an aerodynamic roughness length map derived from extended martian rock abundance data many boundary layer processes simulated within a mars general circulation model mgcm including the description of the processes controlling dust rising from the martian surface are highly sensitive to the aerodynamic roughness length z0 on the basis of rock size frequency distributions inferred from different martian landing sites and earth analog sites we have first established that lognormal modeled rock size frequency distributions are able to reproduce correctly the observed martian rock populations we have validated the hypothesis that the rock abundance of a given area could be estimated at a first order from its thermophysical properties namely its thermal inertia i and its albedo we have demonstrated the possibility of using rock abundance to estimate the roughness density on mars and to retrieve subsequently the aerodynamic roughness length by using semi empirical relationships based on terrestrial wind tunnel and field measurements by combining our methodology with remote sensing measurements of the thermal emission spectrometer aboard mars global surveyor we have derived a global map of the aeolian aerodynamic roughness length with a 1 8 1 8 resolution over the entire martian surface contrary to what is often assumed the martian aeolian aerodynamic roughness length is spatially highly heterogeneous at the fullest resolution the martian aerodynamic roughness length varies from 10 3 cm to 2 33 cm about 84 of the martian surface seems to be characterized by an aeolian aerodynamic roughness length value lower than 1 cm the spatially uniform value that most of the mgcms simulations have assumed recently since the aerodynamic roughness length z0 is a key parameter in deriving the erosion threshold wind velocities we anticipate a significant impact of our findings on the efficiencies for lifting dust in future mgcms

sedimentary record of a fluctuating ice margin from the pennsylvanian of western gondwana parana basin southern brazil abstract the parana basin is a key locality in the context of the late paleozoic ice age lpia because of its location east of the andean proto margin of gondwana and west of contiguous interior basins today found in western africa in this paper we document the sedimentary record associated with an ice margin that reached the eastern border of the parana basin during the pennsylvanian with the aim of interpreting the depositional environments and discussing paleogeographic implications the examined stratigraphic succession is divided in four stacked facies associations that record an upward transition from subglacial to glaciomarine environments deposition took place during deglaciation but was punctuated by minor readvances of the ice margin that deformed the sediment pile tillites well preserved landforms of subglacial erosion and glaciotectonic deformational structures indicate that the ice flowed to the north and northwest and that the ice margin did not advance far throughout the basin during the glacial maximum consequently time equivalent glacial deposits that crop out in other localities of eastern parana basin are better explained by assuming multiple smaller ice lobes instead of one single large glacier these ice lobes flowed from an ice cap covering uplifted lands now located in western namibia where glacial deposits are younger and occur confined within paleovalleys cut onto the precambrian basement this conclusion corroborates the idea of a topographically controlled ice spreading center in southwestern africa and does not support the view of a large polar ice sheet controlling deposition in the parana basin during the lpia

on the origin of the itarare group basal nonconformity and its implications for the late paleozoic glaciation in the parana basin brazil abstract outcrop studies of the itarare group in the alfredo wagner area along the eastern margin of the parana basin in brazil reveal a partially exhumed nnw sse oriented glacial paleovalley the carboniferous rio do sul formation represents the basal interval of the glacially influenced itarare group and rests directly on precambrian basement rocks this nonconformity is thought to represent an areal scouring surface as indicated by the series of glacially carved linear depressions that were incised into the glaciated bedrock during glacial advances these large scale erosional landforms differ in their extent by orders of magnitude and include wide deep troughs as well as narrower shallower gouges glacial troughs and gouges usually display smaller scale subglacial erosion landforms such as polished and grooved striations on their walls and floors these indicate the nnw motion of a warm based grounded glacier in addition large scale erosional features represent local topographic lows within the major glacial paleovalley these local lows are large enough to contain part of the succeeding deglacial deposits which thin toward and onlap the wall of these depressions we grouped these deglacial deposits into six genetic facies associations fa 1 to 6 within three fining upward glacial cycles fa 1 is composed of black shale and represents an initial marine flooding event it can be correlated with the regionally widespread lontras shale fa 2 is composed of sandstones and mudstones associated with flood derived hyperpycnal flows and occasionally with surge type turbidites fa 3 is represented by plastered or injected matrix supported conglomerates emplaced as lodgment tills fa 4 comprises matrix and clast supported conglomerates sandstones and mudstones attributed to ice rafting and concentrated flows in grounding line fan facies this unit also contains ice keel dragging features carved into soft substrate fa 5 is composed of dropstone bearing black shale related to a second major marine flooding event within the paleovalley fa 6 comprises sandstones associated with channelized surge type turbidites forming the second turbidite system in the succession as these deposits are confined to a paleovalley we infer that sedimentation was influenced by topographically controlled ice streams instead of non confined widespread ice lobes as previously suggested reconstructing south america and southern africa in their positions during the lpia suggests that the glacial valleys then present in the namibia area may have extended further into the eastern margin of the parana basin in this scenario confined outlet glaciers would have flowed out from the namibian windhoek ice cap and terminated along the eastern margin of the parana basin

the distribution of dust devil activity on mars 1 martian dust devil activity inferred from their tracks is mapped globally and compared to surface and atmospheric properties dust devils are estimated to lift 2 3 1 1011 kg of dust each year mostly from two narrow bands near 60 n and 60 s during their respective spring and summer approximately 55 of the dust lifted by dust devils in the northern hemisphere is removed from within 45 and 75 n while 65 of that lifted in the south is from within 45 and 75 s the southern band is an order of magnitude more populated with tracks than in the north likely a result of mars orbital eccentricity the equator is the location of a lesser peak while few dust devil tracks are found at the poles and middle latitudes during the fall and winter in both hemispheres few tracks are observed dust devil and track formation does not appear to be controlled significantly by elevation topographic slope dust cover or surface physical properties dust devils annually lift approximately half as much material as local and regional dust storms and are therefore significant contributors of dust into the martian atmosphere

martian surface properties indicated by the opposition effect abstract opposition measurements made by the viking orbiter television cameras in the arabia syrtis major and elysium planitia regions have been combined with observations previously reported to provide a photometric comparison of these areas and several generic features radiative transfer expressions were used to derive average surface particle single scattering albedos phase functions and porosities best functional fit to the data includes consideration of atmospheric scattering two particle populations and surface roughness several findings include the ubiquitous presence of high albedo high porosity surface particles the absence of an opposition surge in the syrtis major region and the largest surface roughness in the chryse areas

late palaeozoic glacial cycles and subcycles in western gondwana correlation of surface and subsurface data of the parana basin brazil the parana basin the largest basin in south america received glacially derived sediments during the late palaeozoic ice age lpia of the gondwana supercontinent despite the importance of this basin for understanding the continental development of the gondwana glaciation and the fact that ca 95 of this basin is not exposed at the surface few attempts have been made to connect the exposed glacial strata to the subsurface record in this paper exposures of glacial cycles in the upper itarare group in santa catarina state southern brazil are analyzed locally correlated and then linked to the three major glacial cycles previously described from subsurface studies along the basin together study areas doutor pedrinho and vidal ramos record five partially comparable shorter glacial subcycles relative to the major glacial cycles these series comprise coarse grained subaqueous outwash deposits turbidite sand sheets marine shales and diamictites the latter mostly derived from delta slope failure and ensuing resedimentation in addition to sedimentological and genetic stratigraphic description and analysis preliminary age determination based on the palynological content is also presented besides a regional correlation of the described succession to the subsurface record is proposed based on well logs and core information all the palynomorph associations identified from the exposed successions which represent the upper third part of the itarare group are related to the subzone protohaploxypinus goraiensis base of the vittatina costabilis zone this zone and correlated ones along the gondwana are considered early permian in age however a first isotopic age recently obtained for the upper itarare group and published elsewhere is considered within a regional stratigraphic framework once it leads to new insights in terms of the lpia time span recorded in the parana basin

mars atmospheric opacity effects observed in the northern hemisphere by viking orbiter imaging observations of changing mars contrasts by viking orbiter television cameras have provided a description of changing atmospheric opacity in the southern hemisphere paper 1 this report extends those measurements into the northern hemisphere over a greater time period and provides a detailed description of photometric changes at the lander sites as seen from orbit reflectivity changes compared with optical depth increases produced by four dust storms indicate mean particle scattering changes with storm evolution as well as substantial opacity in the northern hemisphere for the duration of nearly a martian year

genesis of phosphorite crusts off peru abstract authigenic phosphorite crusts from the shelf off peru 9 40 s to 13 30 s consist of a facies with phosphatic coated grains covered by younger phosphatic laminite the crusts are composed of carbonate fluorapatite which probably formed via an amorphous precursor close to the sediment water interface as indicated by low f p2o5 ratios sr and ca isotopes as well as rare earth element patterns agreeing with seawater dominated fluids small negative ce anomalies and u enrichment in the laminite suggest suboxic conditions close to the sediment water interface during its formation increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and consequently point to episodical development of anoxic conditions during phosphogenesis the peruvian phosphorites formed episodically over an extended period of time lasting from middle miocene to pleistocene individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide rich phosphatic layers coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer phosphatic coated grains were later transported to the place of crust formation where subsequent laminite formation was favored under lower energy conditions a similar succession of phosphatic layers with varying contents of organic matter and sulfide rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

chronology of paleozoic metamorphism and deformation in the blue ridge thrust complex north carolina and tennessee the blue ridge province in northwestern north carolina and northeastern tennessee records a multiphase collisional and accretionary history from the mesoproterozoic through the paleozoic to constrain the tectonothermal evolution in this region radiometric ages have been determined for 23 regionally metamorphosed amphibolites granitic gneisses and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone the garnet ages from the pumpkin patch a thrust sheet 458 455 and 451 ma are similar to those from the structurally overlying spruce pine thrust sheet 460 456 455 and 450 ma both thrust sheets exhibit similar upper amphibolite facies conditions because of the high closure temperature for garnet the garnet ages are interpreted to date growth at or near the peak of taconic metamorphism devonian metamorphic ages are recognized in the spruce pine thrust sheet where sm nd and rb sr garnet ages of 386 and 393 ma and mineral isochron ages of 397 14 and 375 27 ma are preserved hornblendes record similar sup 40 ar sup 39 ar sm nd and rb sr ages of 398 to 379 ma devonian sup 40 ar sup 39 ar hornblende ages are also recorded in the structurally lower pumpkin patch thrust sheet the devonian mineralmore ages are interpreted to date a discrete tectonothermal event as opposed to uplift and slow cooling from an ordovician metamorphic event the mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the alleghanian orogeny the composite thrust stack of the blue ridge complex was subsequently thrust northwestward along the linville falls fault during middle alleghanian orogeny about 300 ma less

precise time and conditions of peak taconian granulite facies metamorphism in the southern appalachian orogen u s a with implications for zircon behavior during crustal melting events abstract the time of peak thermal conditions in the granulite facies zone of the southern appalachian orogen corresponds to the time of formation of garnet bearing leucosomes generated by biotite dehydration melting in garnet sillimanite gneisses leucosomes contain unusually abundant zircon occurring as inclusions in euhedral plagioclase phenocrysts and garnet isotope dilution thermal ionization mass spectrometric analysis of 13 elongate euhedral zircon crystals from the leucosome yields a concordant u pb age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9

formation of monazite via prograde metamorphic reactions among common silicates implications for age determinations three lines of evidence from schists of the great smoky mountains nc indicate that isogradic monazite growth occurred at the staurolite in isograd at 600 c 1 monazite is virtually absent below the staurolite in isograd but is ubiquitous several hundred grains per thin section in staurolite and kyanite grade rocks 2 many monazite grains are spatially associated with biotite coronas around garnets formed via the reaction garnet chlorite muscovite biotite plagioclase staurolite h2o 3 garnets contain high y annuli that result from prograde dissolution of garnet via the staurolite in reaction followed by regrowth and rare monazite inclusions occur immediately outside the annulus and in the matrix but not in the garnet core larger monazite grains also exhibit quasi continuous th zoning with high th cores and low th rims consistent with monazite growth via a single reaction and fractional crystallization during prograde growth common silicates may host sufficient p and lrees that reactions among them can produce observable lree phosphate specifically phosphorus contents of garnet and plagioclase are hundreds of parts per million and dissolution of garnet and recrystallization of plagioclase could form thousands of phosphate grains several micrometers in diameter per thin section lrees may be more limiting but sheet silicates and plagioclase can contain tens to 100 ppm lree so recrystallization of these silicates to lower lree contents could produce hundreds of grains of monazite per thin section monazite ages determined via electron and ion microprobes are 400 ma directly linking prograde barrovian metamorphism of the western blue ridge with the acadian orogeny in contrast to previous interpretations that metamorphism was taconian 450 ma interpretation of ages from metamorphic monazite grains will require prior chemical characterization and identification of relevant monazite forming reactions including reactions previously viewed as involving solely common silicates

a new stratigraphic framework built on u pb single zircon tims ages and implications for the timing of the penultimate icehouse parana basin brazil

structural history of the manitouwadge greenstone belt and its volcanogenic cu zn massive sulphide deposits wawa subprovince south central superior province structural analysis of the manitouwadge greenstone belt integrated with detailed mapping and geochronological and petrographic studies reveals a complex early deformation history that significant

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

monazite u pb dating of staurolite grade metamorphism in pelitic schists a study of the occurrence of and relations between rare earth element ree minerals in pelitic schists indicates that monazite forms at or near the p and t of the staurolite isograd samples at staurolite grade from the silurian perry mountain formation in the rumford quadrangle of maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites the bulk chemistry of the metapelites as seen in the major element abundances and ree patterns does not vary significantly across the study area thus the appearance and disappearance of ree phases is assumed to reflect changes in metamorphic grade in a sample from the biotite zone scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 m scale the texture suggest that metastable detrital monazite breaks down distributing its ree components to allanite from samples below staurolite grade in which monazite is not present our observations suggest that rees are partitioned into allanite at or near the staurolite isograd monazite forms as a metamorphic mineral initiating its role as a geochronometer garnet biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite 525 c 25 c at 3 1 0 25 kbar a total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated each date is remarkably concordant even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes calculations indicate insignificant th disequilibrium in these monazites the conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade two distinct metamorphic events are resolved one at around 400 ma and one at about 370 ma the latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 ma this work suggests that in addition to dating plutonism and high grade metamorphism monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists

patterns of late palaeozoic deglaciation in the parana basin brazil abstract the itarare subgroup middle carboniferous early permian of the parana basin of southeastern brazil contains the thickest most extensive and one of the longest records of late palaeozoic glaciation in all of the gondwana supercontinent glacially controlled deposition started as early as the middle carboniferous when lobate termini of ice streams from the windhoek ice sheet wis and rio grande do sul and asuncion ice caps moved into the parana basin lobes reached sea level and advanced grounded at least as far as 200 km into the basin stabilizing as tidal intertidal margins glaciation and glacial sedimentation occurred penecontemporaneously with the transgressive phase of the late palaeozoic tectonic cycle of the parana basin while gondwana was gradually moving away from the south pole evolution involved complex interaction of tectonic palaeogeographic and palaeoclimatic factors initial flooding of the basin by an epicontinental sea in the middle carboniferous may have provided moisture necessary for the growth of glaciers conversely episodes of sea level rise probably affected the stability of marine glaciers the palaeoclimatic setting for the wis and other ice masses at the margins of the parana basin was mostly temperate multiple ice advances are evidenced by subglacial tillites resting on striated and or glaciotectonized basement and deformed glacigenic sediments identified at different stratigraphic levels in the itarare subgroup glaciation reached its maximum expansion sometime between the late carboniferous and early permian gradually diminishing and vanishing in the late early permian the reconstructed palaeogeographic evolution favors a model of irregular melt back of ice masses particularly along the eastern margin of the parana basin in analogy with the deglacial pattern of the pleistocene laurentide ice sheet in north america retreat of marine glaciers was mostly associated with relatively simple accumulations or complex build ups of ice contact proglacial facies associations in glacial estuarine embayments fast retreat was mainly controlled by decoupling associated with sea level rise and intense calving in the terrestrial environment more fragmentary evidence shows that retreat may have involved several mechanisms including downwasting of stagnant inactive ice margins and destabilization by surging of glaciers moving over deformable sediments highstand sea level following ice wastage is often marked by widespread deposition of relatively thin marine beds at least four stratigraphically different basinwide marine horizons are recognized isostatic rebound of deglaciated margins of the parana basin is recorded by transition from deep marine beds to shallowing coarsening upward prograding sequences of littoral fluvio deltaic facies basinwards sourced from raised basin margins eventually upon emersion these constituted platforms for interglacial and postglacial peat development although glaciation lasted for a long time glacial sedimentation alternated with relatively long periods of intense and widespread subaerial and subaquatic submarine reworking of previously deposited glacigenic sediments and deposition of normal non glacial sediments that comprise the bulk of the itarare subgroup sequence thin coal beds which locally rest on fossil soils denote climatic amelioration during these interglacial phases

shear criteria in the grenville province ontario canada abstract the grenville province of canada is a major proterozoic orogenic belt mapping and structural analysis shows that the central ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones the mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures these structures include rotated tectonic inclusions rotated boudins and pinch and swell structures sheath folds rotated single crystal porphyroclasts single crystal fish and shear band foliation the shear asymmetry indicated by these structures is highly consistent between members of the assemblage and over a large area of the grenville province and shows that the thrust nappes were transported to the northwest the consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady state foliations in the mylonites

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

dust streaks on mars global mapping and photometry of selected areas on mars are used to investigate the nature of bright and dark wind streaks that extend from topographic obstacles occurrence of both bright and dark streaks is strongly latitude dependent and is only weakly correlated with surface properties such as albedo and thermal inertia data on the colors albedos and phase behavior of streaks are consistent with models of bright streaks as mosaics of plains material and brighter redder dust less than 20 of the ground need be covered by the optically thick dust in the brightest parts of the streaks the amount of dust in optically thick layers could be as little as 10 3 g cm2 dark streaks can be interpreted as erosional windows in a patchy dust cover our model of dust deposition in optically thick patches is sedimentologically different from scenarios involving the deposition of ubiquitous optically thin layers it has the advantage that large amounts of dust can be deposited without affecting regional albedos

stratigraphy and tectonics of the virginia north carolina blue ridge evolution of a late proterozoic early paleozoic hinge zone late proterozoic 690 570 ma through ordovician rocks of the virginia north carolina blue ridge and valley and ridge record the evolution of an eastward facing passive margin and its subsequent deformation during continent arc collision taconic orogeny comparison of blue ridge stratigraphy with modern rifts and passive margins suggests that late proterozoic rifting involved substantial crustal attenuation in the eastern blue ridge the axial zone of the blue ridge from central virginia southward is occupied by a major fault system hayesville fries rock fish valley fault contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west during rifting normal faulting and rapid subsidence of attenuating crust in the eastern blue ridge produced rift basins in which deep water clastic and volcanic rocks accumulated lynchburg group ashe formation landward of the hinge zone in the western blue ridge mainly terrestrial to shallow water clastic and volcanic rocks were deposited in a series of subaerial rift basins swift run catoctin grandfather mountain mount rogers formations the hinge zone not only controlled depositional patterns throughout the rift and passive margin history but it was also reactivated during the taconic collision when attenuated crust and its cover were thrust westward over the shelf edge turbidite dominated rift sequences of the eastern blue ridge are unlike typical rift deposits which are characterized by predominantly subaerial and shallow water sedimentation deep water rifts may be common on attenuated crust as in the bay of biscay however they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny

moc observations of the 2001 mars planet encircling dust storm abstract from 15 september 1997 through 21 january 2006 only a single planet encircling martian dust storm was observed by mgs moc the onset of the storm occurred on 26 june 2001 l s 184 7 earliest recorded to date it was initiated in the southern mid to low latitudes by a series of local dust storm pulses that developed along the seasonal cap edge in malea and in hellas basin l s 176 2 184 4 the initial expansion of the storm though asymmetric was very rapid in all directions 3 32 m s 1 the main direction of propagation however was to the east with the storm becoming planet encircling in the southern hemisphere on l s 192 3 several distinct centers of active dust lifting were associated with the storm with the longest persisting for 86 sols syria claritas these regional storms helped generate and sustain a dust cloud haze which reached an altitude of about 60 km and a peak opacity of dust 5 0 by l s 197 0 the cloud had encircled the entire planet between 59 0 s and 60 0 n obscuring all but the largest volcanoes the decay phase began around l s 200 4 with atmospheric dust concentrations returning to nominal seasonal low levels at l s 304 0 exponential decay time constants ranged from 30 117 sols the storm caused substantial regional albedo changes darkening and brightening as a result of the redistribution removal and deposition of a thin veneer of surface dust at least 0 1 11 1 m thick it also caused changes in meteorological phenomena i e dust storms dust devils clouds recession of the polar caps and possibly surface temperatures that persisted for just a few weeks to more than a single mars year the redistribution of dust by large annual regional storms might help explain the long period 30 years between the largest planet encircling dust storms events

evolution of seasonal stratification in the celtic sea during the holocene seasonal thermocline development in the shelf seas of nw europe results in the formation of pronounced frontal regions which separate stratified from well mixed water numerical m 2 palaeotidal modelling of the nw european continental shelf during successive stages of the holocene transgression indicates that seasonal stratification started to develop in the central celtic sea once a critical threshold water depth had been exceeded at around 9000 years bp this hypothesis has been tested through faunal isotopic and geochemical analysis of an ams 14 c dated british geological survey vibrocore from the celtic deep bathymetric basin a record which spans the last 12 000 years marked positive trends in the stable oxygen isotopic data from the benthic foraminifera ammonia batavus and quinqueloculina seminulum between 9000 and about 5000 years bp represent a 4 5 c cooling in summer bottom water temperature caused by the progressive replacement of tidally well mixed by stratified water stable carbon isotopic data from the same species measured through the same core intervals show a negative trend consistent with net oxygen utilization and decomposition of organic matter beneath the productive frontal zone these geological data indicate the progressive evolution of seasonally stratified water in the celtic sea during the earlier part of the holocene modern shelf sea frontal and thermocline development are known to influence biological productivity sedimentation rates the organic content of shelf sediments and the climate of surrounding landmasses hence these palaeodata have significance in defining the role of shelf sea environments in the climate system

mass of dust in the martian atmosphere estimates of the mass of dust suspended in the martian atmosphere are derived from global and regional 9 m opacity maps produced from viking infrared thermal mapper data during the peak of the 1977b storm a total dust mass of approximately 4 3 1014 g was suspended equivalent to 4 3 10 4 g cm2 or a layer 1 4 m thick during a local dust storm near solis planum at ls 227 approximately 1 3 1013 g of dust were lofted equal to about a 6 m layer in that vicinity

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

deformation and metamorphic history of the western blue ridge eastern blue ridge terrane boundary southern appalachian orogen 1 the western eastern blue ridge boundary involved formation by premetamorphic thrusting of an oceanic fragment accretionary wedge island arc upon laurentian basement and metasedimentary cover detailed outcrop scale mapping structural and petrographic analysis reveal both terranes experienced the same deformation at the same metamorphic grade early f1 and f2 isoclinal folds are overprinted by f3 open to tight folds f3 transposes s2 to an axial planar cleavage s3 the youngest penetrative feature prekinematic to synkinematic relative to s2 barrovian metamorphism attained kyanite grade in the western blue ridge wbr upper amphibolite facies in the eastern blue ridge ebr and anatexis in both terranes postkinematic m2b is characterized by appearance of fibrolite throughout the kyanite zone coarsening to granular sillimanite and identification of a kyanite sillimanite isograd in the wbr d3 strain was accommodated by dynamic recrystallization of quartz neocrystallization folding and basal slip of micas pressure solution of plagioclase folding of kyanite and fracturing of garnet scattered lower greenschist facies m4 static retrograde metamorphism resulted in replacement of kyanite and garnet by sericite and chlorite respectively although the juxtaposition of two lithologically distinct terranes implies a fault relationship polymetamorphism and deformation obliterated any microfabrics mineral assemblages or structures that might have developed during premetamorphic thrusting the boundary cannot be an extension of a continent scale dextral strike slip fault system inferred to have developed along laurentian margin in the devonian

north and south subice gas flow and venting of the seasonal caps of mars a major geomorphological agent 1 dark polygons associated with fans and spots appear during the spring on the southern seasonal cap the basal sublimation of the translucent cap and the venting of the co2 gas are responsible for their formation as previously proposed for the spots and fans dark polygons appear when dark material emerges from elongated vents whereas spots and fans form from point sources a class of erosive features etched polygons is associated with depressions a few meters to tens of meters in diameters connected to a network of radiating troughs spiders spiders are shaped by the scouring action of the confined gas converging toward point sources whereas the etched polygons result from the forced migration of the co2 gas over longer distances the minimum age of the spiders is 104 years they result from one of the most efficient erosive processes on mars displacing 2 orders of magnitude more dust per year than a typical dust storm or than all the dust devils during the same time period in the north parts of the seasonal cap are translucent between ls 355 and ls 60 and are associated with spots fans dark polygons and possibly spiders suggesting that the basal sublimation and venting of the cap triggers a subice gas and dust flow that is modifying the morphology of the surface layer however perennial features are extremely uncommon on the north regolith indicating that the conditions for their formation or conservation are not met the reduced basal energy budget of the north cap compared to the south and the shorter seasonal life time of the north translucent ice may explain the relative scarcity of features in the north the polar layered deposits contain the stratigraphic record of climatic changes and catastrophic events both polar deposits may have been locally disrupted by the seasonal subice gas flow and the stratigraphic record may have been partially lost

late paleozoic paleofjord in the southernmost parana basin brazil geomorphology and sedimentary fill abstract in the southernmost part of the parana basin records of the late paleozoic glaciation occur in a discontinuous form preserved in paleovalley systems excavated in the crystalline basement this paper addresses one of these paleovalleys the mariana pimentel which extends over 60 km with nw se valley direction and a constant width of 2 5 km with the objective of demonstrating that the paleovalley worked as a fjord during the glaciation period its origin as well as sedimentary fill and morphology were analyzed the paleovalley morphology was obtained through electrical resistivity electrical sounding and lateral mapping and mathematical modeling in four transverse sections the morphology of the paleovalley documented by the u shape steady width and high depth reaching up to 400 m are typical features of modern glacial valleys the sedimentary facies that fill the base of the paleovalley such as rhythmites and dropstones with thickness up to 70 m and diamictites with faceted pebbles up to 5 m thick are signs of its glacial origin during the glaciation period the paleovalley had a connection to the epicontinental sea located to the northwest extended toward namibia and was excavated by glaciers from the highlands of this region thus the evidence attests that the mariana pimentel paleovalley was a fjord during the late paleozoic glaciation the duration of the late paleozoic glaciation which is longer than the quaternary glaciation the apatite fission track that suggests erosion up to 4 km thick in the study area and the lack of preserved hanging valleys in the mariana pimentel indicate that the paleovalley once featured a higher dimension furthermore the existence of paleofjords excavated in the border of the basement corroborates the idea of small ice centers controlled by topography during the late paleozoic glaciation

sillimanite potash feldspar assemblages in graphitic pelites strontian area scotland graphitic pelites of the western moinian were metamorphosed at the time of emplacement of the strontian granodiorite intrusion at a late stage of the caledonian orogeny producing a metamorphic zonation the sillimanite zone in which k feldspar does not occur with sillimanite is succeeded by the muscovite sillimanite k feldspar zone sillimanite k feldspar zone without primary muscovite and cordierite k feldspar zone secondary muscovite from retrograde hydration of sillimanite k feldspar is distinguished texturally from primary muscovite but is compositionally similar primary porphyroblastic muscovite inherited from the regional metamorphic textural evolution of the rocks disappears abruptly at the muscovite out isograd migmatites of earlier regional origin with recrystallized textures are distinguished from those associated with the late caledonian metamorphism which are confined to the sillimanite k feldspar and cordierite k feldspar zones muscovite compositions are inferred to be very low in fe3 there are no marked changes in muscovite composition at the entry of sillimanite k feldspar higher na contents than in some other muscovites coexisting with sillimanite k feldspar are interpreted in terms of relatively low p in the strontian area andalusite is found at two localities from cordierite garnet sillimanite biotite k feldspar quartz assemblages a p estimate of 4 1 0 4 kbar is obtained with the aqueous fluid having x text h text 2 text o approx 0 5 and the t at the cordierite k feldspar isograd is estimated as 690 c t at the muscovite out isograd is inferred to the maximum for muscovite quartz sillimanite k feldspar equilibrium with graphite at p 4 1 kbar t 645 c with x text h text 2 text o approx 0 84 the well defined lower boundary of the muscovite sillimanite k feldspar zone is attributed to regionally rather homogeneous fluid composition at x text h text 2 text o approx 0 7 the low p prevented melting in the muscovite sillimanite k feldspar zone migmatites in the higher zones are attributed to partial melting which accounts for the low x text h text 2 text o necessary to produce the cordierite assemblages obscurity of these migmatites is attributed to deformation during and after the migmatization a breccia structure in and near the cordierite k feldspar zone where pelites have flowed around disrupted pieces of psammite and amphibolite can be explained by presence of grain boundary melts in the pelites

long term wind erosion on mars abstract wind erosion and deposition are powerful agents of surface change on mars erosion is sensitive to the atmospheric density so feedback between orbit variations and atmospheric density can enhance the sensitivity of erosion rates to orbital parameters we use statistics derived from a 1 gyr integration of the spin axis of mars coupled with runs of the nasa ames 3 d general circulation model gcm at a variety of orbital conditions and pressures to explore this feedback we find that wind erosion in the gcm is associated with two factors baroclinic zone winds and strong cross equator solstice flows both of these factors are influenced by topography producing an asymmetry in the erosion pattern between the north and the south the erosion model averaged over 1 gyr produces potential erosion rates of 1 7 10 5 m yr 1 in the north and 1 0 10 5 m yr 1 in the south which increase by an order of magnitude in an early 28 mbar atmosphere the stability of these potential erosion patterns over geological time indicates that the lowland regions of mars are continuously eroded and that wind is capable of revealing ancient sedimentary deposits or eroding substantial deposits that may have otherwise been preserved

lontras shale parana basin brazil insightful analysis and commentaries on paleoenvironment and fossil preservation into a deglaciation pulse of the late paleozoic ice age abstract the lontras shale is a fossil site located in mafra santa catarina south of brazil this late paleozoic pennsilvanian cisularian stratum holds an important place with great potential for the synthesis of knowledge about paradigmatic events that happened on gondwana this marine fossil site preserved into a 1 1 m black shale comprises a combination of different kinds of preservation in broad taphonomical spectra including hard and or soft tissues of aquatic e g fishes poriferans and ammonoids and terrestrial biotas e g insect and woods among other organisms that were well preserved under still unrevealed process and conditions considering the prevailing depositional settings and aiming to present fossil diagenetic aspects that led to some special preservation modes we performed chemical analyses of sponges and insects by means of energy dispersive spectroscopy eds synchrotron micro x ray fluorescence micro xrf and raman spectroscopy to date we have identified the majority of fossils preserved as carbonaceous compressions or phosphatized forms the well preserved fossils have so far permitted new insights into metazoan evolution especially regarding insecta porifera and conodonta our findings not only allow suggestions regarding the chemical remains of soft tissues on these specimens but also can provide background data for future analyses of these groups in similar depositional settings

age constraints of the glaciation in the parana basin evidence from new u pb dates the absence of absolute dating in glacial deposits of the parana basin prevents better understanding of the complete history of the late palaeozoic ice age in western gondwana and the climatic change that took place in the late carboniferous in this study new u pb radiometric ages date the topmost glacial deposits of the itarare group at 307 7 3 1 ma kasimovian moscovian and the base of post glacial deposits of the rio bonito formation at 298 8 1 9 ma ghzelian asselian correlation of glacial and post glacial deposits of the parana basin with other southwestern gondwana basins shows that the end of the late palaeozoic ice age is asynchronous supplementary material laser ablation inductively coupled plasma mass spectrometry u pb zircon data and methods for the ash sample of the itarare group and the tonstein sample of the rio bonito formation are available at https doi org 10 6084 m9 figshare c 3278273

mechanism for burgess shale type preservation exceptionally preserved fossil biotas of the burgess shale and a handful of other similar cambrian deposits provide rare but critical insights into the early diversification of animals the extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft bodied fossil assemblages has remained enigmatic since walcott s initial discovery in 1909 here we demonstrate the mechanism of burgess shale type preservation using sedimentologic and geochemical data from the chengjiang burgess shale and five other principal burgess shale type deposits sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation low sulfate concentrations in the global ocean and low oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability subsequently rapid entombment of fossils in fine grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments a permeability barrier provided by bed capping cements that were emplaced at the seafloor is a feature that is shared among burgess shale type deposits and resulted from the unusually high alkalinity of cambrian oceans thus burgess shale type preservation of soft bodied fossil assemblages worldwide was promoted by unique aspects of early paleozoic seawater chemistry that strongly impacted sediment diagenesis providing a fundamentally unique record of the immediate aftermath of the cambrian explosion

carboniferous glaciotectonized sediments in the southernmost parana basin brazil ice marginal dynamics and paleoclimate indicators abstract carboniferous glaciogenic strata itarare gp in the southernmost parana basin brazil exhibit soft sediment deformation features previously interpreted as glaciotectonism these sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation depositional environments and paleoclimate in this region during the carboniferous five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts a medium sandstone facies a fine grained silt clay rhythmite and mudstone facies with dropstones and diamictite pellets a sandy clinoform facies and a folded sandstone with interbedded mudstone facies the depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice proximal transitional terrestrial to estuarine setting rb k values from the rhythmites reflect a transition from a freshwater to brackish environment and the chemical index of alteration cia of the rhythmites ranges from 65 to 73 reflecting a relative increase in the degree of chemical weathering through time deformation features include widespread folding thrust faults hydrofractures decollement surfaces and piggyback fold thrust complexes the deformation is interpreted as evidence of a push moraine complex formed by at least two complete ice advance retreat cycles the occurrence of decollement surfaces plastically deformed proglacial sediments and hydrofractures indicate a dynamic warm based or polythermal glacier abundant outwash sediments rhythmites with dropstones and the shift to average cia values all support a temperate paleoclimate deformation structures indicate a nw direction of ice shove that is in agreement with the regional scale hypothesis that a nnw flowing lobe extended out of uruguay during the carboniferous and terminated in the southernmost parana basin this study demonstrates that late paleozoic glaciation in this region was more dynamic than previously understood with high frequency fluctuations in ice marginal positions

taphonomy of lacustrine fish fossils of the parnaiba basin northeastern brazil spatial and causative relations of konservat lagerstatten in west gondwana during jurassic cretaceous abstract fossil bonanzas called konservat lagerstatten provide geological windows to the ecology of ancient life communities these deposits often occur as cluster of fossil localities with similar geographic locations geological ages and facies as the fossils were generally preserved during special periods and large scale events in earth history within environments with exceptional taphonomic conditions this work applied petrological and geochemical analyses to evaluate the taphonomic pathway of a konservat lagerstatten in the muzinho shale upper jurassic lower cretaceous of the parnaiba basin northeastern brazil these lacustrine deposits include interbedded limestones and fossiliferous black shales deposited in anoxic non euxinic and saline environment black shales contain articulated skeletons of fish fossils characterised by three dimensional preservation of the skeletal tissue the polytypical assemblage includes diverse ontogenetic stages and is concentrated in a specific stratigraphic level mass mortality was triggered by anoxia probably established by stratified water column high c flux and thermocline demise the specimens are encased in kerogen bearing siliciclastic laminae and cementstone microfacies both microbially induced the fossils were cemented by eodiagenetic poikilotopic calcite that filled the skeletal articulation including bone trabeculae and voids originally filled by bone marrow the short distance similar fossil assemblages ages and coincident palaeoenvironments suggest that muzinho shale parnaiba basin and crato formation araripe basin compose a cluster of exceptional preservation in west gondwana lava flows and outgassing related to the central atlantic magmatic province camp resulted in the subaereal exposure of volcanic plains acid rains and subsequent co2 greenhouse crisis camp magmatism was especially voluminous in northern brazilian basins enhancing nutrient rich waters and lacustrine eutrophication due to intensified chemical weathering in this area probably other konservat lagerstatten remain unreported in west gondwana despite similar causative events the individual taphonomic pathways of muzinho shale and crato formation are very different suggesting that in these cases local phenomena affected authigenesis and diagenesis

the wayah granulite facies metamorphic core southwestern north carolina high grade culmination of taconic metamorphism in the southern blue ridge the wayah granulite facies metamorphic core in the eastern blue ridge southwestern north carolina contains the metamafic assemblage hornblende orthopyroxene clinopyroxene which defines the hornblende lower granulite facies the metamorphic progression from the kyanite almandine zone into the wayah granulite core is continuous across the hayesville fault a major premetamorphic peak tectonostratigraphic boundary this metamorphic transition includes a distinct muscovite absent alkali feldspar second sillimanite zone retrogression is limited this entire range in metamorphic grade apparently was produced during a single prograde paleozoic likely taconic metamorphic event the hayesville fault crosses the study area mostly within the first sillimanite zone northwest of the wayah granulite core both supracrustal sequences juxtaposed along this fault consist predominantly of metasedimentary rocks probably deposited during the late proterozoic although paragneiss pre dominates the southeastern terrane is distinguished by the presence of metamorphosed felsic mafic and ultramafic igneous rocks no evidence supports the presence of grenville basement within the study area the second of four deformational events synchronous with the metamorphic peak created northeast southwest to east west trending upright isoclinal folds which dominate the outcrop pattern macroscopic f 2 folds deformed the hayesville fault application of mineralogic thermometry and barometry yields conditions which range from 585 c 5 5 kb at the kyanite sillimanite isograd to 842 c 9 8 kb in the hornblende granulite facies macroscopic distributions of migmatite and perthite published experimental data and barometric calculations suggest that t 669 c p 6 52 kb and x h 2 o 0 62 at the second sillimanite isograd results indicate that the wayah granulite facies core constitutes the high grade culmination of paleozoic metamorphism in the southern appalachian blue ridge

carboniferous permian carbon isotope stratigraphy of successions from china yangtze platform usa kansas and russia moscow basin and urals abstract the pennsylvanian and early permian were times of extreme sea level changes of 100 to perhaps up to 200 m for example glacio eustatic sea level changes are well documented from the us midcontinent moscow basin and yangtze platform where more than 100 stacked subtidal to supratidal sequences have been described in contrast carbonates developed in the ural mountains and south guizhou of the yangtze platform were deposited in ramp and slope settings 13c values of whole rock samples from the us midcontinent and moscow basin were intensively affected by diagenesis only well preserved brachiopod shells maintained their original carbon isotope ratios in contrast limestones deposited in the deeper water slope environments were not affected by meteoric diagenesis and are interpreted to have mainly retained their carbon isotope ratios mean 13c values of slope carbonates of the yangtze platform increase from about 3 during the visean and serpukhovian to 5 at the carboniferous permian boundary and decrease to values around 2 in the early kungurian this positive 13c excursion coincides with low 13c values in the yangtze platform successions which were reset by meteoric diagenesis the highest 13c values recorded in the slope succession coincide with the inferred maximum glaciation that caused pronounced sea level lowstands short lived but significant negative 13c excursions in the chinese slope succession are interpreted to reflect changes in ocean circulation due to sea level rises which caused enhanced production and or preservation of organic matter which influenced the subsequent early diagenetic cementation

nested paleozoic successor basins in the southern appalachian blue ridge field studies in the southern appalachian blue ridge and its southwest extension the talladega belt indicate that in at least three regions polydeformed and metamorphosed turbidite dominated sequences unconformably overlie rifted margin continental terrace wedge clastic rocks and overlying carbonate platform deposits these sequences are 1 the talladega group in the talladega belt 2 the walden creek group along the west flank of the blue ridge and 3 the mineral bluff formation within the core of the blue ridge paleontologic evidence indicates that the talladega and walden creek groups are in part as young as silurian devonian the presence of these anomalously young sequences unconformably above the trailing margin stratigraphy in the blue ridge brings into question conventional ideas of the timing and nature of the tectonic evolution of the ancient continental margin

retrogressive dissolution of garnet effect on garnet biotite geothermometry cation exchange geothermometry is predicated on the basis of chemical equilibration between phases involved in ion exchange reactions if chemical equilibrium is attained between the phases at any temperature and subsequently locked in temperature determinations should be consistent throughout a given sample the attainment of chemical equilibrium is commonly inferred where phases used for thermobarometry are in mutual contact or where sharp straight grain boundaries exist between grains thermometric determinations on garnet biotite pairs in a metapelite from the cheyenne belt wyoming illustrate the problem of using these criteria to infer chemical equilibrium in one example garnet biotite pairs from different parts of a single zoned garnet grain yield apparent temperatures that range over 150 c and are anomalously higher than documented metamorphic conditions almandine garnets from low to intermediate grade pelitic rocks commonly show strong compositional zoning retrograde modification coupled with strain induced dissolution of garnet may result in meaningless temperatures because of nonequilibration of phases used for geothermometry apparent textural equilibrium or mutual contact of grains may not be indicative of chemical equilibrium dissolution of garnet under conditions of biotite stability can produce garnet biotite pairs with sharp straight grain boundaries suggestive of textural equilibrium in the absence of visible truncation of compositional zones compositional maps or microprobe traverses both parallel and normal to schistosity are necessary to evaluate the possibility of dissolution failure to do so may result in the incorrect interpretation of t p determinations

origin of paleovalleys on the rio grande do sul shield brazil implications for the extent of late paleozoic glaciation in west central gondwana abstract the location longevity and geographic extent of late paleozoic ice centers in west central gondwana remain ambiguous paleovalleys on the rio grande do sul shield of southernmost brazil have previously been interpreted as fjords carved by outlet glaciers that originated in africa and emptied into the parana basin brazil in this study the sedimentology stratigraphy and provenance of sediments infilling two such paleovalleys the mariana pimentel and leao paleovalleys were examined in order to test the hypothesis that an ice center over present day namibia drained across southernmost brazil during the carboniferous and permian contrary to previous findings the facies assemblage from within the paleovalleys is inconsistent with a fjord setting and no clear evidence for glaciation was observed the facies show a transition from a non glacial lacustrine estuarine environment to a fluvial dominated setting and finally to a restricted marine estuarine environment detrital zircon results present a single population of neoproterozoic ages c 800 550 ma from the paleovalley fill that matches the ages of underlying igneous and metamorphic basement dom feliciano belt and is incongruent with african sources that contain abundant older mesoproterozoic paleoproterozoic and archean zircons furthermore results suggest that the formation of the paleovalleys and the deposition of their fill were controlled by the reactivation of neoproterozoic basement structures during the carboniferous and permian the lack of evidence for glaciation in these paleovalleys highlights the need for detailed studies of supposed late paleozoic glacial deposits these results are supportive of the hypothesis that well established glacial sediments on the rio grande do sul shield southern margin of the parana basin may be the product of a separate lobe extending north across uruguay rather than a single massive ice sheet draining west from africa

southeastern margin of the middle paleozoic shelf southwesternmost appalachians regional stability bracketed by acadian and alleghanian tectonism lower devonian lower mississippian metasedimentary rocks in the southernmost appalachian orogen near the southeast corner of laurentia record a period of continental margin stabilization bracketed by orogenic events best assigned to the early acadian and earliest alleghanian orogenies these strata are in the talladega belt a far traveled allochthon forming the southwestward extent of the western blue ridge in the allochthon lower paleozoic trailing margin carbonate shelf rocks are unconformably overlain by a 2 5 km thick early acadian silurian lower devonian clastic wedge derived from basement cored uplifts to the south and southeast deposition of this clastic wedge was followed by continental margin stabilization and deposition of a starved basin sequence containing widespread lower devonian lower mississippian chert near the outer shelf margin the chert sequence grades northeastward through a shelf slope basin plain transition into a thick black shale basinal facies these units were deposited during the classic acadian orogeny in new england and the catskill delta s progradation across the central appalachian foreland the major clastic response to acadian uplift the talladega sequence subsequently was subjected to low grade regional metamorphism and pervasive deformation possibly as late as middle mississippian during what were probably the initial stages of the alleghanian orogeny deposition of the lower devonian lower mississippian sequence of the talladega belt thus represents a significant hiatus in deformation along southeast laurentia bracketed first by orogenic activity related to early acadian silurian early devonian foundering of the early paleozoic shelf and deposition of a clastic wedge and later by earliest alleghanian regional dynamothermal events

the burnsville fault evidence for the timing and kinematics of southern appalachian acadian dextral transform tectonics there has been a conspicuous absence of documented acadian structures in the blue ridge province of the southern appalachians even though a growing body of evidence suggests that the middle paleozoic acadian orogeny affected the rocks of this region new mapping along with structural petrologic and geochronologic data from western north carolina show that the contact between the metasedimentary and metavolcanic rocks of the ashe metamorphic suite and the grenville basement rocks of the western blue ridge corresponds to a devonian high grade dextral strike slip fault zone the burnsville fault timing of motion on the burnsville fault is constrained by field relationships metamorphic fabrics and assemblages and radiometric ages a u pb zircon crystallization age for a pegmatite sheared by the burnsville fault shows that the last movement on the fault must he younger than 377 ma previously published 4 0 ar 3 9 ar cooling ages show that the burnsville fault must be older than ca 360 ma we have mapped the burnsville fault for 100 km in northwest ern north carolina the gossan lead fault of northwesternmost north carolina and southwestern virginia is the likely continuation of the burnsville fault to the north east southwest of asheville north carolina the burnsville fault may connect to the devonian dahlonega shear zone or may be cut by post devonian thrust faults associated with the alleghanian orogeny diachroneity of acadian elastic wedge the presence of silurian devonian pull apart basins the presence of devonian high grade dextral shearing in the inner piedmont and recent plate reconstructions for the middle paleozoic support the interpretation that the late phase of the acadian orogeny in the southern appalachians was primarily a dextral transpressional event the burnsville fault and the inner piedmont formed the boundaries of a dextral transform margin that may have extended the length of the appalachian orogen this interpretation requires that models for the paleozoic tectonics of the appalachians incorporate major pre alleghanian dextral displacement

inference of dust opacities for the 1977 martian great dust storms from viking lander 1 pressure data abstract a eddington radiative transfer algorithm is used to compute the thermal tidal heating of a dusty martian atmosphere for a given set of dust optical depth effective single scattering albedo and phase function asymmetry parameter the resulting thermal tidal forcing is used in a classical atmospheric tidal model to compute the amplitudes of the surface pressure oscillations at the viking lander 1 site for the two 1977 martian great dust storms parametric studies show that the dust opacities and optical parameters derived from the viking lander imaging data are roughly representative of the global dust haze needed to reproduce the tidal surface pressure amplitudes also observed at lander 1 except that the model inferred asymmetry parameter is smaller during the onset of a great storm the observed preferential enhancement during dust storm onset of the semidiurnal tide at viking lander 1 relative to its diurnal counterpart is shown to be due primarily to the elevation of the tidal heating source in a very dusty atmosphere although resonant enhancement of the main semidiurnal tidal mode makes an important secondary contribution

deformational behaviour of migmatites and problems of structural analysis in migmatite terrains an understanding of the response of migmatites to deformation is crucial to an interpretation of their structures and in anatectic and intrusive migmatite terrains due consideration must be given to the modification of deformation processes imposed by melts in partially molten systems containing more than 30 melt the classical theories of fold formation are inapplicable the folds produced are of the viscous fold type their geometry is unpredictable and may mimic structures due to refolding this is particularly true at very high degrees of melting where flow of melt carrying rafts of solids begins failure to distinguish viscous folds from the products of refolding will lead to errors in interpreting the structural and metamorphic history of an area and to consequent misinterpretation of the true structural relations between geometrically complex high grade and more regularly deformed low grade areas

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

exceptionally preserved fossil assemblages through geologic time and space abstract geologic deposits containing fossils with remains of non biomineralized tissues i e konservat lagerstatten provide key insights into ancient organisms and ecosystems such deposits are not evenly distributed through geologic time or space suggesting that global phenomena play a key role in exceptional fossil preservation nonetheless establishing the influence of global phenomena requires documenting temporal and spatial trends in occurrences of exceptionally preserved fossil assemblages to this end we compiled and analyzed a dataset of 694 globally distributed exceptional fossil assemblages spanning the history of complex eukaryotic life 610 to 3 ma our analyses demonstrate that assemblages with similar ages and depositional settings commonly occur in clusters each signifying an ancient geographic region up to hundreds of kilometers in scale which repeatedly developed conditions conducive to soft tissue preservation using a novel hierarchical clustering approach we show that these clusters decrease in number and shift from open marine to transitional and non marine settings across the cambrian ordovician interval conditions conducive to exceptional preservation declined worldwide during the early paleozoic in response to transformations of near surface environments that promoted degradation of tissues and curbed authigenic mineralization potential we propose a holistic explanation relating these environmental transitions to ocean oxygenation and bioturbation which affected virtually all taphonomic pathways in addition to changes in seawater chemistry that disproportionately affected processes of soft tissue conservation after these transitions exceptional preservation rarely occurred in open marine settings excepting times of widespread oceanic anoxia when low oxygen levels set the stage with these patterns non marine cluster count is correlated with non marine rock quantity and generally decreases with age this result suggests that geologic processes which progressively destroy terrestrial rocks over time limit sampling of non marine deposits on a global scale future efforts should aim to assess the impacts of such phenomena on evolutionary and ecological patterns in the fossil record

new dust opacity mapping from viking infrared thermal mapper data global dust opacity mapping for mars has been carried forward using the approach described by martin 1986 for viking ir thermal mapper data new maps are presented for the period from the beginning of viking observations until ls 210 in 1979 1 36 mars years this range includes the second and more extensive planet encircling dust storm observed by viking known as storm 1977b improvements in approach result in greater time resolution and smaller noise than the earlier work a strong local storm event filled the hellas basin at ls 170 prior to the 1977a storm dust is retained in equatorial regions following the 1977b storm far longer than in mid latitudes minor dust events appear to raise the opacity in northern high latitudes during northern spring additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation the first evidence of the start of the 1977b storm is pushed back to ls 274 2 preceding signs of the storm in images by about 15 hours

high spatial and temporal resolution sampling of martian gas abundances from crism spectra 1 the compact reconnaissance imaging spectrometer for mars on the mars reconnaissance orbiter spacecraft has been collecting spectra in the visible to near infrared wavelength range for over 5 years approximately 3 martian years observations consist of image cubes with two main selections of spectral sampling 74 and 545 spectral channels and two main selections of spatial sampling approximately 20 and 200 m pixel retrievals of gas abundances specifically co2 h2o and co are performed from spectra collected in all observation modes the retrievals are efficiently performed using a look up table where the strength of gas absorption features are precalculated for an n dimensional discrete grid of known input parameters season location environment viewing geometry etc and the one unknown parameter to be retrieved gas abundance a reverse interpolation in the look up table is used to match the observed strength of the gas absorption to the gas abundance this algorithm is extremely fast compared to traditional radiative transfer computations that seek to recursively fit calculated results to an observed spectral feature and can therefore be applied on a pixel by pixel basis to the tens of thousands of compact reconnaissance imaging spectrometer for mars images to examine cross scene structure as well as to produce climatological averages