buried topography of utopia mars persistence of a giant impact depression knobs partially buried craters ring fractures and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials these features are widely distributed in the utopia area but are absent in a large roughly circular region centered at about 48 degree n 240 degree w this implies the existence of a circular depression about 3 300 km in diameter buried beneath utopia planitia that is here interpreted to represent the central part of a very large impact basin the presence of buried curved massifs around part of this depression and a roughly coincident mascon lend further support present topography areal geology and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of mars

observations of periglacial landforms in utopia planitia with the high resolution imaging science experiment hirise 1 the region of western utopia planitia 80 105 e 40 55 n displays several types of landforms similar to earth periglacial features including scallop shaped depressions and networks of polygonal terrains the scalloped depressions have been proposed to originate from thermokarstic processes such as sublimation and or melting of near surface ground ice using hirise imagery we characterize these depressions and several associated distinct polygon networks in unprecedented morphologic and topographic detail and investigate support for an ice based degradation process the scalloped depressions and interior polygons and ridges are found to evolve together mainly influenced by sublimation local proximity of ground ice to the surface and obliquity variations

geochronology of the mount read volcanics tasmania australia the late middle cambrian mount read volcanics tasmania australia comprise submarine rhyolite to basalt lavas intrusions and volcaniclastic rocks which host significant polymetallic massive sulfide deposits isotopic u pb and 40 ar 39 ar dating of magmatic zircons and hornblendes respectively from different parts of the mount read volcanics yields a concordant age of 502 6 or 3 5 ma 1sigma which constrains the time of volcanogenic massive sulfide mineralization the comstock tuff from the tyndall group in the upper part of the volcanic sequence and the mount black dacite from the central volcanic complex have weighted mean 206 pb 238 u ages of 494 4 or 3 8 ma and 494 9 or 4 3 ma respectively and may be slightly younger than the other rocks fauna in the mount read volcanics is diagnostic of a late middle cambrian middle to late menevian stage age and the isotopic dates therefore imply that the late cambrian is younger than 500 ma inherited zircons in the mount read volcanics have ages similar to those found in granites and sediments in the tasman fold belt southeastern australia which suggests similar source material for the two areas

facies architecture of mineralized submarine volcanic sequences cambrian mount read volcanics western tasmania the middle to late cambrian mount read volcanics western tasmania comprise compositionally and texturally diverse lavas and volcaniclastic rocks most of which were emplaced in submarine environments below wave base the facies architecture reflects the contrasting character and geometry of primary volcanic and volcaniclastic facies which are strongly controlled by eruption style and eraplacement processes in the northern mount read volcanics the principal volcanic facies are 1 silicic intermediate and mafic lavas 2 juvenile volcaniclastic deposits generated in association with the extrusion of lava flows or else produced by explosive eruptions and 3 synvolcanic intrusions involving silicic and mafic magmas emplaced into unconsolidated host sequences distribution and lithofacies characteristics suggest that deposits from both intrabasinal submarine and extrabasinal subaerial and or shallow marine volcanic centers are present the volcanic facies are interbedded with a sedimentary facies association comprising black mudstone and or graded bedded sandstone of mixed volcanic and precambrian basement provenance the predicted facies geometry of the principal volcanic facies has guided our approach to correlation in the mount read volcanics mass fiow emplaced pumiceous volcaniclastic facies provide the best framework for correlation because these are produced in large volumes erupted infrequently eraplaced rapidly and are widely distributed distinctive units of this type that occur near hellyer in the southwell subgroup and 40 km to the south at white spur howards road white spur formation could each be part of the same regionally extensive volcaniclastic facies association a similar volcaniclastic facies association hosts the hercules and rosebery massive sulfide deposits and clearly demonstrates that such associations are prospective for this style of mineralization

formation of patterned ground and sublimation till over miocene glacier ice in beacon valley southern victoria land antarctica a thin glacial diamicton informally termed granite drift occupies the floor of central beacon valley in southern victoria land antarctica this drift is 40 ar 39 ar analyses of presumed in situ ash fall deposits that occur within granite drift at odds with the great age of this ice are high centered polygons that cut granite drift if polygon development has reworked and retransported ash fall deposits then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice our results show that the surface of granite drift is stable at polygon centers and that enclosed ash fall deposits can be used to define the age of underlying glacier ice in our model for patterned ground development active regions lie only above polygon troughs where enhanced sublimation of underlying ice outlines high centered polygons the rate of sublimation is influenced by the development of porous gravel and cobble lag deposits that form above thermal contraction cracks in the underlying ice a negative feedback associated with the development of secondary ice lenses at the base of polygon troughs prevents runaway ice loss secondary ice lenses contrast markedly with glacial ice by lying on a d versus 18 o slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess the latter indicates that secondary ice lenses likely formed by melting downward percolation and subsequent refreezing of snow trapped preferentially in deep polygon troughs the internal stratigraphy of granite drift is related to the formation of surface polygons and surrounding troughs the drift is composed of two facies a nonweathered matrix supported diamicton that contains 25 striated clasts in the 16 mm fraction and a weathered clast supported diamicton with varnished and wind faceted gravels and cobbles the weathered facies is a coarse grained lag of granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies the concentration of cosmogenic 3 he in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice these profile patterns and the 3 he surface exposure ages of 1 18 0 08 ma and 0 18 0 01 ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr although granite drift is stable at polygon centers low frequency slump events occur at the margin of active polygons slumping together with weathering of surface clasts creates the large range of cosmogenic nuclide surface exposure ages observed for granite drift maximum rates of sublimation near active thermal contraction cracks calculated by using the two 3 he depth profiles range from 5 m m y to 90 m m y sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates nevertheless these rates are orders of magnitude lower than those computed on theoretical grounds during eruptions of the nearby mcmurdo group volcanic centers ash fall debris collects at the surface of granite drift either in open thermal contraction cracks or in deep troughs that lie above contraction cracks these deposits subsequently lower passively as the underlying glacier ice sublimes the fact that some regions of granite drift have escaped modification by patterned ground for at least 8 1 ma indicates long term geomorphic stability of individual polygons once established polygon toughs likely persist for as long as 10 5 10 6 yr our model of patterned ground formation which applies to the hyperarid cold desert polar climate of antarctica may also apply to similar sized polygons on mars that occur over buried ice in utopia planitia

widespread loess like deposit in the martian northern lowlands identifies middle amazonian climate change consistently mappable units critical to distinguishing the style and interplay of geologic processes through time are sparse in the martian lowlands this study identifies a previously unmapped middle amazonian ca 1 ga unit middle amazonian lowland unit mal that postdates the late hesperian and early amazonian lowland plains by 2 b y the unit is regionally defined by subtle marginal scarps and slopes has a mean thickness of 32 m and extends 3 1 10 6 km 2 between lat 35 n and 80 n pedestal type craterforms and nested arcuate ridges thumbprint terrain tend to occur adjacent to unit mal outcrops suggesting that current outcrops are vestiges of a more extensive deposit that previously covered 16 10 6 km 2 exposed layers surface pits and the draping of subjacent landforms allude to a sedimentary origin perhaps as a loess like deposit emplaced rhythmically through atmospheric fallout we propose that unit mal accumulated coevally with and at the expense of the erosion of the north polar basal units identifying a major episode of middle amazonian climate driven sedimentation in the lowlands this work links ancient sedimentary processes to climate change that occurred well before those implied by current orbital and spin axis models

chemical remobilization of volcanic hosted sulphide deposits at rosebery and mt lyell tasmania abstract devonian hydrothermal activity has resulted in localised chemical remobilization of the cambrian massive sulphide deposits at rosebery and mt lyell in the mt read mineral province of western tasmania at rosebery part of the massive pyrite sphalerite galena chalcopyrite quartz phyllosilicate lens has been replaced volume for volume by a pyrite pyrrhotite chalcopyrite magnetite phlogopite tourmaline quartz assemblage with complete removal of cambrian sphalerite and galena the replacement is later than folding of probable devonian age the likely temperature the presence of tourmaline and the relatively reduced nature of the secondary assemblage suggest that the related fluids were derived from devonian granitoids which on independent evidence tourmaline veins pyrrhotite deposits metamorphic biotite and shallow granites nearby appear to extend beneath the rosebery mice lead and zinc dissolved from the cambrian sulphide lens may have been redeposited as veins and replacements in younger rocks the resulting lead is likely to yield anomalous isotopic ages and be associated with sulphur having the isotopic characteristics of cambrian massive sulphide deposits at mt lyell altered felsic volcanics host large disseminated pyrite chalcopyrite quartz phyllosilicate lenses and minor stratabound exhalative massive sulphide bodies all of probable cambrian age in the north lyell fault zone late cambrian early ordovician siliciclastic sediments have been replaced by cherty quartz bornite chalcopyrite chalcocite sericite pyrophyllite ores of likely devonian age this assemblage probably formed from acid pyrophyllite oxidized barite solutions that may have derived high a cu a fe by dissolution of cambrian chalcopyrite disseminated pyrite chalcopyrite ores of cambrian age have a well developed devonian cleavage in which hematite has crystallized thereby indicating circulation of oxidized fluids during or after cleavage development precipitation mechanisms for the bornite ores may have involved fluid sulphide and or fluid sediment reactions the fluids are unlikely to have been derived directly from devonian granitoids but they may have been modified groundwaters circulating either during cleavage growth or as a result of heating by post kinematic devonian granitoid plutons

thermal contraction crack polygons on mars a synthesis from hirise phoenix and terrestrial analog studies thermal contraction crack polygons are complex landforms that have begun to be deciphered on earth and mars by the combined investigative efforts of geomorphology environmental monitoring physical models paleoclimate reconstruction and geochemistry thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice ranging in ice content from weakly cemented soils to debris covered massive ice relative to larger topographic features polygons may form rapidly and reflect climate conditions at the time of formation preserving climate information as relict landforms in the geological record polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer and to delimit subsurface ice conditions analysis of martian polygon morphology and distribution indicates that geologically recent thermal contraction crack polygons on mars form predominantly in an ice rich latitude dependent mantle more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content deposition and modification history and to assess microclimate variation on timescales of ka to ma analyses of martian polygon morphology guided by investigations of terrestrial analog thermal contraction crack polygons strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons implying the presence of an ice rich substrate that was fractured during and subsequent to obliquity driven depositional periods and continually modified by ongoing vapor equilibration processes

antarctic dry valleys microclimate zonation variable geomorphic processes and implications for assessing climate change on mars the antarctic dry valleys adv are generally classified as a hyper arid cold polar desert the region has long been considered an important terrestrial analog for mars because of its generally cold and dry climate and because it contains a suite of landforms at macro meso and microscales that closely resemble those occurring on the martian surface the extreme hyperaridity of both mars and the adv has focused attention on the importance of salts and brines on soil development phase transitions from liquid water to water ice and ultimately on process geomorphology and landscape evolution at a range of scales on both planets the adv can be subdivided into three microclimate zones a coastal thaw zone an inland mixed zone and a stable upland zone zones are defined on the basis of summertime measurements of atmospheric temperature soil moisture and relative humidity subtle variations in these climate parameters result in considerable differences in the distribution and morphology of 1 macroscale features e g slopes and gullies 2 mesoscale features e g polygons including ice wedge sand wedge and sublimation type polygons as well as viscous flow features including solifluction lobes gelifluction lobes and debris covered glaciers and 3 microscale features e g rock weathering processes features including salt weathering wind erosion and surface pitting equilibrium landforms are those features that formed in balance with environmental conditions within fixed microclimate zones some equilibrium landforms such as sublimation polygons indicate the presence of extensive near surface ice identification of similar landforms on mars may also provide a basis for detecting the location of shallow ice landforms that today appear in disequilibrium with local microclimate conditions in the adv signify past and or ongoing shifts in climate zonation understanding these shifts is assisting in the documentation of the climate record for the adv a similar type of landform analysis can be applied to the surface of mars where analogous microclimates and equilibrium landforms occur 1 in a variety of local environments 2 in different latitudinal bands and 3 in units of different ages documenting the nature and evolution of the adv microclimate zones and their associated geomorphic processes is helping to provide a quantitative framework for assessing the evolution of climate on mars

identification of sublimation type thermal contraction crack polygons at the proposed nasa phoenix landing site implications for substrate properties and climate driven morphological evolution 1 we identify a surface within the nasa phoenix landing site area d characterized by boulder topped polygonally patterned mounds comparable in radius and cross sectional morphology to terrestrial sublimation polygons found in antarctica both martian and antarctic polygons display topographic asymmetry with shallow equator facing slopes and steep pole facing slopes interpreted to indicate insolation dependent differential sublimation of buried ice on the basis of morphological similarities we classify the phoenix box 1 polygons as sublimation polygons terrestrial sublimation polygons form where ice volumes exceeding available pore space occur in the shallow subsurface and where near surface conditions are too cold and dry to permit the development of saturated active layers conditions comparable to those recently modeled for mars in the phoenix landing site area d box 1 the identification of sublimation polygons on mars would provide direct evidence for shallow ground ice exceeding pore ice volumes and shed light on the emplacement mechanism of this ice

the first 20 years 1978 1979 to 1998 1999 of ice wedge growth at the illisarvik experimental drained lake site western arctic coast canada in august 1978 a large tundra lake was drained to study the aggradation of permafrost into newly exposed lake bottom sediments ice wedge growth which started in the first winter following drainage had ceased in most of the lake bottom within about twelve years the gradual cessation of thermal contraction cracking can be attributed to rapid vegetation growth snow entrapment an increase in winter ground temperatures and a decrease in the linear coefficient of thermal contraction associated with freeze thaw consolidation of the initially saturated lake bottom sediments the tilt and separation of markers in the active layer revealed gradual convergence towards the troughs even after ice wedge growth had ceased for the first few years the ice wedge growth rate was up to 3 cm a as determined by excavation drilling separation of the bottoms of benchmarks installed into permafrost and divergence of free floating inductance coils placed on the sides of ice wedges well below the bottom of the active la

foliation boudinage control on the formation of the rosebery pbzn orebody tasmania abstract the rosebery pb zn ag au massive sulphide deposit occurs within the middle cambrian mount read volcanics of western tasmania it is hosted by a tuffaceous slate lens that is surrounded by felsic volcanics the volcanic belt and its ordovician cover were deformed and metamorphosed during an early to middle devonian orogeny tabberabberan orogeny during this orogeny d 3 e w shortening was partially accommodated by reverse bedding parallel shearing within the slate unit and immediately underlying volcanic rocks this caused reactivation and intensification of an early foliation s 2 and the development of asymmetric chocolate tablet foliation boudinage structures with boudin necks parallel to the x and y directions the mineralization occurs along extensional shear planes that are associated with foliation boudinage mineralogical zoning in the orebody can be directly related to dilational bends in shears associated with foliation boudin necks it is proposed that these structures gaped during the d 3 deformation allowing infiltration of metalliferous hydrothermal fluids along shear plane and cleavage directions into the fractured wall rock progressive metasomatic replacement of the wall rock resulted in the formation of a sulphide sulphate silicate orebody the orebody is completely discordant to folds in the host rock this observation confirms a structurally controlled origin and refutes earlier interpretations of the deposit as a tightly folded chemical sediment isotope data indicate that the orebody was probaby derived from volcanogenic sulphides within the mount read volcanics the rosebery orebody is best regarded as a metamorphogenic deposit that formed by dissolution upward transport concentration and re precipitation of primary sulphides the proposed macroscopic model complements separate microstructural evidence by the author indicating a syn d 3 timing of mineralization

clastic patterned ground in lomonosov crater mars examining fracture controlled formation mechanisms the area surrounding lomonosov crater on mars has a high density of seemingly organised boulder patterns these form seemingly sorted polygons and stripes within kilometre scale blockfields patches of boulder strewn ground which are common across the martian high latitudes several hypotheses have been suggested to explain the formation of clastic patterned ground on mars it has been proposed that these structures could have formed through freeze thaw sorting or conversely by the interaction of boulders with underlying fracture polygons in this investigation a series of sites were examined to evaluate whether boulder patterns appear to be controlled by the distribution of underlying fractures and test the fracture control hypotheses for their formation it was decided to focus on this suite of mechanisms as they are characterised by a clear morphological relationship namely the presence of an underlying fracture network which can easily be evaluated over a large area it was found that in the majority of examples at these sites did not exhibit fracture control although fractures were present at many sites there were very few sites where the fracture network appeared to be controlling the boulder distribution in general these were not the sites with the best examples of organization suggesting that the fracture control mechanisms are not the dominant geomorphic process organising the boulders in this area

high resolution thermal inertia derived from the thermal emission imaging system themis thermal model and applications 1 thermal inertia values at 100 m per pixel are determined using nighttime temperature data from the thermal emission imaging system themis on the mars odyssey spacecraft producing the highest resolution thermal inertia data set to date themis thermal inertia values have an overall accuracy of 20 a precision of 10 15 and are consistent with both thermal emission spectrometer orbital and miniature thermal emission spectrometer surface thermal inertia values this data set enables the improved quantification of fine scale surface details observed in high resolution visible images in the tharsis region surface textures and crater rims observed in visible images have no corresponding variation in the themis thermal inertia images indicating that the dust mantle is pervasive at themis scales and is a minimum of a few centimeters and up to 1 2 m thick the thermal inertia of bed form material indicates particle diameters expected for aeolian sediments and these materials are likely currently saltating variations in the thermal inertia within interior layered deposits in hebes chasma can be distinguished and the thermal inertia is too low to be consistent with bedrock or a lava flow thus a secondary emplacement of volcanic material or a volcanic ash deposit is a more likely method of formation higher resolution themis thermal inertia enables the identification of exposed bedrock on the martian surface in nili patera and ares vallis bedrock material corresponds to distinct compositional and morphologic surfaces indicating that a specific unit is exposed and is likely currently being kept free of unconsolidated material by aeolian processes

thermokarst in siberian ice rich permafrost comparison to asymmetric scalloped depressions on mars 1 on earth the thawing of permafrost deposits with high ground ice content results in massive surface subsidence and the formation of characteristic large thermokarst depressions slope asymmetries within thermokarst depressions suggest lateral growth which occurs due to thermoerosion and gravimetric mass wasting along these slopes it has been proposed that rimless asymmetrically shaped depressions called scalloped depressions on mars were formed by insolation driven ground ice sublimation we investigated a large thermokarst depression in ice complex deposits in the siberian arctic as a terrestrial analogue for scalloped depressions in martian volatile rich mantle deposits our results from field studies insolation modeling and geomorphometric analyses suggest lateral thermokarst development in a northern direction this conclusion is obvious due to steeper slope angles of the south facing slopes insolation and surface temperatures are crucial factors directly influencing thermokarst slope stability and steepness comparative analyses of martian scalloped depressions in utopia planitia were conducted using high resolution high resolution imaging science experiment context camera and thermal infrared thermal emission imaging system satellite data by direct analogy we propose that the lateral scalloped depression development on mars was primarily forced on the steep pole facing slopes in the equator ward direction insolation modeling confirms that this must have happened in the last 10 ma during an orbital configuration of higher obliquity than today development would have been maximized if the orbit was both highly oblique and highly eccentric and or the martian summer coincided with perihelion relatively short events of increasing sublimation or even thawing of ground ice led to fast slumping processes on the steep pole facing slopes

the role of sublimation for the formation of the northern ice cap results from the mars orbiter laser altimeter abstract the evolutionary history of the martian ice caps is practically unknown and the caps are key to understanding the climatic history of mars sublimation and ice flow are two hypotheses that have recently been suggested to explain the observed form of the northern ice cap here we present a sublimation model and compare the shape of the cap obtained from this model with mars orbiter laser altimeter mola observations of the north polar region this model can reproduce the shape of the cap and individual troughs sublimation is an important process as are flow and wind erosion we were able to constrain the time scales of sublimation only to about two orders of magnitude 1 100 million years this estimate depends on many unknown parameters such as friction velocity and wind speed over the ice caps these can be determined only by direct measurement over the surface

modeling the development of martian sublimation thermokarst landforms abstract sublimation thermokarst landforms result from collapse of the surface when ice is lost from the subsurface on mars scalloped landforms with scales of decameters to kilometers are observed in the mid latitudes and considered likely thermokarst features we describe a landscape evolution model that couples diffusive mass movement and subsurface ice loss due to sublimation over periods of tens of thousands of mars years under conditions similar to the present the model produces scallop like features similar to those on the martian surface starting from much smaller initial disturbances the model also indicates crater expansion when impacts occur in surfaces underlain by excess ice to some depth with morphologies similar to observed landforms on the martian northern plains in order to produce these landforms by sublimation substantial quantities of excess ice are required at least comparable to the vertical extent of the landform and such ice must remain in adjacent terrain to support the non deflated surface we suggest that martian thermokarst features are consistent with formation by sublimation without melting and that significant thicknesses of very clean excess ice up to many tens of meters the depth of some scalloped depressions are locally present in the martian mid latitudes climate conditions leading to melting at significant depth are not required

thermokarst lakes and ponds on mars in the very recent late amazonian past the history of water is fundamental to understanding the geological evolution of mars and to questions concerning the possible development of life on the red planet today mars is cold and dry it

polygon pattern geomorphometry on svalbard norway and western utopia planitia mars using high resolution stereo remote sensing data abstract polygonal systems formed by thermal contraction cracking are complex landscape features widespread in terrestrial periglacial regions the manner in which cracking occurs is controlled by various environmental factors and determines dimension shape and orientation of polygons analogous small scale features are ubiquitous in martian mid and high latitudes and they are also inferred to originate from thermal contraction cracking we studied the geomorphometry of polygonally patterned ground on svalbard to draw a terrestrial analogy to small scale polygonal structures in scalloped terrain in martian mid latitudes we performed a comparative quantitative terrain analysis based on high resolution stereo remote sensing data hrsc ax and hirise in combination with terrestrial field data and multivariate statistics to determine the relationship of polygon geomorphometry to local environmental conditions results show that polygonal structures on svalbard and in utopia planitia on mars are similar with respect to their size and shape a comparable thermal contraction cracking genesis is likely polygon evolution however is strongly related to regional and local landscape dynamics individual polygon dimensions and orthogonality vary according to age thermal contraction cracking activity and local subsurface conditions based on these findings the effects of specific past and current environmental conditions on polygon formation on mars must be considered on both earth and mars the smallest polygons represent young recently active low centered polygons that formed in fine grained ice rich material small low centered martian polygons show the closest analogy to terrestrial low centered ice wedge polygons the formation of composite wedges could have occurred as a result of local geomorphological conditions during past martian orbital configurations larger polygons reflect past climate conditions on both earth and mars the present degradation of these polygons depends on relief and topographical situation on svalbard the thawing of ice wedges degrades high centered polygons in contrast the present appearance of polygons in utopia planitia is primarily the result of contemporary dry degradation processes

the effects of martian orbital variations upon the sublimation and relaxation of north polar troughs and scarps abstract the north polar layered deposits pld of mars are climatologically significant because they represent the largest actively exchanging reservoir of martian water the kilometer scale topography of the north pld is dominated by troughs and scarps which exhibit highly correlated surface slopes and total depths the most widespread theories of pld evolution presume that the asymmetrical nature of north pld troughs characterized by equatorward facing slopes that are generally steeper than poleward facing slopes is the result of preferential h 2 o sublimation from equatorward facing trough walls however our orbitally modulated simulations indicate that there is no long term sublimation advantage of equatorward facing trough walls because of the effects of obliquity upon the slope dependence of sublimation rate in contrast we propose that viscous relaxation of subsurface water is consistent with the slope and depth distributions of north pld troughs and scarps the results of our finite element simulations suggest that a mere 2 k difference in the subsurface temperatures of opposing trough walls is sufficient to produce the observed slope disparity due to the slower rate of uplift of colder poleward facing trough walls both our sublimation and relaxation simulations indicate that present day north pld troughs have formed since 5 ma and are not sites of long term deposition additionally our results imply that glacial flow may govern the large scale evolution of the north pld especially at high obliquity

the distribution of putative periglacial landforms on the martian northern plains in this investigation hirise images were surveyed across three regions of the northern plains of mars acidalia arcadia and utopia planitiae a sample of hirise images within each of these areas was examined the aim of the investigation was to determine the distribution of three varieties of landform clastic patterned ground lobate hill slope features and scalloped depressions these features are of importance as they are morphologically similar to terrestrial periglacial features examples of these landforms were found across the study areas scalloped depressions are common at mid latitudes in each of the three regions sorted patterned ground was not as widespread some examples of sorted patterned ground were found in all three study areas but the main concentration occurs in the north western region of acidalia planitia lobate hill slope features were the least common of the three landform types despite occurring infrequently several possible examples of lobate features were found to occur in proximity to other putative periglacial features it was found that lobate hill slope features and clastic patterns occurred further south than expected beyond the southern extent of many previous studies these features and by extension the processes that form them could thus occur more widely than previously thought it also appears that although putative periglacial features are often found in crater interiors they are not limited to these environments extensive areas of patterned ground were found on the inter crater plains lobate features and sorted stripes were often found on crater walls which would support the hypothesis that they are analogous to hill slope features on earth scalloped depressions were most often found in crater interiors or on crater ejecta although examples on the inter crater plains were not uncommon

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

evidence age and thickness of a frozen paleolake in utopia planitia mars abstract recent mapping studies west of elysium mons mars have pinpointed subice features that suggest the existence of a frozen paleolake in utopia planitia as recently as 1 8 billion years ago the subice features are interpreted to be hyaloclastic ridges and hills table mountains associated jokulhlaup deposits and fluvial channels photoclinometric studies of these features and of a basal scarp around the northwest flank of elysium mons interpreted to have been an ice sheet boundary indicate that the maximum thickness of ice within the basin may have been about 180 m this thickness of ice during a relatively late stage of martian geologic history would have important implications concerning the atmospheric the climatic and possibly the exobiologic history of the planet

40ar 39ar age constraints on deformation and mineralization rosebery zn pb cu and mount lyell cu deposits tasmania australia the 40ar 39ar dating of alteration muscovite from the rosebery zn pb cu and mount lyell cu deposits tasmania australia has determined a succession of deformation events which occurred from 400 378 ma and comprises the devonian tabberabberan orogeny the dates from rosebery range from 400 390 ma are a minimum age for mineralization indicate the time of deformation and provide a maximum age limit for granitoid emplacement in the vicinity of the deposit the ages from the mount lyell field range from 400 378 ma are a minimum for mineralization and date cleavage development the north lyell cu mineralization which was probably broadly coeval with deformation may have formed at 400 ma all pre devonian alteration micas in the rosebery and mount lyell areas have been recrystallized or reset the tabberabberan deformation in western tasmania was broadly contemporaneous with widespread crustal shortening in southeastern australia as established from the dating of alteration minerals associated with deformation related precious and base metal deposits

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

small scale polygonal features on mars seasonal thermal contraction cracks in permafrost a time dependent viscoelastic model of thermal stress in martian ice rich permafrost is developed to test the hypothesis that small scale polygonal features observed from orbit and by the viking lander 2 are the result of thermal contraction cracking as commonly occurs in terrestrial permafrost results indicate that significant tensile stress occurs in martian ice rich permafrost as a result of seasonal cycles in the ground temperature using conservative rheological parameters appropriate for ice at low temperatures tensile stresses poleward of about 20 to 30 latitude easily exceed the tensile strength assumed to be 2 to 3 mpa and fractures should readily form in the equatorial regions special conditions may allow tensile stresses to approach the tensile strength these results support a thermal contraction origin of observed small scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice

co evolution of polygonal and scalloped terrains southwestern utopia planitia mars abstract thermal contraction crack polygons and scalloped depressions two of the most common landforms found in utopia planitia mars have previously been linked to the presence of ice rich deposits in the subsurface although the formation and evolution of these features individually are relatively well understood little to no effort has been directed towards elucidating possible interactions that occur between them during their development thus the overarching goal of this research was to investigate if there is an evolutionary link between polygonal and scalloped terrains by correlating metrics representing polygon and scallop maturity a variety of statistical analyses were performed using hirise and mola datasets to quantify interactions between four sets of polygonal and scalloped terrains our results demonstrate the existence of a negative relationship between polygonal subdivision and surface elevation indicating that polygon networks become more evolved as the surface subsides these results suggest that the permafrost landscape in utopia planitia may once have been extremely ice rich and that multiple geomorphic processes may be responsible for its evolution ultimately this work demonstrates that landscape reconstruction is more complete when a system approach is followed quantifying interactions between landforms as opposed to examining an individual landform in isolation

evidence of ancient continental glaciation in the martian northern plains whorled ridges spaced about 2 6 km and forming lobate patterns with lobe widths of about 150 km occur at many locations in the northern plains of mars commonly in close association with sinuous troughs that contain medial ridges these landforms resemble moraines tunnel channels and eskers found in terrestrial glacial terrains such as the midcontinent of north america some martian landscapes may have formed by disintegration of continental glaciers that covered much of the northern plains into the early amazonian i e late in martian geologic history meltwater processes apparently were important in the collapse of these hypothesized ice sheets hence the glaciers apparently were wet based in part whereas striking similarities exist among areas of the northern plains and some glaciated pleistocene terrains on earth there are also important differences notably drumlin fields such as those in many glacial landscapes on earth are rare absent or not yet resolved in images of the martian northern plains another major difference is that postglacial fluvial and other water related modifications especially erosion of pleistocene terrains are substantial but similar modifications are not observed in the northern plains a virtually complete and sudden decline in the activity of liquid surface water following glaciation in the northern plains seems to be implied the climatic implications of the hypothesized martian glaciers and their decline are unclear we investigate two possibilities alternatively involving a relatively warm paleoclimate and the modern martian climate the hypothesized ice sheets in the basins within the northern plains generally at elevations lower than 1 km suggest a relationship of these frozen bodies of water with former regional lakes or seas which may have formed in response to huge discharges of water from martian outflow channels this possible relationship has been modeled glaciers may have evolved from seas by their progressive freezing and then grounding and sublimational redistribution of sea ice the transition to glaciation may have taken several million years if the climate was very cold comparable to today s or tens of thousands of years if the climate was as warm as modern antarctica a glacierized sea may have involved an extended period of glaciolacustrine and ice shelf processes

stratigraphical evidence of late amazonian periglaciation and glaciation in the astapus colles region of mars abstract recent modeling of the meteorological conditions during and following times of high obliquity suggests that an icy mantle could have been emplaced in western utopia planitia by atmospheric deposition during the late amazonian period costard f m forget f madeleine j b soare r j kargel j s 2008 lunar planet sci 39 abstract 1274 madeleine b forget f head j w levrard b montmessin f 2007 lunar planet sci 38 abstract 1778 astapus colles aba is a late amazonian geological unit located in this hypothesized area of accumulation that comprises an icy mantle tens of meters thick tanaka k l skinner j a hare t m 2005 us geol surv sci invest map 2888 for the most part this unit drapes the early amazonian vastitas borealis interior unit abvi to a lesser degree it overlies the early amazonian vastitas borealis marginal unit abvm and the early to late hesperian up plains unit hbu2 tanaka k l skinner j a hare t m 2005 us geol surv sci invest map 2888 landscapes possibly modified by late amazonian periglacial processes costard f m kargel j s 1995 icarus 114 93 112 mcbride s a allen c c bell m s 2005 lunar planet sci 36 abstract 1090 morgenstern a hauber e reiss d van gasselt s grosse g schirrmeister l 2007 j geophys res 112 doi 10 1029 2006je002869 e06010 seibert n m kargel j s 2001 geophys res lett 28 899 902 soare r j kargel j s osinski g r costard f 2007 icarus 191 95 112 soare r j osinski g r roehm c l 2008 earth planet sci lett 272 382 393 and glacial processes milliken r e mustard j f goldsby d l 2003 j geophys res 108 e6 doi 10 1029 2002je002005 5057 mustard j f cooper c d rifkin m k 2001 nature 412 411 414 tanaka k l skinner j a hare t m 2005 us geol surv sci invest map 2888 have been reported within the region researchers have assumed that the periglacial and glacial landscapes occur within the same geological unit the aba i e morgenstern a hauber e reiss d van gasselt s grosse g schirrmeister l 2007 j geophys res 112 doi 10 1029 2006je002869 e06010 tanaka k l skinner j a hare t m 2005 us geol surv sci invest map 2888 in this study we use hirise high resolution image science experiment mars reconnaissance orbiter imagery to identify the stratigraphical separation of the two landscapes and show that periglacial landscape modification has occurred in the geological units that underlie the aba not in the aba itself moreover we suggest that the periglacial landscape extends well beyond the perimeter of the aba and could be the product of wet cold climate processes these processes involve freeze thaw cycles and intermittently stable liquid water at or near the surface by contrast we propose that the aba is a very recent late amazonian geological unit formed principally by dry cold climate processes these processes comprise accumulation by atmospheric deposition and ablation by sublimation

high latitude patterned grounds on mars classification distribution and climatic control patterned grounds such as polygonal features and slope stripes are the signature of the presence of ground ice and of temperature variations in cold regions on earth identifying similar features on mars is important to understand its past climate as well as the ground ice distribution in this study young patterned grounds are classed and mapped from the systematical analysis of mars observer camera high resolution images these features are located poleward of 55 latitude which fits the distribution of ground ice found by the neutron spectrometer onboard mars odyssey thermal contraction due to seasonal temperature variations is the predominant process of formation of polygons formed by cracks which sizes vary from 15 to 300 m the small 40 m widespread polygons are very recent and degraded by the desiccation of ground ice from the cracks which enhances the effect of ice sublimation the large polygons 50 to 300 m located only around the south co2 polar cap indicate the presence of ground ice and thus outline the limit of the co2 ice cap they could be due to the blanketing of water ice deposits by the advances and retreats of the residual co2 ice cap during the last thousand years large 50 250 m and homogeneous polygons similar to ice wedge polygons hillslope stripes and solifluction lobes may indicate that specific environments such as crater floors and hillslopes could have been submitted to freeze thaw cycles possibly related to higher summer temperatures in periods of obliquity higher than 35 these interpretations must be strengthened by higher resolution images such as those of the hirise mission of the mars reconnaissance orbiter because locations with past seasonal thaw could be of major interest as potential landing sites for the phoenix mission

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

mars nature and evolution of young latitude dependent water ice rich mantle 1 high resolution altimetry and imaging have revealed the presence of a meters thick sedimentary layer at middle to high northern and southern latitudes presently covering at least 23 of the planet the layer is interpreted to be water ice rich and to undergone degradation recently its activity very likely coincided with the last major obliquity excursion a few hundred thousand years ago the majority of the layer at higher latitudes however persisted for a much longer time in the late amazonian stratigraphic analysis suggests a complex history of successive episodes of deposition and removal repeated deposition and removal of the mantles are interpreted to be responsible for the unusual statistical properties of kilometer scale topography in the transitional mid latitude zones

stratigraphic volcanic setting of massive sulfide deposits in the cambrian mount read volcanics tasmania the cambrian mount read volcanics of western tasmania host the volcanic hosted massive sulfide vhms deposits at hellyer que river rosebery hercules and mount lyell the volcanics were erupted along and partly onto the western margin of a precambrian basement terrane now represented by the tyennan region the volcanic belt is asymmetrical in section with a main eastern zone of relatively massive lava rich sequences accompanied by abundant intrusions and a broader western zone of volcano sedimentary sequences rich in volcaniclastic mass flow sandstone and breccia the rocks have been strongly deformed regionally metamorphosed to lower greenschist facies and hydrothermally altered in many areas stratigraphic and facies relationships within the highly complex volcanic belt are critically reviewed and reassessed the main volcanic zone is considered to comprise two interfingering lava rich associations the quartz feldspar porphyritic eastern sequence and the largely feldspar porphyritic central volcanic complex this lava rich zone interfingers with the western volcano sedimentary sequences herein referred to as the yolande river sequence the dundas group and the mount charter group new term recent volcanologic studies indicate that the lava rich sequences and indeed the bulk of the preserved mount read volcanics are submarine in character despite the presence of abundant ignimbritelike pumiceous deposits the early volcanism was predominantly rhyolitic dacitic in composition but was followed by a period of active andesitic basaltic volcanism during which the massive sulfide deposits at hellyer and que river and some of those at mount lyell were formed this widespread andesitic basaltic volcanism appears to coincide with a period of active extension and rifting along the belt focused to some extent on the henty fault system the rifting episode is evidenced by abundant tholeiitic dikes along the henty fault and throughout the northern central volcanic complex and by tholeiitic pillow lavas within the henty fault wedge the andesitic basaltic volcanism appears to have culminated in distinctive p 2 o 5 and light rare earth element lree enriched basalts at hellyer and similar rocks at lynch creek and howards plains near queenstown may be correlates the hellyer basalts were buried by black shale and the final volcanic phase was marked by a return to active felsic volcanism which produced widespread crystal and pumice rich mass flow deposits of the southwell subgroup and tyndall group

metallogenic episodes of the tasman fold belt system eastern australia precise dating of alteration minerals and host rocks to au and base metal deposits in the tasman fold belt system eastern australia has established a number of distinct metallogenic epochs which correspond to episodes of magmatism or to deformation events base and precious metal mineralization was emplaced in the mount read volcanics tasmania at about 500 ma and other hydrothermal or deformational events may have occurred in the hellyer deposit between approximately 480 and 440 ma copper and au mineralization was emplaced in the lachlan fold belt new south wales at about 440 ma and appears to be related to the end stages of ordovician magmatism that spans a period from approximately 480 through to 440 ma gold deposits or base metals which formed in new south wales and tasmania at about 420 to 380 ma are essentially coeval with deformation and or are broadly related to siluro devonian magmatism that largely occurred from about 420 to 390 ma gold mineralization in the anakie inlier northeast drummond basin north queensland was essentially coeval with the emplacement of the host drummond sequence at about 350 ma

the stratigraphic and structural setting of the mt lyell volcanic hosted sulfide deposits volcanic hosted sulfide deposits at mt lyell are confined to a complex silicic volcanic unit mine sequence which is up to 800 m thick and includes laterally impersistent lavas ash flow and ash fall deposits as well as abundant open framework volcanic mass flow breccias pervasive hydrothermal alteration involving na depletion is associated with the sulfide mineralization and persists with minor disseminated pyrite mineralization over a strike length of 8 km within the mine sequence stratigraphic relations together with lithofacies and thickness variations in volcanic units suggest that the sulfide mineralization and hydrothermal alteration occurred at a major eruptive center and that ore deposition occurred over an extended time interval during the accumulation of the mine sequence two orientation groups of middle devonian folds are present in the area first generation d 1 folds are closed to tight and upright north northwest trending structures which largely control the distribution and orientation of rock units in the volcanic sequence but have not been associated with significant microstructural modification of the ores or host rocks open west to west northwest trending upright second generation d 2 folds are well developed in sedimentary sequences overlying the volcanic sequence but are poorly developed in the volcanic sequence itself d 2 strain has been accommodated in the volcanic sequence largely by the development of a mesoscopically penetrative west to west northwest trending upright foliation s 2 and associated downdip lineation l 2 shortenings of up to 60 percent perpendicular to s 2 and elongations of up to 150 percent parallel to l 2 have modified primary sulfide microstructures and are responsible for many of the sulfide deposits in the area being elongate in the l 2 direction

organic matter accumulation of late ordovician sediments in north guizhou province china sulfur isotope and trace element evidences abstract the organic rich upper ordovician sediments wufeng and guanyinqiao formations on the yangtze platform are considered to be one of the main source rocks here we present geochemical proxies including redox indicator s c ratios and sulfur isotopes and productivity indices toc mo and ba contents from nanbazi section in north guizhou province south china in order to investigate the mechanism of organic matter accumulation the geochemical data suggest a stagnant and anoxic environment predominated the yangtze sea during the wufeng period whereas ventilated and oxygenated marine conditions pervaded the yangtze sea during the guanyinqiao period variations in the concentration of toc mo and ba indicate that higher organic carbon export in the wufeng intervals than those in the guanyinqiao intervals these variations in redox and productivity during the late ordovician were associated with different mechanisms and forcing processes the abrupt change from anoxic to oxygenated condition at the beginning of the guanyinqiao was concomitant with the global glacial period likely resulted from the glacio eustatic sea level fall and subsequent circulation of cold dense oxygenated waters upon the shelf seabed the productivity variations were related to the change of nutrient supply which is consistent with volcanic activities and runoff to the yangtze sea redox changes together with primary productivity fluctuations could have played a significant role on the variation of organic matter accumulation during the late ordovician in south china

resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground 1 to help interpret the polygonal patterned ground on mars we present recent findings about a similar form of patterned ground in a particularly cold and arid region on earth the dry valleys of antarctica in this region distinct arrays of interconnected polygons which we refer to herein simply as patterned ground characterize many surfaces reflecting a subsurface network of interconnected subvertical wedges of sand that grow incrementally as sand progressively fills soil fractures the fractures form initially as thermoelastic stresses arise during periods of rapid cooling of frozen ground and they continue to open and close in response to thermal cycles we describe the initiation and maturation of the patterned ground using data for the growth of sand wedges and for the evolution of crack patterns and microrelief over time scales ranging up to 106 years

landscape evolution in martian mid latitude regions insights from analogous periglacial landforms in svalbard periglacial landforms on spitsbergen svalbard norway are morphologically similar to landforms on mars that are probably related to the past and or present existence of ice at or near the surface many of these landforms such as gullies debris flow fans polygonal terrain fractured mounds and rock glacier like features are observed in close spatial proximity in mid latitude craters on mars on svalbard analogous landforms occur in strikingly similar proximity which makes them useful study cases to infer the spatial and chronological evolution of martian coldclimate surface processes the analysis of the morphological inventory of analogous landforms on svalbard and mars allows the processes operating on mars to be constrained different qualitative scenarios of landscape evolution on mars help to better understand the action of periglacial processes on mars in the recent past

high precision u pb zircon chronostratigraphy of the mount read volcanic belt in western tasmania australia implications for vhms deposit formation the middle cambrian mount read volcanics of western tasmania australia host several world class volcanic hosted massive sulfide vhms deposits representing a wide range of deposit styles although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures rapid lateral and vertical facies changes and faults with uncertain sense and magnitude of displacement have made it impossible to correlate stratigraphy across the belt previous dating studies in the area have yielded relatively imprecise crystallization ages we have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy id tims u pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the mount read volcanics and underlying mafic ultramafic complexes the new data permit detailed resolution of age relationships within the belt the study establishes an age of 516 0 0 9 ma for the mcivor hill gabbro which is part of a mafic ultramafic complex interpreted to underlie the mount read volcanics magmatism in the central mount read volcanics lasted at least 12 7 m y from 506 8 1 0 ma for a massive dacite unit in the lower part of the central volcanic complex to 496 0 0 9 ma for a welded ignimbrite in the lower tyndall group together with previous age constraints results of the study provide a precise chronostratigraphic framework for magmatism and vhms deposit formation within the mount read volcanics the precise age data indicate that north of the henty fault magmatism occurred in three discrete pulses at least two of which were separated by periods of sedimentation we demonstrate that vhms deposits in the mount lyell roseberry hercules and que hellyer districts comprising the majority of the known significant vhms deposits in the belt formed within a narrow time interval at 500 1 ma at a relatively late stage in the evolution of the belt some of the larger intrusions in the belt e g the bonds range porphyry 500 4 0 8 ma were emplaced contemporaneously with vhms deposit formation however other bodies such as the murchison granite 497 3 0 9 ma are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested these conclusions will aid explorers in targeting vhms mineralization in the mount read volcanics

outwash plains and thermokarst on mars abstract the spatial distribution of different types of rampart craters on mars suggests a hemispheric asymmetry in the distribution of ground ice the northern plains especially major topographic depressions near the terminations of outflow channels have high percentages of rampart craters two of these basins acidalia and utopia planitiae received extraordinarily large amounts of water and sediment from the chryse and elysium outflow channels in both regions the analysis of high resolution viking pictures 12 m pixel indicates a concentration of kilometer scale depressions that are similar in size and form to thermokarstic features in yakutia siberia and parts of the arctic coastal plain of north america accordingly we infer that 1 utopia planitia and acidalia planitia may contain thick laterally continuous ice rich sedimentary deposits related to outflow channel forming floods and 2 these areas of mars may have experienced thermokarstic processes similar to modern thermokarstic processes in some periglacial regions of earth

utopia basin mars characterization of topography and morphology and assessment of the origin and evolution of basin internal structure recently obtained mars orbiter laser altimeter mola topography has permitted a new assessment of the morphology structure and history of utopia planitia the new topographic data convincingly demonstrate that the utopia region is an impact basin as originally proposed by mcgill 1989 whose major topographic expression is a circular 1 3 km deep depression 3200 km in diameter utopia basin is the largest easily recognizable impact structure in the northern hemisphere of mars and is the only portion of the northern lowlands that exhibits a distinct large scale impact signature its presence there and its ancient age verify that a significant part of the northern lowlands dates back to the noachian period using slope maps derived from gridded topography we have mapped a series of linear slope anomalies within the basin and have analyzed their origin these features can be classified into two groups one oriented dominantly radial to the basin and the other oriented dominantly circumferential the circumferential features are tens of kilometers wide and hundreds of kilometers long and appear to remain relatively constant with respect to elevation we explore end member hypotheses 1 that the features formed by the direct action of water ice or 2 that they are tectonic wrinkle ridges we interpret the majority of these features to be due to tectonic deformation primarily wrinkle ridge formation modified by later sedimentation associated with the emplacement of the outflow channels and the formation of the vastitas borealis formation on the basis of the characteristics of these features and their association with polygonal terrain thumbprint terrain smoothness of units and evidence of sedimentary deposits we find that our results support earlier hypotheses proposing that a significant volume of water occupied utopia basin during the late hesperian period of outflow channel formation utopia basin differs significantly in depth and degradation state from the deeper and fresher appearing hellas basin although the two are both noachian in age a significant part of this difference seems reasonably interpreted to be the result of the extensive sedimentary and volcanic infilling history of the northern lowlands

the frequency of ice wedge cracking 1967 1987 at garry island western arctic coast canada the frequency of ice wedge cracking has been studied at garry island northwest territories for the 1967 1987 period sites have included low centre polygons intermediate centre polygons and polygons that do not fit any classification system analyses of crack frequency have included trough characteristics polygon characteristics and ice wedge types the results show that crack frequencies are highly variable within one site and also between two adjacent sites the correlation between crack frequency and a low air temperature is poor crack frequencies for a site with 59 wedges ranged from 8 to 42 between 1967 and 1979 and for a nearby site with 32 wedges from 22 to 75 between 1967 and 1987 in view of the wide range in crack frequencies at a given site the use of mean ice wedge growth rates for estimating ages of ice wedges and their casts in environmental reconstruction may be hazardous the data show that the common twofold classification into active and inactive wedges is difficult to apply be

distribution and evolution of scalloped terrain in the southern hemisphere mars abstract scalloped depressions are a unique martian surface morphology found in the northern and southern hemisphere latitude dependent dust and ice rich surface mantles these features exhibit a distinct asymmetric north south slope profile characterized by steep pole facing scarps flat floors and gentle equator facing slopes we examined high resolution stereo camera hrsc images of the southern hemisphere to determine their longitudinal distribution which revealed that a majority of scalloped terrain is located in the region of the southern wall of the hellas basin and northern malea planum a detailed map of this area was produced where scallops were found to contour the southern wall of the basin and where the ice rich mantle was seen to be thickest scalloped terrain is concentrated along the topographic highs near the amphitrites and peneus paterae and areal extent and depth decreases with increasing depth into the basin we also examined existing hypothesis for the formation and evolution of scalloped depressions using high resolution imaging science experiment hirise images and data from the thermal emission imaging system infrared themis ir and the thermal emission spectrometer tes our approach provides regional context for the development of scalloped terrains within the southern hemisphere and offers detailed evidence of scallop depressions forming around small cracks presumably caused by thermal contraction morphometric measurements show that scalloped depressions can be as much as 40 m deep with typical depths of between 10 and 20 m our observations of scallop formation and development in the southern hemisphere support a solar insolation model proposed by previous researchers e g morgenstern a hauber e reiss d van gasselt s grosse g schirrmeister l 2007 j geophys res 112 citeid e06010 lefort a russell p s thomas n mcewen a s dundas c m kirk r l 2009a j geophys res 114 e04005 lefort a russell p s thomas n 2009b icarus in press observations made using hirise images suggest that scalloped depressions most likely form from small cracks in the mantle which become larger and deeper through sublimation of interstitial ice from within the mantle sublimation is likely enhanced on equator facing slopes because of increased solar insolation which accounts for the asymmetric slope profile and hemispherical orientation and is demonstrated by themis ir images we suggest that sublimation lag deposits can possibly be removed by dust devils or strong slope winds related to the hellas basin offering an explanation as to why scalloped terrain is so abundant only in this area of the southern hemisphere daytime maximum summer temperatures suggest that sublimation in the study area of malea planum is possible under current conditions if the sublimation lag is removed while it cannot be ruled out that scalloped terrain in malea planum is presently evolving we attribute the extensive distribution to geologically recent obliquity excursions when conditions were more conducive to mesoscale modification of the ice rich mantle

scalloped terrains in the peneus and amphitrites paterae region of mars as observed by hirise abstract the peneus and amphitrites paterae region of mars displays large areas of smooth geologically young terrains overlying a rougher and older topography these terrains may be remnants of the mid latitude mantle deposit which is thought to be composed of ice rich material originating from airfall deposition during a high obliquity period less than 5 ma ago within these terrains there are several types of potentially periglacial features in particular there are networks of polygonal cracks and scalloped shaped depressions which are similar to features found in utopia planitia in the northern hemisphere this area also displays knobby terrain similar to the so called basketball terrains of the mid and high martian latitudes we use recent high resolution images from the high resolution imaging science experiment hirise along with data from previous mars missions to study the small scale morphology of the scalloped terrains and associated polygon network and knobby terrains we compare these with the features observed in utopia planitia and attempt to determine their formation process while the two sites share many general features scallops in peneus amphitrites paterae lack the diverse polygon network i e there is little variation in the polygon sizes and shapes and large curvilinear ridges observed in utopia planitia this points to a more homogeneous ice content within the substrate in the peneus amphitrites paterae region and implies that scallop formation is independent of polygon formation this work shows that as in utopia planitia sublimation of interstitial ice is a likely process explaining the formation of the scalloped depressions in the region of peneus amphitrites paterae therefore we provide a simplified scallop formation model based on sublimation of interstitial ice as proposed for utopia planitia we also show that the differences in scallop morphologies between the two regions may be explained by differences in near surface ice content sublimation rates and age of formation of the scalloped terrains

distribution of mid latitude ground ice on mars from new impact craters new impact craters at five sites in the martian mid latitudes excavated material from depths of decimeters that has a brightness and color indicative of water ice near infrared spectra of the largest example confirm this composition and repeated imaging showed fading over several months as expected for sublimating ice thermal models of one site show that millimeters of sublimation occurred during this fading period indicating clean ice rather than ice in soil pores our derived ice table depths are consistent with models using higher long term average atmospheric water vapor content than present values craters at most of these sites may have excavated completely through this clean ice probing the ice table to previously unsampled depths of meters and revealing substantial heterogeneity in the vertical distribution of the ice itself

amazonian northern mid latitude glaciation on mars a proposed climate scenario recent geological observations in the northern mid latitudes of mars show evidence for past glacial activity during the late amazonian similar to the integrated glacial landsystems in the dry valleys of antarctica the large accumulation of ice many hundreds of meters required to create the observed glacial deposits points to significant atmospheric precipitation snow and ice accumulation and glacial flow in order to understand the climate scenario required for these conditions we used the lmd laboratoire de meteorologie dynamique mars gcm general circulation model which is able to reproduce the present day water cycle and to predict past deposition of ice consistent with geological observations in many cases prior to this analysis however significant mid latitude glaciation had not been simulated by the model run under a range of parameters in this analysis we studied the response of the gcm to a wider range of orbital configurations and water ice reservoirs and show that during periods of moderate obliquity 25 35 and high dust opacity sdust 1 5 2 5 broad scale glaciation in the northern mid latitudes occurs if water ice deposited on the flanks of the tharsis volcanoes at higher obliquity is available for sublimation we find that high dust contents of the atmosphere increase its water vapor holding capacity thereby moving the saturation region to the northern mid latitudes precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems producing surface ice distribution and amounts that are consistent with the geological record ice accumulation rates of 10 mm yr

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

thermal contraction crack polygons on mars classification distribution and climate implications from hirise observations morphology is shown to be consistent with thermal contraction cracking of an ice rich mantling unit consistent with observations of sediment wedge thermal contraction crack polygons forming in ice cemented sediment at the phoenix landing site polygon groups are distributed symmetrically in both northern and southern hemispheres suggesting strong climate controls on polygon morphology northern hemisphere polygonally patterned surfaces are found to decrease in age from low to high latitude spanning surface ages from 1 to 0 1 ma suggesting more recent deposition of ice rich material at high latitudes than at low latitudes six of the seven classes of polygons are interpreted to be capable of forming because of the combined effects of thermal contraction cracking and differential sublimation suggesting that sublimation and sand wedge polygons dominate martian high latitudes gully polygon systems present at midlatitudes suggest that small amounts of liquid water may have been involved in thermal contraction crack polygon processes producing composite wedge polygons no evidence is found for the presence of pervasive small scale ice wedge polygons

the mars observer laser altimeter investigation the primary objective of the mars observer laser altimeter mola investigation is to determine globally the topography of mars at a level suitable for addressing problems in geology and geophysics secondary objectives are to characterize the 1064 nm wavelength surface reflectivity of mars to contribute to analyses of global surface mineralogy and seasonal albedo changes to assist in addressing problems in atmospheric circulation and to provide geodetic control and topographic context for the assessment of possible future mars landing sites the principal components of mola are a diode pumped neodymium doped yttrium aluminum garnet laser transmitter that emits 1064 nm wavelength laser pulses a 0 5 m diameter telescope a silicon avalanche photodiode detector and a time interval unit with 10 ns resolution mola will provide measurements of the topography of mars within approximately 160 m footprints and a center to center along track foot print spacing of 300 m along the mars observer subspacecraft ground track the elevation measurements will be quantized with 1 5 m vertical resolution before correction for orbit and pointing induced errors mola profiles will be assembled into a global 0 2 deg x 0 2 deg grid that will be referenced to mars center of mass with an absolute accuracy of approximately 30 m other data products will include a global grid of topographic gradients corrected individual profiles and a global 0 2 deg x 0 2 deg grid of 1064 nm surface reflectivity

a prelanding assessment of the ice table depth and ground ice characteristics in martian permafrost at the phoenix landing site 1 we review multiple estimates of the ice table depth at potential phoenix landing sites and consider the possible state and distribution of subsurface ice a two layer model of ice rich material overlain by ice free material is consistent with both the observational and theoretical lines of evidence results indicate ground ice to be shallow and ubiquitous 2 6 cm below the surface undulations in the ice table depth are expected because of the thermodynamic effects of rocks slopes and soil variations on the scale of the phoenix lander and within the digging area which can be advantageous for analysis of both dry surficial soils and buried ice rich materials the ground ice at the ice table to be sampled by the phoenix lander is expected to be geologically young because of recent climate oscillations however estimates of the ratio of soil to ice in the ice rich subsurface layer suggest that that the ice content exceeds the available pore space which is difficult to reconcile with existing ground ice stability and dynamics models these high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity initially high porosity soils or the migration of water along thin films measurement of the d h ratio within the ice at the ice table and of the soil to ice ratio as well as imaging ice soil textures will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable

recent ice rich deposits formed at high latitudes on mars by sublimation of unstable equatorial ice during low obliquity observations from the gamma ray spectrometer instrument suite on the mars odyssey spacecraft have been interpreted as indicating the presence of vast reservoirs of near surface ice in high latitudes of both martian hemispheres1 2 3 4 5 ice concentrations are estimated to range from 70 per cent at 60 latitude to 100 per cent near the poles possibly overlain by a few centimetres of ice free material in most places4 this result is supported by morphological evidence of metres thick layered deposits that are rich in water ice6 7 8 9 and periglacial like features10 11 found only at high latitudes diffusive exchange of water between the pore space of the regolith and the atmosphere has been proposed to explain this distribution12 but such a degree of concentration is difficult to accommodate with such processes9 13 14 alternatively there are suggestions that ice rich deposits form by transport of ice from polar reservoirs and direct redeposition in high latitudes during periods of higher obliquity9 13 but these results have been difficult to reproduce with other models here we propose instead that during periods of low obliquity less than 25 high latitude ice deposits form in both hemispheres by direct deposition of ice as a result of sublimation from an equatorial ice reservoir that formed earlier during a prolonged high obliquity excursion using the ice accumulation rates estimated from global climate model simulations we show that over the past ten million years large variations of mars obliquity have allowed the formation of such metres thick sedimentary layered deposits in high latitude and polar regions

the distribution and behavior of martian ground ice during past and present epochs mars undergoes significant oscillations in its orbit which will have a pronounced effect on its climate and in particular on the behavior of subsurface water ice we explore and map the behavior of ice in the martian near surface regolith over the past 1 m y using a diffusion and condensation model presented in an earlier paper with two primary modifications to include orbitally induced variations in insolation and atmospheric water abundance we find that the past behavior of ground ice differs significantly from that at the present epoch primarily the result of high amplitude oscillations in obliquity presently 25 in midlatitude and equatorial regions ground ice will condense from atmospheric water during times of higher obliquity filling the top few meters of the regolith with significant amounts of ice at an obliquity of 32 ground ice becomes stable globally during times of lower obliquity ground ice will sublime and diffuse back into the atmosphere dessicating the regolith to a depth of about 1 to 2 m equatorward of 60 to 70 latitude in the high latitude regions these oscillations are considerably subdued below this depth of cyclic saturation and dessication a long term stability of ice exists in some geographic regions we present a map of the distribution of ice expected at the present epoch cyclic exchange of water between the global regolith and polar regions will have significant implications for surface geology and the polar layered deposits

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

spatial variation in sediment fluxes redox conditions and productivity in the permian triassic panthalassic ocean abstract two permian triassic boundary sections in central japan provide a rare window into environmental conditions within the panthalassic ocean which encompassed more than half the earth s surface at 252 ma integration of petrographic geochemical and time series data provides new insights regarding the fluxes of major and trace components to the sediment as well as environmental conditions in both the deep and intermediate water masses at each study site the ubara section was located in a high productivity peri equatorial location whereas the gujo hachiman section was located in a moderate productivity location at some distance from the paleoequator an upward transition from gray organic poor cherts to black siliceous mudstones at both sites occurred in conjunction with increased primary productivity intensified euxinia within the oxygen minimum zone omz and decimation of the radiolarian zooplankton community euxinia in the omz of the equatorial panthalassic ocean developed episodically for a 200 250 kyr interval during the late permian followed by an abrupt intensification and lateral expansion of the omz around the permian triassic boundary throughout the study interval bottom waters at both sites remained mostly suboxic a finding that counters hypotheses of development of a superanoxic permo triassic deep ocean as a consequence of stagnation of oceanic overturning circulation

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

the geochemical characteristics and factors controlling the organic matter accumulation of the late ordovician early silurian black shale in the upper yangtze basin south china abstract organic rich black shale of the upper yangtze basin from the late ordovician and early silurian is considered an excellent source rock in south china the formation and preservation conditions of this resource are revealed by its geochemical characteristics in this study geochemical indices including redox indices v v ni v cr v sc and ni co and primary productivity indices p ti and ba al and paleoclimate clastic flux and sedimentary rate analyses are presented to investigate the accumulation mechanism of organic matter redox indices suggest that a stagnant anoxic environment predominated in the upper yangtze basin during accumulation of wufeng and longmaxi formations in contrast ventilated and oxygenated marine conditions pervaded the upper yangtze basin during deposition of linxiang and guanyinqiao formations the concentrations of v and u demonstrate that accumulation of organic matter was mainly controlled by redox conditions besides such factors as clastic fluxes fresh water inflows or a mixed deposition with a rapid sedimentary rate cannot be ignored due to their influences on organic matter enrichment and preservation however weak co variance relationship of toc content and productivity proxies including p ti and ba al demonstrates that the accumulation of organic matter was not controlled by primary productivity results of the present study suggest a depositional model that stresses the importance of tectonic movements and glacial events on the accumulation and preservation of organic matter the model shows that the upper yangtze basin was a semi restricted basin system influenced by the isolation of xuefeng but also it implies that oxygen depleted bottom water of the basin favored the accumulation and preservation of sedimentary organic matter resulting in the formation of organic rich black shale

the global topography of mars and implications for surface evolution elevations measured by the mars orbiter laser altimeter have yielded a high accuracy global map of the topography of mars dominant features include the low northern hemisphere the tharsis province and the hellas impact basin the northern hemisphere depression is primarily a long wavelength effect that has been shaped by an internal mechanism the topography of tharsis consists of two broad rises material excavated from hellas contributes to the high elevation of the southern hemisphere and to the scarp along the hemispheric boundary the present topography has three major drainage centers with the northern lowlands being the largest the two polar cap volumes yield an upper limit of the present surface water inventory of 3 2 to 4 7 million cubic kilometers

convergence and intraplate deformation in the lachlan fold belt of southeastern australia abstract the lachlan fold belt of southeastern australia is dominated by a widespread oceanic association including cambrian submarine mafic volcanics and an overlying widespread ordovician quartz rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres these rocks are overlain by or in fault contact with deep marine to continental silurian to early carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks major deformation achieved by thrusting and folding affected the fold belt during the silurian to middle devonian and the early carboniferous shortening estimates vary throughout the belt but the widespread ordovician quartz turbidite succession usually has at least 60 shortening which implies an original width of the fold belt of about 1700 km strike slip faulting was important on a local to regional scale but large scale strike slip displacements appear unlikely the large values of shortening are consistent with development of the belt in a convergent margin setting at least for the silurian to carboniferous the nature of the lower crust of the lachlan fold belt has always presented a problem of interpretation but we favour a late proterozoic quasi continental lower crustal layer that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during silurian carboniferous convergence and provided a source for silurian devonian granites

the periglacial landscape at the phoenix landing site 1 the mars scout mission phoenix landed in the northern high latitudes of mars in an area thought to be dominated by subsurface ground ice and has provided a unique detailed examination of periglacial morphology and topography that is not visible from orbit the landing site and surrounding terrain is dominated by polygonal patterned ground typical of the martian northern plains the polygons exhibit a gentle u shaped trough and rounded mound topography with an average trough depth of 17 cm and an average surface slopes of only 6 7 polygons an average 4 3 m across form a pattern superimposed on all other geologic landforms including larger 20 25 m polygons and kilometer long linear ridges the polygon morphology and the predominance of subsurface ice cemented soil are consistent with the formation of subsurface sand wedges in a permanently frozen arid climate small scale soil furrows and cracks into the ice cemented soil indicate that the thermal contraction process responsible for the formation of the polygons is active today rocks appear to be sorted within the polygon troughs indicating a process of cryoturbation within the permanently frozen polygons the complexity of the patterns crosscutting relations and rock distributions indicate advanced maturity of the polygon networks and shifting crack patterns possibly facilitated by crack and wedge annealing during times of low obliquity

secular distribution of biogenic silica through the phanerozoic comparison of silica replaced fossils and bedded cherts at the series level abstract the modern marine silica cycle is dominated by silica secreting phytoplankton principally diatoms but this cycle has evolved considerably during the phanerozoic we analyzed the temporal distribution of silica replaced fossils and bedded chert to determine the influence of factors such as extinctions and climate change on siliceous facies trends in silica replacement of fossils match faunal radiations during the ordovician and siluro devonian with peaks in silica replacement in the late ordovician and middle devonian corresponding to peaks in the abundance and diversity of siliceous sponges sharp drops in the abundance of silica replaced fossils and or bedded cherts coincide with four of the five major mass extinctions no discernible decrease marks the extinction at the end of the triassic we expected peaks in bedded chert deposition during glacial episodes because enhanced ocean circulation should favor more and stronger upwelling orogenic episodes which may trigger continental glaciati

deposition and degradation of a volatile rich layer in utopia planitia and implications for climate history on mars 1 we investigate the surface morphology of a study area in western utopia planitia mars which is characterized by a variety of landforms that resemble those of terrestrial periglacial landscapes polygonally fractured ground and thermokarst like depressions are found to be located in a young mantling deposit with a thickness of several tens of meters we observe a latitudinal dependence of the degradation of this mantling deposit larger depressions whose floors reveal the underlying basement rocks cover a higher fraction of the total terrain in the southern part of the study area than in the northern part indicating a more intense degradation of the mantle in the south all depressions have an asymmetric cross section in north south direction interpreted to be the result of the different solar radiation on differently oriented slopes on the basis of our morphological observations we develop a conceptual model for landscape evolution in western utopia planitia it involves subaerial deposition of a layered ice rich mantle and its subsequent degradation by polygon formation and sublimation a terrestrial analog to the polygonally fractured mantling deposit and its thermokarst like depressions is the siberian ice complex or yedoma which consists of subaerial ice rich deposits and is connected to nonglaciated landscapes with highly continental cold climatic environmental conditions our comparison suggests that no unusual or exotic processes need to be invoked to explain the current morphology of western utopia however the young age of the deposition and degradation implies climatic excursions in the very recent past on mars

thermokarst processes and the origin of crater rim gullies in utopia and western elysium planitia abstract we have identified a number of gullies that could be aqueous in origin near or at the rim of several impact craters in utopia planitia and western elysium planitia 30 0 59 0 n 241 0 291 0 w based on the sharpness of their incisions and the general absence of superposed craters we ascribe a relatively recent origin to the gullies scalloped depressions are commonplace throughout the region as well as on the crater walls rims and floors near the areas of gully issuance occasionally the depressions cross cut the gully debris aprons suggesting that the formation of some depressions is even more recent than that of the gullies previous research has proposed that the depressions are collapse basins formed by thermokarst processes on earth thermokarst landforms occur in areas of low gradient topography where the permanently frozen ground permafrost is ice rich and has undergone a change in thermal equilibrium this change can be triggered by long term or episodic cyclic climate change and accompanying rises in mean temperatures towards 0 c as well as by rises in seasonally sustained summer temperatures well above 0 c in order to explain the origin of the rim or near rim gullies we invoke high obliquity and the possibility that this region of mars experienced obliquity driven rises in temperature atmospheric pressure and humidity sufficient to keep surface water and near surface ground ice stable for extended periods of time we propose that gully formation is closely related to local freeze thaw processes that in turn generate a thermokarst landscape of which the gullies are a part this geological and climatological scenario comprises the following steps 1 an inundation of meltwater at high obliquity due to the thawing of an atmospherically deposited snowpack or ice sheet and the subsequent saturation of the underlying regolith to tens of metres of depth 2 loss of water on the surface perhaps as obliquity decreases slightly followed by the progressive freezing of the saturated regolith this creates an aggrading mass of ice rich regolith 3 obliquity induced temperature rises that engender the thaw drainage and partial evaporation of the near surface ice rich regolith 4 localised formation of thermokarst collapse basins alases as water is evacuated from these basins 5 formation of gullies near or at some impact crater rims as the result of meltwater migration from nearby alases through the thawed regolith to the areas of gully issuance although the plains materials in this region are in part very old possibly hesperian or even noachian the mantling deposits and their deformation by thermokarst processes appears to be relatively young this suggests that recent climatic conditions could have been episodically warmer and wetter than had been previously thought

isotopic dating of basin inversion the palaeozoic cobar basin lachlan orogen australia abstract the age of inversion of the early devonian cobar basin is poorly defined by field geological data which have to be extrapolated from the coeval shelf west of the basin in order to better constrain the age of deformation a whole rock isotopic dating program was undertaken and yields the following results rb sr isotopic dates predate the depositional age giving a regional isochron of 442 4 ma three k ar dates from zone 1 are mutually indistinguishable 402 8 402 8 405 8 ma suggesting a mid to late early devonian age of cleavage formation four whole rock 40ar 39ar spectra from the high strain zone 1 give intermediate temperature plateaux ranging from ca 389 ma to 404 ma in the lower strain zone 2 total gas ages from three samples range from ca 401 ma to ca 423 ma an approximate intermediate temperature plateau for one sample indicates an age ca 405 ma the other sample yields a good plateau of ca 437 ma these data are interpreted as reflecting a low grade cleavage forming inversion event which took place at ca 395 400 ma isotopic data from zone 1 suggest near complete rejuvenation of detrital phases during the 395 400 ma event in contrast the older zone 2 rb sr ages and some of the 40ar 39ar ages reflect at least in part basement provenance ages preserved within detrital micas and feldspars which did not undergo complete rejuvenation because of lower deformational strain and or temperature of metamorphism the intense 395 400 ma late early devonian inversion event in zones 1 and 2 of the former cobar basin largely bypassed the southwestern part of the basin and the adjacent shelf to the west these only underwent minor block faulting and uplift at this time but were more strongly deformed along with the overlying fluviatile mulga downs group in the carboniferous