider on average than those of the species pool from whichtheyweredrawn buttherangesofnorthernextraprovincials resemble those of their parent pool this contrast is primarily a consequence of the biogeographic structure of the eastern pacific fauna water massboundariesaremoreeffectivebarriersforsouthern species migrating north in response to changing climatic conditions than for northern species moving south our analysis of pleistocenemarinemollusksprovidesabiologicalandenvironmental context for species response to environmental change and permits predictions about the movement of eastern pacific species relative to major environmental barriers in the face of future global change

a new concept on tectonic correlation between korea china and japan histories from the late proterozoic to cretaceous abstract the dabie sulu collision belt in china extends to the hongseong odesan belt in korea while the okcheon metamorphic belt in korea is considered as an extension of the nanhua rift within the south china block the hongseong odesan belt divides korea s gyeonggi massif into northern and southern portions the southern gyeonggi massif and the yeongnam massif are correlated with china s yangtze and cathaysia blocks respectively while the northern gyeonggi massif is part of the southern margin of the north china block the southern and northern gyeonggi massifs rifted from the rodinia supercontinent during the neoproterozoic to form the borders of the south china and north china blocks respectively subduction commenced along the southern and eastern borders of the north china block in the ordovician and continued until a triassic collision between the north china and south china blocks while subduction was occurring on the margin of the north china block high p t metamorphic belts and accretionary complexes developed along the inner zone of southwest japan from the ordovician to the permian during the subduction the hida belt in japan grew as a continental margin or continental arc collision between the north and south china blocks began in korea during the permian 290 260 ma and propagated westwards until the late triassic 230 210 ma creating the sinistral tanlu fault in china and the dextral fault in the hida and hida marginal belt in japan phanerozoic subduction and collision along the southern and western borders of the north china block led to formation of the qinling dabie sulu hongseong hida yanji belt

nature and distribution of surficial deposits in chryse planitia and vicinity mars color images of bright red dust deposits at the mutch memorial station were acquired at variable incidence angles during sol 611 subsolar longitude 70 northern spring season after removing effects due to atmospheric scattering and absorption the data were used to estimate the independent variables in the hapke 1986 photometric function in blue green and red coordinates the vector representing the space radiance factor of the landing site extracted from viking orbiter images acquired on sol 609 is separated by a euclidean distance of only 0 022 units and an angle of only 1 5 from the vector estimated from the station data for the orbiter lighting and viewing geometries this result implies that light reflected from dust exposures dominates the orbiter signal multiplicative and additive atmospheric terms cancel one another and surface roughness is a second order effect in the orbiter data dust radiance factors computed from station data are most like laboratory spectra for fine grained hawaiian palagonite and are indistinguishable from earth based spectra of classical bright areas color composites of orbiter images show that the dust is found immediately south of acidalia planitia and in association with topographic barriers such as craters and cliffs examination of viking infrared thermal mapper data shows that the dust deposits typically do not have distinctive thermal inertia signatures implying that the deposits are optically thick hundreds of micrometers but thinner than the diurnal thermal skin depth centimeters dark gray material with thermal inertia values 8 12 10 3 cal cm 2 s 1 2 k 1 indicative of sand grain sizes 0 5 1 5 mm dominates the acidalia planitia lowlands and parts of kasei vallis this material also occurs as dark streaks extending from craters in xanthe terra and oxia palus space radiance factors of dark gray material are similar to spectra of mafic rock mixed with a minor amount of palagonitelike material material that is darker than but just as red as the dust deposits occurs in lunae planum xanthe terra and oxia palus areas of intermediate elevation multiple phase angle orbiter images suggest that the dark red exposures are a mixture of bright red and dark gray materials with the dark red exposures being relatively rough at a subpixel scale as compared to exposures of the other two materials thus only two types of materials can be detected in the data covering the study area palagonitelike dust and mafic rock fragments thermal inertia values 4 8 10 3 cal cm 2 s 1 2 k 1 for the dark red material are consistent with the presence of fine to medium sand size particles 0 10 0 5 mm such material should be easily moved by winds the lack of aeolian features implies that the dark red deposits are not composed of loose material rather they are probably more eroded versions of blocky soil or duricrust cemented dust and rock fragments exposed at the station both bright red dust aeolian suspension load and dark gray materials saltation traction loads migrate over the dark red substrate the overall distribution of the surficial units is controlled by topography at a variety of length scales lower wind threshold friction velocities associated with higher atmospheric densities in lowlands keep dust from accumulating and duricrust from forming while higher threshold velocities in highlands lead to net accumulation of dust local topographic obstacles craters walls ridges perturb wind flow and lead to local accumulation or erosion at a variety of elevations in addition the bright red dust ubiquitously found between exposures of dark gray and dark red materials may accumulate as the wind velocity gradient decreases at the transition from smoother dark gray exposures to rougher dark red exposures the distribution of the materials must also be modulated by climatic variations induced by quasi periodic oscillations in obliquity eccentricity and spin axis direction constraining these surficial deposits to be 1 ma

cretaceous reactivation of the deokpori thrust taebaeksan basin south korea constrained by k ar dating of clayey fault gouge 1 the deokpori thrust juxtaposing the early paleozoic yeongwol type joseon supergroup yeongwol group against the duwibong type joseon supergroup taebaek group in the okcheon belt is an important regional structure in the phanerozoic tectonic evolution of korea although the initiation of the thrust has recently been conjectured to occur during the middle jurassic daebo tectonic event in a continental magmatic arc setting its movement history has never been studied we report timing of brittle reactivation of the thrust obtained from k ar dating of fault gouges in the core of the deokpori thrust zone k ar illite age data of fault gouges suggest reactivation of the thrust at ca 72 ma after a contact metamorphism around 94 ma our result is the first documentation of the reactivation of a major jurassic daebo structure and the reactivation may represent a significant contractional tectonic event during the late cretaceous in the korean peninsula

magnetic changes accompanying the thermal decomposition of nontronite in air and its relevance to martian mineralogy thermal treatment of nontronite in air for long periods at 700 c or short periods at 900 c results in destruction of the nontronite structure a distinct reddening in color and a spectacular increase in magnetic susceptibility and saturation magnetization up to 4 4 am2 kg magnetic property measurements and calculations suggest that the magnetism is due to the presence of ultrafine particles of or fe2o3 the precise identity has not yet been resolved the highly magnetic thermally treated nontronite is amorphous to x rays consistent with an ultrafine grain size prolonged heating results in the growth of fe2o3 as suggested by x ray ir and magnetic measurements reflectivity spectra of a sample heated for 1 hour at 900 c indicates in addition to fe2o3 the presence of an opaque magnetite like phase given the composition of the starting material and the experimental conditions the presence of fe3o4 is unlikely the cause of this opacity is as yet unknown thermally treated nontronite has chemical color and magnetic properties akin to those found by viking on mars these results favor an origin for the fine grained martian surface material by repeated impacts into an fe rich smectite charged regolith weldon et al 1980