the modi taung nankwe gold district slate belt central myanmar mesothermal veins in a mesozoic orogen abstract the recently discovered quartz gold veins comprising the modi taung nankwe gold district in central myanmar are largely hosted by mudstones of the late palaeozoic slate belt mesozoic rocks of the paunglaung mawchi zone separate the slate belt from the shan scarp and shan plateau to the east at modi taung 5 km of exploration adits indicate that the veins within steeply dipping oblique reverse slip shear zones are displaced by movements along the shears intruded by late jurassic calc alkaline dykes and offset on conjugate cross faults drill intercepts show that mineralization extends vertically for more than 500 m coarse visible gold book and ribbon texture and stylolitic laminations and trace metal values are consistent with mineralization at mesothermal depths of several kilometres in the brittle ductile regime our field observations suggest that mineralization took place in the early jurassic following collision of myanmar on the passive western margin of a greater shan thai continental block with an oceanic arc on the overriding plate to the west collision generated intra continental thrusting in and east of the paunglaung mawchi zone metamorphism of plateau rocks thrust west beneath the slate belt to form the mogok metamorphics and ascent of mineralizing fluids expelled from dehydrating underthrust rocks reversal in orogenic polarity initiated late jurassic eastward subduction of oceanic crust beneath myanmar and generation of a magmatic arc with dykes cutting the quartz gold veins renewed east directed thrusting translated the slate belt over arc clastics and probably resulted in exhumation of the mogok metamorphics and offsets of the veins on conjugate faults young k ar rb 87 sr 86 and ar 40 ar 39 ages on the mogok metamorphic belt imply renewed tertiary uplift and possibly intrusion of mid tertiary or palaeogene granites

lithospheric processes and products in the southern canadian cordillera a lithoprobe perspective analyses of lithoprobe and other data from southwestern canada provide new insights on how this portion of the cordillera formed during plate convergence along the western margin of north america crustal rocks are detached from their mantle lithosphere which must have been consumed during subduction detachment occurred at or near the base of the crust beneath the intermontane and or omineca belts probably along the tips of tectonic wedges while the rocks were still outboard of the relatively cool mechanically rigid north american craton during the late cretaceous and early tertiary rotation of detached rocks caught between the north american craton and the oceanic plates accounts for some apparently conflicting results between paleomagnetic data that indicate large northward translation of rocks in the western cordillera and regional geological features that appear to preclude comparable amounts of translation of rocks in the eastern cordillera during the same time interval transpression assoc

accretion rate of cosmic spherules measured at the south pole micrometeorites are terrestrially collected extraterrestrial particles smaller than about 1 mm which account for most of the mass being accreted to the earth1 2 compared with meteorites micrometeorites more completely represent the earth crossing meteoroid complex3 4 and should include fragments of asteroids comets mars and our moon as well as pre solar and interstellar grains3 6 previous measurements of the flux of micrometeoroids that survive to the earth s surface have large uncertainties owing to the destruction of particles by weathering7 8 9 inefficiencies in magnetic collection or separation techniques7 8 9 low particle counts10 11 poor age constraint7 8 9 12 13 or highly variable concentrating processes12 13 here we describe an attempt to circumvent these problems through the collection of thousands of well preserved and dated micrometeorites from the bottom of the south pole water well which supplies drinking water for the scott amundsen station using this collection we have determined precise estimates of the flux and mass distribution for 50 700 m cosmic spherules melted micrometeorites allowing for the expected abundance of unmelted micrometeorites14 in the samples our results indicate that about 90 of the incoming mass of submillimetre particles evaporates during atmospheric entry our data indicate the loss of glass rich and small stony spherules from deep sea deposits7 8 and they provide constraints for models describing the survival probability of micrometeoroids15 16

middle jurassic exhumation along the western flank of the selkirk fan structure thermobarometric and thermochronometric constraints from the illecillewaet synclinorium southeastern british columbia new thermobarometric and thermochronometric data from the illecillewaet synclinorium a broad regional southwest verging structure in the western flank of the selkirk fan structure indicate that accretion of the intermontane superterrane resulted in deep burial 20 25 km of the outer margin of ancestral north america and that subsequent southwest verging folding and thrusting were accompanied by rapid exhumation southwest verging folding of lower paleozoic rocks in the illecillewaet synclinorium was initiated during regional peak metamorphic conditions at 6 7 kbar crystallization of late synkinematic granitoid plutons that intrude the illecillewaet synclinorium also occurred at pressures of 6 7 kbar however equilibration of the contact aureoles of the plutons occurred at 3 3 5 kbar during the latter stages of the southwest verging deformation evidently a decompression on the order of 3 kbar occurred concurrently with southwest verging folding and thrusting in the illecillewaet synclinorium thermochronometric analyses further indicate that plutonic rocks from the illecillewaet synclinorium cooled rapidly between times of crystallization and times of ar closure in biotite regional relationships indicate that the accretion of the intermontane superterrane to the edge of the north american margin in southeastern british columbia occurred between 187 and 173 ma toarcian bajocian subsequent southwest verging deformation occurred approximately between 173 and 168 ma bajocian bathonian thus the removal of at least 10 km of rocks from above the illecillewaet synclinorium inferred from thermobarometric data occurred approximately between 173 and 168 ma rapid denudation of the selkirk fan structure was probably the result of synorogenic extension or extensional collapse contemporaneous with the development of southwest verging structures at deeper levels of the orogen the data presented here indicate that all significant deformation and most of the exhumation along the western flank of the selkirk fan structure were complete by late middle jurassic time after the middle jurassic the selkirk fan structure must have been transported passively northeastward along the basal decollement of the cordilleran foreland thrust and fold belt together with the intermontane superterrane as the fold and thrust belt tectonically overrode the margin of the north american craton

relationship between sea ice freeboard and draft in the arctic basin and implications for ice thickness monitoring we have confirmed our earlier finding that the probability density function pdf of ice freeboard in the arctic ocean can be converted to a pdf of ice draft by applying a simple coordinate transformation based on the measured mean draft and mean elevation this applies in each of six 50 km sections north of greenland of joint airborne laser and submarine sonar profile obtained along nearly coincident tracks from the arctic basin north of greenland and tested for this study detailed differences in the shape of the pdf can be explained on the basis of snow load and can in principle be compensated by the use of a more sophisticated freeboard dependent transformation the measured density ratio r actually mean draft mean elevation ratio for each section was found to be consistent over all sections tested despite differences in the ice regime indicating that a single value of r might be used for measurements done in this season of the year the mean value r from all six sections is 7 89 on the assumption that all six values are drawn from the same population the standard deviation is 0 55 for a single 50 km section and thus 0 22 for 300 km of track in attempting to infer ice draft from laser measured freeboard we would therefore expect an accuracy of about 28 cm in 50 km of track if mean draft is about 4 m and about 11 cm in 300 km of track these accuracies are compatible with the resolution of predictions from numerical models a simple model for the variability of r with season and with mean ice thickness gives results in reasonable agreement with observations they show that although there is a large seasonal variability due to snow load there is a stable period from november to april when the variability is chiefly dependent on the mean ice thickness alone thus in principle r can be mapped over the arctic ocean as a basis for interpreting survey data better field data are needed on the seasonal and spatial variability of three key quantities area averaged snow load mean density of first year and multiyear ice including the effect of ridging with these two ice regimes and density of near surface water

40ar 39ar thermochronology of the thor odin pinnacles area southeastern british columbia tectonic implications of cooling and exhumation patterns the thor odin dome is a basement cored tectonothermal culmination in southern british columbia containing high grade metamorphic rocks that were polydeformed during the cordilleran orogenesis a north south 40ar 39ar thermochronology transect was carried out throughout a 7 km thick tilted section in the thor odin dome and structurally overlying rocks to construct thermochronological histories using existing u pb geochronology data with new 40ar 39ar data and to determine the nature of the boundary between the dome and overlying rocks at cariboo alp hornblende cooling dates are 62 58 ma at the highest structural level 57 55 ma in the middle and 57 53 ma at cariboo alp on the upper boundary of the dome muscovite and biotite cooling dates are 53 50 5 ma identical throughout the dome margin and overlying panel the cariboo alp area separating the thor odin dome from overlying rocks did not accommodate major post cooling extensional deformation rather it is a late cretaceous to paleocene compressional shear zone these domains cooled at different rates from 700 to ca 300 c with upper structural levels cooling at rates of ca 20 c ma and the lowest levels at rates in excess of 120 c ma all levels passed through the closure temperature for argon in biotite here calculated to be 320 330 c together at ca 52 51 ma differential cooling rates are the result of interaction between northeast directed compressional transport of rocks towards the foreland of the orogen overlapping with activity on the columbia river fault zone reflecting crustal scale extension that reached a peak in the eocene

tectonics of the western margin of the shan plateau central myanmar implication for the india indochina oblique convergence since the oligocene abstract we present new data and interpretations on the neogene tectonics of the shan scarp area central myanmar and its relationship with the india indochina oblique convergence we describe ductile and brittle fabrics associated with the major features in this area the mogok metamorphic belt mmb the shan scarp and the sagaing fault from these data we identify a succession of two tectonic regimes first a dominant nnw sse trending extension marked by ductile stretching that characterizes the mmb and associated n70e brittle normal faults later from middle or upper miocene to the present these structures were cross cut by brittle right lateral faults among which the most important are the n20w transpressive shan scarp fault zone and the n s sagaing fault to explain this transition from a dominant transtensive to a transpressive stress regime that occurred during miocene we place our data within a larger geodynamic context we suggest that like the intraplate deformation in the indian ocean the end of spreading in the south china sea the opening of the andaman basin or the end of subduction within the indo burma range the change in the tectonic regime in central myanmar could be in response to a major miocene regional plate kinematic reorganization

rock relationships in the mogok metamorphic belt tatkon to mandalay central myanmar abstract the mogok metamorphic belt mmb over 1450 km long and up to 40 km wide consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the western margin of the shan plateau in central myanmar and continuing northwards to the eastern himalayan syntaxis exposures in quarries allow correlation of palaeozoic meta sedimentary early mesozoic meta igneous and late mesozoic intrusive rocks within a 230 km long northerly trending segment of the mmb from tatkon to kyanigan north of mandalay and with the mogok gemstone district 100 km to the northeast relationships among the metamorphic and intrusive rocks with sparse published radiometric age controls indicate at least two metamorphic events one before and one after the intrusion of late jurassic to early cretaceous calc alkaline rocks these relationships can be explained by either of two possible tectonic histories one constrained by correlation of mid permian limestones across myanmar requires early permian and early jurassic regional metamorphic events prior to an early tertiary metamorphism in the western part of but within a shan thai western myanmar block the second not compatible with a single laterally continuous permian limestone requires pre upper jurassic regional metamorphism and orogenic gold mineralization in the mergui group and western myanmar early cretaceous collision of an east facing mergui western myanmar island arc with the shan plateau and early tertiary metamorphism in the mmb related to reversal in tectonic polarity following the arc plateau collision

the relation between arctic sea ice surface elevation and draft a case study using coincident auv sonar and airborne scanning laser 1 data are presented from a survey by airborne scanning laser profilometer and an auv mounted upward looking swath sonar in the spring beaufort sea the air snow surface elevation and water ice draft surfaces were mapped at 1 1 m resolution over a 300 300 m area data were separated into level and deformed ice fractions using the surface roughness of the sonar data the relation r d f between draft d and surface elevation f was then examined correlation between top and bottom surfaces was essentially zero at full resolution requiring averaging over patches of at least 11 m diameter to constrain the relation largely because of the significant error 15 cm of the laser instrument level ice points were concentrated in two core regions corresponding to level fy ice and refrozen leads with variations in r attributed primarily to positive snow thickness variability deformed ice displayed a more diffuse cloud with draft having a more important role in determining r because of wider deformed features underwater averaging over footprints similar to satellite altimeters showed the mean surface elevation typical of icesat to be stable with averaging scale with r 3 4 level and r 4 2 deformed the minimum elevation within a footprint characteristic reported for cryosat was less stable significantly overestimating r for level ice r 5 and deformed ice r 6 the mean draft difference between measurements and isostasy suggests 70 m as an isostatic length scale for level ice the isostatic scale for deformed ice appears to be longer than accessible with these data 300 m

ion microprobe zircon u pb age and geochemistry of the myanmar jadeitite combined geochemistry and geochronology of the myanmar jadeitite were determined bulk rock trace element compositions display u shaped ree patterns with pronounced positive eu anomalies the total ree abundances are very low less than half chondritic and the high field strength elements and some large ion lithophile elements are moderately enriched these features indicate a metasomatic origin there are three groups of zircons with different interior characteristics cathodoluminescence mineral inclusions chemical compositions and sensitive high resolution ion microprobe u pb ages group i zircons with a mean age of 163 2 3 3 ma mostly have distinct oscillatory zoning highest u and th contents and na free mg rich mineral inclusions and thus indicate an igneous formation of oceanic crust or hydrothermal serpentinization and or rodingitization event in the middle jurassic group ii zircons with a mean age of 146 5 3 4 ma have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions suggesting that formation of the myanmar jadeitites as well as subduction of the eastern indian oceanic plate occurred in the late jurassic group iii zircons have an age of 122 2 4 8 ma which represents a later unknown thermal event discovery of the middle jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern indo burman range

comparison of sea ice thickness measurements under summer and winter conditions in the arctic using a small electromagnetic induction device drillhole determined sea ice thickness was compared with values derived remotely using a portable smalloffset loop loop steady state electromagnetic em induction device during expeditions to fram strait and the siberian arctic under typical winter and summer conditions simple empirical transformation equations are derived to convert measured apparent conductivity into ice thickness despite the extreme seasonal differences in sea ice properties as revealed by ice core analysis the transformation equations vary little for winter and summer thus the em induction technique operated on the ice surface in the horizontal dipole mode yields accurate results within 5 to 10 of the drillhole determined thickness over level ice in both seasons the robustness of the induction method with respect to seasonal extremes is attributed to the low salinity of brine or meltwater filling the extensive pore space in summer thus the average bulk ice conductivity for summer multiyear sea ice derived according to archie s law amounts to 23 ms m compared to 3 ms m for winter conditions these mean conductivities cause only minor differences in the em response as is shown by means of 1 d modeling however under summer conditions the range of ice conductivities is wider along with the widespread occurrence of surface melt ponds and freshwater lenses underneath the ice this causes greater scatter in the apparent conductivity ice thickness relation this can result in higher deviations between em derived and drillhole determined thicknesses in summer than in winter

top bottom multisensor remote sensing of arctic sea ice the arctic sea ice cover has been studied using near simultaneous observations by passive and active synthetic aperture radar sar microwave sensors upward looking and sidescan sonars a lidar profilometer and an infrared sensor data from two aircraft and a submarine over an approximately 100 km track of central arctic sea ice were registered and analyzed to evaluate the characteristics of the ice cover and the utility of each sensor in ice studies the results of comparative and correlation analyses are as follows the probability density functions of ice draft from sonar and elevation from lidar were found to be almost identical when isostasy is taken into account this result suggests that the basic ice thickness distribution can be derived from the surface topography measurements alone reasonable correlation was found between sar backscatter and ice draft or elevation especially when scales were adjusted such that 15 20 sar pixels were averaged however surface roughness derived directly from standard deviations in the lidar elevation data was found to be poorly correlated to the sar backscatter these results indicate that the sar values are affected more by scattering from the ice than from the snow covered surface the active and passive microwave sensors are shown to generally complement each other as the two sensors are especially sensitive to different physical properties of the sea ice undeformed first year ice showed low backscatter values but high brightness temperatures while some multiyear ice showed high backscatter values and low brightness temperature however surfaces identified as multiyear ice by the passive system have a large spread in the unaveraged sar backscatter indicating limitations when using a one channel sar for ice type identification at the highest resolution also ridged ice identified by sonar and sar data covers a large range of passive microwave emissivity suggesting considerable variability in the age and salinity of this type of ice significant variations about 0 11 in the minimum emissivity of consolidated multiyear ice are observed in different regions of the arctic using the high resolution 30 m passive microwave data this suggests that regional variations in texture and scattering characteristics of multiyear ice in the arctic are present likely influenced by different histories of formation of the ice in different regions

sea ice thickness from airborne laser altimetry over the arctic ocean north of greenland 1 we present a new method to measure ice thickness of polar sea ice freeboard heights using airborne laser altimetry combined with a precise geoid model giving estimates of thickness of ice through isostatic equilibrium assumptions in the paper we analyze a number of flights from the polar sea off northern greenland and estimate accuracies of the estimated freeboard values to be at a 13 cm level corresponding to about 1 m in absolute thickness

comparison of coincident snow freeboard and sea ice thickness profiles derived from helicopter borne laser altimetry and electromagnetic induction sounding 1 sea ice thickness plays a critical role in global climate change but it cannot be measured directly from space alternatively sea ice freeboard is measured and converted to sea ice thickness with assumptions made for snow depth and snow ice densities this paper investigates the relationship between snow freeboard ice freeboard plus snow and total thickness ice thickness plus snow and addresses the uncertainties that arise from the unknown snow depth and snow ice densities a unique data set of coincident measurements of snow freeboard and total thickness was collected in the arctic and antarctic snow freeboard was determined by laser altimetry and total thickness was determined by electromagnetic induction with a helicopter borne instrument obtained total thickness snow freeboard ratios range from 2 to 12 in the arctic and from 2 to 8 in the antarctic the principal finding is that the ratios vary greatly within each region and a fixed ratio per profile should not be used as this can induce incorrect ice thicknesses the ratio uncertainties can induce a relative thickness error of 5 4 and 4 9 in the first year and multiyear ice mode additionally the coincident measurements allow the calculation of snow depth that can be used to densify existing in situ measurements to assess accuracy calculated snow depths were compared to in situ measurements and agree within 5 cm this increases if measurements and calculations differ spatially the method of deriving snow freeboard from laser altimetry is briefly described and the variability of the total thickness snow freeboard ratio is shown for one profile in the lincoln sea and one in the weddell sea

investigation of impact spherules a new promising method for the correlation of quaternary deposits abstract during the course of the last hundred years microscopic magnetite and glassy spherules of different genesis extraterrestrial volcanic industrial biogenic diagenic explosive etc have often been found in various geological formations alongside classical geological methods research into extraterrestrial and explosive spherules can provide valuable information and facilitates stratigraphical correlations over large areas during the history of the earth there seem to have been several peaks in the global occurrence of spherules the best known being recorded on the k t boundary several tens of impact craters ranging from neoproterozoic to holocene in age and from less than 100 m to more than 50 km in diameter occur both in the crystalline basement and in the sedimentary cover of the old east european craton in the fennoscandian baltic region in the baltic states alone at least 18 impact craters and a large number of meteorite falls have been recorded the oldest palaeometeorite matter known in the baltic states to date is enclosed in lower cambrian sandstone this paper presents some promising results of spherule studies which were carried out surrounding holocene meteorite craters in estonia on the basis of spherule studies impact events can be precisely dated the spherule bearing layers serve as excellent stratigraphical markers for the correlation of sedimentary sequences

spherules with pure iron cores from myanmar jadeitite type i deep sea spherules abstract here we report spherules in myanmar jadeitite a rock forming from jadeitic fluids within mantle derived serpentinized rocks in subduction zones under high pressure conditions 1 0 gpa and rather low temperatures of about 250 370 c the spherules have off centre iron nuclei and dendritic wustite cortexes with tiny wustite crystals perpendicular to the surface of iron core within the spherules are vesicles occupied by calcite jadeite albite or mixtures of these phases and the cortexes contain about 10 wt sio 2 al 2 o 3 na 2 o filling materials within wustite the spherules are in direct contact with jadeite crystals contrasting patterns of some individual spherules are obvious between a front area with a crowd of hill like prominences and a rear zone with one or more rings on the surface such surface features and internal textures suggest that they experienced movement at high temperature and then rapid cooling chemical compositions of the nuclei are homogenous and consist of nearly pure iron with minor cr 56 fe and 57 fe in the core and cortex are 0 51and 0 78 respectively such features suggest that there is a very low possibility of origin associated with volcanic explosive eruption impact ejecta chemical reduction or oxidation of iron on seafloor since biological reduction processes are not significant under high p t condition in subduction zones this origin is excluded considering their low ni contents it is more likely that they belong to the minor type i deep sea cosmic spherules dusts of low isotope fractionation this discovery shows that such spherules could remain stable under low temperature and high pressure conditions during recycling processes and therefore could be found in rocks related to slab derived sediments within subduction zones this also suggests that subducted oceanic slab sediments contribute to the formation of jadeitite coupled with dehydration of sediments and altered oceanic crust

carbonate hosted nonsulphide zn pb mineralization of southern british columbia canada many carbonate hosted sulphide deposits in the salmo district of southern british columbia have near surface zn and pb bearing iron oxide rich gossans the gossans formed when carbonate hosted base metal sulphides were subjected to intense supergene weathering processes and metals were liberated by the oxidation of sulphide minerals two types of supergene carbonate hosted nonsulphide deposits direct replacement red ore and wallrock replacement white ore are present in the salmo district the direct replacement deposits formed by the oxidation of primary sulphides the base metals passed into solution and were redistributed and trapped within the space occupied by the oxidized portion of the sulphide protore depending on the extent of replacement of the sulphides by zn pb and fe bearing oxides silicates carbonates and phosphates the resulting ore can be called mixed sulphides and nonsulphides or simply nonsulphide the wallrock replacement deposits formed when base metals liberated by the oxidation of sulphides were transported by circulating supergene solutions down and or away from the sulphides to form wallrock replacement deposits the direct replacement nonsulphide zones of the salmo district overlay the sulphide bodies in which they replaced the sulphides and carbonates forming large irregular replacement masses encrustations and open space fillings they consist predominantly of hematite goethite hemimorphite zn4si2o7 oh 2 h2o minor hydrozincite zn5 co3 2 oh 6 cerussite pbco3 and traces of willemite zn2sio4 the wallrock replacement zones consist mainly of hemimorphite with local occurrences of iron oxides hopeite zn3 po4 2 4h2o and tarbuttite zn2 po4 oh no remnants of sulphides were observed in the replacement zones the salmo nonsulphide deposits were formed by prolonged weathering of mississippi valley type mvt mineralization that underwent dissolution and oxidation of the pyrite sphalerite and galena protore the weathering also leached out highly mobile zn less mobile pb and left behind the iron oxides precipitating zn and pb silicates within the protore or at a distance from the protore

mineralogy of carbonaceous chondritic microclasts in howardites identification of c2 fossil micrometeorites abstract seventy one carbonaceous chondritic microclasts of average size 150 m have been found in three howardites yamato 793497 jodzie kapoeta all carbonaceous chondritic microclasts are made of a fine grained phyllosilicate rich matrix supporting a variety of minerals such as olivine pyroxene spinel iron oxides iron nickel sulfides and calcium carbonates such a mineralogy is typical of chondritic c2 matter half of the carbonaceous chondritic microclasts are tochilinite rich and have been tentatively called cm2 microclasts the other half are magnetite rich and have been tentatively called cr2 microclasts the absence of a correlation between the cm2 cr2 ratio in carbonaceous chondritic microclasts and in numerous millimeter sized clasts found in the same sections argues for carbonaceous chondritic microclasts being true micrometeorites rather than fragments of larger objects dynamical simulations show that it is possible for asteroidal dust to encounter vesta the howardite s putative parent asteroid at velocity low enough 1 to prevent fragmentation because the micrometeorite flux in the inner solar system has been decreasing with time we argue that carbonaceous chondritic microclasts have been trapped in vesta s regolith early in the history of the solar system and are fossil micrometeorites because both microclasts and clasts found in howardites are related to c2 chondritic matter we propose that c2 matter represents the bulk or at least a significant fraction of the primordial howardite parent asteroid considering the abundance of c2 matter among fossil micrometeorites we speculate that the c2 fossil micrometorites are the so far unidentified agent of the late chondritic veneer that endowed the earth s mantle with an excess of siderophile elements relative to the contents predicted by the core mantle separation models the discovery that c2 fossil micrometeorites are similar to c2 modern antarctic micrometeorites supports recent models proposing a micrometeoritic origin for the earth s oceans and volatile species

the origin of mineralized fractures at the bluebell mine site riondel british columbia the bluebell pb zn deposit is located on the east side of kootenay lake in southeastern british columbia it is a fracture controlled replacement deposit hosted in lower cambrian marble of the badshot and mohican formations the orebodies trend west northwest parallel to a prominent set of fractures that controlled mineralization these fractures are interpreted to have formed as part of a conjugate set contrary to some earlier suggestions folding played no role in fracture formation the youngest generation of folds are reinterpreted as having formed through development of shear bands during extension of the layering prior to fracture development formation of the fractures also postdated motion on a number of early tertiary normal faults that are associated with regional extension and alkaline magmatism west northwest trending fractures which formed as part of a conjugate set were reactivated and controlled the intrusion of mafic dikes mineralizing fluids and minor late faulting fracturing faulting mafic dike intrusion and mineralization may be associated with a long lived basement structure that runs below the area which could have facilitated the ascent of deep crustal or mantle derived magma and mineralizing fluids the results of this study provide a new perspective on exploration strategies for hydrothermal ore deposits in this part of southeastern british columbia

thinning of the arctic sea ice cover comparison of sea ice draft data acquired on submarine cruises between 1993 and 1997 with similar data acquired between 1958 and 1976 indicates that the mean ice draft at the end of the melt season has decreased by about 1 3 m in most of the deep water portion of the arctic ocean from 3 1 m in 1958 1976 to 1 8 m in the 1990s the decrease is greater in the central and eastern arctic than in the beaufort and chukchi seas preliminary evidence is that the ice cover has continued to become thinner in some regions during the 1990s

synoptic airborne thickness surveys reveal state of arctic sea ice cover 1 while summer arctic sea ice extent has decreased over the past three decades it is subject to large interannual and regional variations methodological challenges in measuring ice thickness continue to hamper our understanding of the response of the ice thickness distribution to recent change limiting the ability to forecast sea ice change over the next decade we present results from a 2400 km long pan arctic airborne electromagnetic em ice thickness survey in april 2009 the first ever large scale em thickness dataset obtained by fixed wing aircraft over key regions of old ice in the arctic ocean between svalbard and alaska the data provide detailed insight into ice thickness distributions characteristic for the different regions comparison with previous em surveys shows that modal thicknesses of old ice had changed little since 2007 and remained within the expected range of natural variability

modeling the summertime evolution of sea ice melt ponds 1 we present a mathematical model describing the summer melting of sea ice we simulate the evolution of melt ponds and determine area coverage and total surface ablation the model predictions are tested for sensitivity to the melt rate of unponded ice enhanced melt rate beneath the melt ponds vertical seepage and horizontal permeability the model is initialized with surface topographies derived from laser altimetry corresponding to first year sea ice and multiyear sea ice we predict that there are large differences in the depth of melt ponds and the area of coverage between the two types of ice we also find that the vertical seepage rate and the melt rate of unponded ice are important in determining the total surface ablation and area covered by melt ponds

silicate microspherules intercepted in the plume of etna volcano a possible volcanic origin has been suggested for the micrometre sized spherules that have been discovered in polar snows or ice cores1 5 and in the stratosphere6 7 however although such particles especially when black and magnetic have been identified among the tephra deposits of some volcanoes3 8 12 their volcanic emission has never been directly observed here we describe micro spherules intercepted in the plume of mount etna sicily during moderate recurring volcanic activity their study using analytical transmission and scanning electron microscopy atem and asem demonstrates the simultaneous presence in the plume of glassy silicate microspherules of various chemical compositions 47 98 sio2

arctic ocean gravity geoid and sea ice freeboard heights from icesat and grace 1 icesat laser measurements provide a high resolution mapping of the sea ice surface of the arctic ocean which can be inverted to determine gravity anomalies and sea ice freeboard heights by a lowest level filtering scheme in this paper we use updated terrestrial gravity data from the arctic gravity project in combination with grace gravity field models to derive an improved arctic geoid model this model is then used to convert icesat measurements to sea ice freeboard heights with a coarse lowest level surface method the derived freeboard heights show a good qualitative agreement to the coverage of multi year sea ice however comparison to an airborne lidar underflight north of greenland shows that the lowest level filtering scheme may introduce a bias we finally use the icesat and grace results to derive new gravity anomalies by fourier inversion the satellite only gravity field shows all major tectonic features of the arctic ocean and has an accuracy of 6 mgal compared to recent airborne gravity data illustrating the usefulness of icesat data for gravity field determination

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

tectonic significance of the kootenay terrane southeastern canadian cordillera an alternative model new field observations from the northern termination of the kootenay arc show that lower paleozoic rocks assigned to the kootenay terrane are depositionally linked to and conformable with underlying rocks that form part of the north american cordilleran miogeocline accordingly models involving an allochthonous origin for the kootenay terrane should be reevaluated the presence of coarse clastic rocks and mafic volcanic rocks within the lower paleozoic sequence of the kootenay arc is explained in terms of intermittent early paleozoic extensional deformation such as is well documented elsewhere along the cordilleran margin of the north american craton

comparison of freeboard retrieval and ice thickness calculation from als asiras and cryosat 2 in the norwegian arctic to field measurements made during the n ice2015 expedition we present freeboard measurements from airborne laser scanner als the airborne synthetic aperture and interferometric radar altimeter system asiras and cryosat 2 siral radar altimeter ice thickness measurements from both helicopter borne and ground based electromagnetic sounding and point measurements of ice properties this case study was carried out in april 2015 during the n ice2015 expedition in the area of the arctic ocean north of svalbard the region is represented by deep snow up to 1 12 m and a widespread presence of negative freeboards the main scattering surfaces from both cryosat 2 and asiras are shown to be closer to the snow freeboard obtained by als than to the ice freeboard measured in situ this case study documents the complexity of freeboard retrievals from radar altimetry we show that even under cold below 15 c conditions the radar freeboard can be close to the snow freeboard on a regional scale of tens of kilometers we derived a modal sea ice thickness for the study region from cryosat 2 of 3 9 m compared to measured total thickness 1 7 m resulting in an overestimation of sea ice thickness on the order of a factor 2 our results also highlight the importance of year to year regional scale information about the depth and density of the snowpack as this influences the sea ice freeboard the radar penetration and is a key component of the hydrostatic balance equations used to convert radar freeboard to sea ice thickness

satellite based estimates of sea ice volume flux through fram strait sea ice volume fluxes through fram strait arctic ocean are estimated for the two icesat measurement periods in february march and october november 2003 by combining sea ice area fluxes determined from space borne microwave observations with estimates of the sea ice thickness distribution inferred from measurements of icesat s geoscience laser altimeter system glas instrument the thickness is derived from icesat data by converting its surface elevation measurements into an ice freeboard estimate combined with prior information about ice density and snow depth and density the freeboard is converted into ice thickness uncertainties in freeboard estimates due to geoid model errors are reduced through the use of the recent geoid from the arctic gravity project missing information about the ocean circulation and ocean tides is approximated locally by interpolating the sea surface height linearly between open leads meridional ice volume fluxes estimated for 798 n using ice drift observed by amsr e quikscat amount to 168 km 3 236 km 3 and 62 km 3 77 km 3 for 30 day periods in february march and october november 2003 respectively these values lie in the range of previous results from similar studies but are considerably smaller than the average ice flux during the 1990s most likely because of a smaller ice drift speed during 2003

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem

selkirk fan structure southeastern canadian cordillera tectonic wedging against an inherited basement ramp a revised cross section through the selkirk fan structure provides the basis for a new model for the middle jurassic tectonic evolution of the southern omineca belt canadian cordillera palinspastic restoration of this cross section shows that the southwest verging structures along the west flank of the selkirk fan structure formed as a result of tectonic wedging of distal north american strata clachnacudainn complex beneath more proximal north american strata and that the selkirk fan structure developed outboard from a crustal ramp dogtooth high inherited from late proterozoic early paleozoic rifting along the western margin of north america the first episode of mesozoic deformation in southeastern british columbia occurred between 187 and 173 ma and involved the northeastward juxtaposition of the intermontane superterrane over the outer part of the north american continental terrace wedge it resulted in deep burial 20 25 km of the outer margin of north america a crustal ramp localized along the western edge of the late proterozoic early paleozoic dogtooth high impeded the northeastward propagation of the orogenic wedge comprising the intermontane superterrane and the imbricate underlying northeast verging thrust sheets of north american supracrustal rocks tectonic wedging involving southwest verging deformation occurred within the orogenic wedge and the resulting crustal thickening established sufficient topography and gravitational potential to drive the propagation of the deformation eastward into the dogtooth range and the rocky mountains the southwest verging structures along the west flank of the selkirk fan developed between approximately 173 and 168 ma concurrent with synorogenic extension and 10 km of exhumation the initial subsidence of the foreland basin during kimmeridgian time ca 154 ma provides the first indication of tectonic loading and lithospheric flexure of the north american plate it is interpreted to mark the time at which the orogenic wedge overrode the crustal ramp of the dogtooth high and advanced onto relatively thick and rigid continental lithosphere the tectonic model proposed for the selkirk fan structure illustrates how the configuration of the rifted margin influenced the style of crustal thickening during subsequent compressional deformation

an assessment of state of the art mean sea surface and geoid models of the arctic ocean implications for sea ice freeboard retrieval state of the art arctic ocean mean sea surface mss models and global geoid models ggms are used to support sea ice freeboard estimation from satellite altimeters as well as in oceanographic studies such as mapping sea level anomalies and mean dynamic ocean topography however errors in a given model in the high frequency domain primarily due to unresolved gravity features can result in errors in the estimated along track freeboard these errors are exacerbated in areas with a sparse lead distribution in consolidated ice pack conditions additionally model errors can impact ocean geostrophic currents derived from satellite altimeter data while remaining biases in these models may impact longer term multi sensor oceanographic time series of sea level change in the arctic this study focuses on an assessment of five state of the art arctic mss models ucl13 04 dtu15 13 10 and a commonly used ggm egm2008 we describe errors due to unresolved gravity features inter satellite biases and remaining satellite orbit errors and their impact on the derivation of sea ice freeboard the latest mss models incorporating cryosat 2 sea surface height measurements show improved definition of gravity features such as the gakkel ridge the standard deviation between models ranges 0 03 0 25 m the impact of remaining mss ggm errors on freeboard retrieval can reach several decimeters in parts of the arctic while the maximum observed freeboard difference found in the central arctic was 0 59 m ucl13 mss minus egm2008 ggm the standard deviation in freeboard differences is 0 03 0 06 m