effects of plasticity on convection in an ice shell implications for europa abstract europa s surface exhibits numerous pits uplifts and disrupted chaos terrains that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in an ice shell including the effects of plasticity which provides a simple continuum representation for brittle or semibrittle deformation along discrete fractures plastic deformation occurs when stresses reach a specified yield stress at lower stresses the fluid flow follows a newtonian temperature dependent viscosity four distinct modes of behavior can occur for yield stresses exceeding 1 bar plastic effects are negligible and stagnant lid convection with no surface motion and minimal topography results at intermediate yield stresses a stagnant lid forms but deforms plastically leading to surface velocities up to several millimeters per year slightly smaller yield stresses allow episodic catastrophic overturns of the upper conductive lid with transient stagnant lids forming in between overturn events the smallest yield stresses allow continual recycling of the upper lid with simultaneous gradual ascent of warm ice to the surface and descent of cold near surface ice into the interior the exact yield stresses over which each regime occurs depend on the ice shell thickness melting temperature viscosity and activation energy for viscous creep to form hummocky matrix and translate chaos plates by several kilometers substantial surface strain must accompany chaos formation and the three plasticity dominated convection modes described here can provide such deformation our simulations suggest that if yield stresses of 0 2 1 bar are relevant to europa then convection in europa s ice shell can produce chaos like structures at the surface however our simulations have difficulty explaining europa s numerous pits and uplifts when plasticity forces the upper lid to participate in the convection dynamic topography of 50 100 m amplitude results but the topographic structures generally have diameters of 30 100 km an order of magnitude wider than typical pits and uplifts none of our simulations produced isolated pits or uplifts of any diameter

the timing of ordovician magmatism in the english lake district and cross fell inliers volcanic and hypabyssal intrusive rocks of the lower palaeozoic english lake district and cross fell inliers are elements of the ordovician destructive plate margin system of microcontinental avalonia two igneous sheets within the marine sedimentary skiddaw group of these inliers previously described as lavas are reinterpreted as sills sedimentary rocks enclosing these sills are of late tremadoc early arenig c 493 ma and early llanvirn 476 ma age and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions previous interpretation of the breccias on the older sheet as sediment deposited on the eroded top of a lava flow implied an early ordovician onset of arc magmatism such early magmatism would have been virtually coincident with the latest tremadoc initiation of arc magmatism in wales but evidence for such a near synchronous response tothe putative onset of subduction is lacking respective onsets of magmatismwere probably separated by at least 17 m y and possibly by as much as 29 m y the apparent contemporaneity of mid and late ordovician volcanic episodes in england and wales and similarities in extensional tectonic style suggest that the two areas then were part of the same subduction system responding similarly to plate scale magma generating and tectonic processes the early ordovician situation is uncertain but the absence of arc volcanic rocks of this age in the english lake district suggests that this area and wales are not tectonically juxtaposed elements of a former simple linear arc

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

evidence for subduction in the ice shell of europa despite widespread evidence for extension there have been few signs of contraction on the icy surface of jupiter s europa evidence for a subduction like convergent boundary suggests that europa may have active plate tectonics

estimates of europa s ice shell thickness from elastically supported topography 1 the thickness of europa s solid ice shell is uncertain and has important implications for europa s habitability and thermal history and the design of future spacecraft missions here we obtain an estimate of the ice shell thickness from observations of a plateau sw of cilix impact crater stereo topographic profiles suggest that the plateau is flexurally supported with an effective elastic thickness te of 6 2 5 km for a conductive temperature profile this te value implies a solid ice shell thickness of 15 9 20 km if the shell is convecting this estimate is a lower bound combined with independent estimates we infer a probable shell thickness of 25 km the shell thickness is likely to be uniform over the entire satellite

hydration during uplift is recorded by reset rb sr whole rock ages the garn fadryn sub volcanic intrusion of n wales gives a u pb zircon age of 4533 2 2 ma this is the first reliable emplacement age from the welsh caradoc volcanic province proving that the rb sr whole rock age of 392 32 ma does not date intrusion but reflects resetting during early devonian low grade metamorphism resetting in this setting is chemically controlled and is a function of disturbance during water rock interaction it is not analogous to the predominantly thermally controlled resetting of k ar systems post volcanism rb sr whole rock secondary ages in this region have a mean age of 399 9 ma and record a specific event which is the time of hydration it is proposed that this event was triggered by uplift which opened fractures increased permeability facilitated formation of secondary hydrated minerals and reset the rb sr system this model for resetting should have widespread applicability to other volcanic provinces affected by low grade metamorphism

ice melting and downward transport of meltwater by two phase flow in europa s ice shell with its young surface very few impact craters and the abundance of tectonic and cryovolcanic features europa has likely been subjected to relatively recent endogenic activity morphological analyses of chaos terrains and double ridges suggest the presence of liquid water within the ice shell a few kilometers below the surface which may result from enhanced tidal heating a major issue concerns the thermal gravitational stability of these water reservoirs here we investigate the conditions under which water can be generated and transported through europa s ice shell we address particularly the downward two phase flow by solving the equations for a two phase mixture of water ice and liquid water in one dimensional geometry in the case of purely temperate ice we show that water is transported downward very efficiently in the form of successive porosity waves the time needed to transport the water from the subsurface region to the underlying ocean varies between 1 and 100 kyr depending mostly on the ice permeability we further show that water produced in the head of tidally heated hot plumes never accumulates at shallow depths and is rapidly extracted from the ice shell within less than a few hundred kiloyears our calculations indicate that liquid water will be largely absent in the near subsurface with the possible exception of cold conductive regions subjected to strong tidal friction recently active double ridges subjected to large tidally driven strike slip motions are perhaps the most likely candidates for the detection of transient water lenses at shallow depths on europa

intra ordovician deformation in southeast ireland evidence from the geological setting geochemical affinities and u pb zircon age of the croghan kinshelagh granite the croghan kinshelagh alkali granite intrudes a cleaved volcano sedimentary sequenceon the border of counties wicklow and wexford in southeast ireland u pb dating of zircons fromthe granite indicate a mid caradoc emplacement age of 454 1 ma the duncannon group hostrocks form the southwestern end of the avoca volcanic belt a mid ordovician caradoc sequenceof acid and intermediate lavas and volcaniclastics dolerite dykes intrude the granite elsewhere in theregion dolerites are generally associated with volcanic rocks the main dl deformation within theduncannon group rocks is manifest as a steep dl cleavage generally regarded as a product of latecaledonian regional deformation in southeast ireland the croghan kinshelagh granite showsstrong geochemical coherence with subalkaline varieties of the caradoc volcanic rocks relativelyhigh th y nb and ree contents set it apart from any other known granite type in southeastireland together with the geochemical evidence the age determination of 454 ma indicates that thecroghan kinshelagh granite was generated and emplaced during ordovician volcanism in southeastireland volcanism was closely followed by penetrative deformation and emplacement of the granite the intra ordovician deformation may have been a consequence of closure of the iapetus ocean ormore localized events such as accretion on the hanging wall of the subduction zone the age of thecroghan kinshelagh granite provides an important datum for ordovician volcanism and subductionin southeast ireland

thermal equilibrium states of europa s ice shell implications for internal ocean thickness and surface heat flow abstract ice shell thickness and ocean depth are calculated for steady state models of tidal dissipation in europa s ice shell using the present day values of the orbital elements the tidal dissipation rate is obtained using a viscoelastic maxwell rheology for the ice the viscosity of which has been varied over a wide range and is found to strongly increase if an inviscid internal ocean is present to determine steady state values the tidal dissipation rate is equated to the heat transfer rate through the ice shell calculated from a parameterized model of convective heat transfer or from a thermal conduction model if the ice layer is found to be stable against convection although high dissipation rates and heat fluxes of up to 300 mwm 2 are in principle possible for europa these values are unrealistic because the states for which they are obtained are thermodynamically unstable equilibrium models have surface heat flows around 20 mwm 2 and ice layer thicknesses around 30 km which is significantly less than the total thickness of the h2o layer these results support models of europa with ice shells a few tens of kilometers thick and around 100 km thick subsurface oceans

lower palaeozoic and precambrian igneous rocks from eastern england and their bearing on late ordovician closure of the tornquist sea constraints from u pb and nd isotopes the u pb isotope ages and nd isotope characteristics of asuite of igneous rocks from the basement of eastern england show that ordovician calc alkaline igneous rocks are tectonically interleaved with late precambrian volcanic rocks distinct from precambrian rocks exposed in southern britain new u pb ages for the north creake tuff zircon 449 13 ma moorby microgranite zircon 457 20 ma and the nuneaton lamprophyre zircon and baddeleyite 442 3 ma confirm the presence ofan ordovician magmatic arc tectonically interleaved precambrian volcanic rocks within this arc are verified by new u pb zircon ages for tuffs at glinton 612 21 ma and orton 616 6 ma initial end values for these basement rocks range from 4 to 6 consistent with generation of both c 615 ma and c 450 ma groups of rocksin continental arc settings the u pb and sm nd isotope data support arguments for an ordovician fold thrust belt extending from england to belgium and that the ordovician calc alkaline rocks formed in response to subductionof tornquist sea oceanic crust beneath avalonia

estimates of ice thickness in the conamara chaos region of europa europa the second galilean moon outward from jupiter is similar in size to earth s moon but has an outer water ice layer on the order of 100 km thick images acquired by the galileo spacecraft reveal domical features and iceberg like blocks in some areas that suggest the ice was regionally thin at the time the features formed this paper applies buoyancy and flexural models to some of the features to estimate ice thickness assuming that the ice blocks were floating in a liquid sublayer the buoyancy model suggests an ice layer 0 2 3 0 km thick also the flexural models suggest that only the upper 0 1 0 5 km of that ice layer responded elastically to stresses acting as an elastic lithosphere

flanking fractures and the formation of double ridges on europa abstract europa a satellite of jupiter is one of the most intriguing worlds in the solar system its dearth of impact craters and plethora of surface morphologies point to a dynamic evolution of its icy shell in geologically recent times double ridges are a common landform and appear to have formed over a significant fraction of the satellite s observed geologic history thus understanding their formation is critical to unraveling europa s history and many models have been proposed to explain their creation a clue to the formation of ridges may lie in evidence for flexure of the lithosphere in response to a load imposed by the ridge itself marginal troughs and subparallel flanking fractures when this flexure has been modeled a simple elastic lithosphere has typically been assumed however the generally thin lithospheres suggested by these models require very high heat flows that are inconsistent with europa s expected thermal budget of order 1 w m 2 vs of order 10 mw m 2 each of the proposed formational models however predicts a thermal anomaly that may facilitate the flexure of europa s lithosphere here we simulate this flexure in the presence of these anomalies as a means to evaluate the different models of ridge formation we find that nearly all models of double ridge formation are inconsistent with the observation of flexure specifically the flanking fractures except for a cryovolcanic model in which the growing ridge is underlain by a cryomagmatic sill that locally heats and thins the lithosphere

ganderia laurentia collision in the caledonides of great britain and ireland during terrane convergence an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition in the caledonides of great britain and ireland units accreted to laurentia during the early palaeozoic era include peri gondwanan terrane assemblages that earlier separated from west gondwana however the southern uplands terrane contains detrital zircon populations apparently derived entirely from laurentia characterized by a large asymmetric mesoproterozoic peak and a scarcity of zircon at 600 ma and 2 1 ga in contrast cambrian and ordovician rocks from the lake district and the leinster massif of ireland show abundant grains with these ages together with a range of mesoproterozoic zircon these characteristics are shared with the monian terrane of anglesey and with ganderia in the appalachians indicating probable derivation from amazonia in west gondwana silurian sandstones from the lake district show an influx of laurentia derived zircon and lack the peri gondwanan signal this indicates that in the caledonides ganderia was not accreted to the laurentian margin until c 430 ma in contrast to the ordovician accretion of ganderian fragments recorded in the appalachians suggesting that the configuration of the closing iapetus ocean varied significantly along the strike of the orogen supplementary material details of sample coordinates analytical procedure and u pb detrital zircon analytical data are available at www geolsoc org uk sup18739

sm nd garnet age for the ordovician borrowdale volcanic group english lake district garnets separated from andesites and dacites of the calc alkaline borrowdale volcanic group are highly enriched in heavy rare earth elements hree relative to their host rocks d gt yb from 9 in andesite to 37 in dacite sm nd ratios and d gt lree show a wide range probably resulting from small quantities of an ree rich impurity in the separates the four garnet host rock pairs analysed yield sm nd ages identical within analytical error of a weighted mean of 457 4 ma the borrowdale group overlies upper llanvirn sediments and is unconformably overlain by mid caradoc the age of 457 ma is considered to be the best estimate available for the age of the llandeilo as the samples analysed are from the lower part of the group the age suggests that a lower palaeozoic timescale intermediate between those of gale et al 1979 and mckerrow et al 1980 might be the most appropriate

convection in ice i shells and mantles with self consistent grain size 1 the viscosity of ice i is grain size dependent for temperature and stress conditions appropriate for ice i shells and mantles of large and midsized icy satellites satellite thermal evolution heat flux critical shell thickness for convection brittle ductile transition temperature and potential for surface deformation are therefore grain size dependent using measured grain sizes from terrestrial ice sheets experiencing temperature and strain rate conditions similar to convecting ice shells in icy satellites we develop two end member models of grain size and its evolution in the absence of non water ice impurities grain size in a convecting ice shell of a large icy satellite will evolve over time to an equilibrium value which we determine to be between 30 and 80 mm because of dynamic recrystallization such large grains imply viscous ice so convection in an ice shell thinner than 35 km in a large icy satellite such as ganymede and callisto would be sluggish at best and may cease after a few convective overturns if it starts soluble ions and silicate microparticles may keep grains small however of order 1 5 mm and permit convection in shells as thin as 30 km in large icy satellites the dynamic recrystallization model provides a plausible upper limit on grain size in a geodynamically evolved ice shell whereas grain sizes from dust contaminated terrestrial ice cores provide a plausible lower limit on grain size in an icy satellite on the basis of these constraints grain sizes in evolved ice shells of large water ice satellites in the absence of tidal forcing should be of order 1 mm to 10 cm

active formation of chaos terrain over shallow subsurface water on europa the galileo spacecraft revealed a number of chaos regions on jupiter s moon europa where the surface terrain appears to have been disrupted from below in many places the surface contains sharp edged blocks or rafts of ice that have at some point been flipped or rotated some characteristics of these regions have been hard to explain such as the fact that the archetypal conamara chaos stands above its surroundings and contains matrix domes schmidt et al apply lessons learned from analogous processes within earth s subglacial volcanoes and ice shelves to an analysis of archival data that suggests chaos terrain forms above liquid water lenses that are perched only 3 kilometres deep within the ice shell the data suggest that ice water interactions and freeze out give rise to the varied morphology of chaos terrains implying that more water is involved than has been previously appreciated for instance the sunken topography of thera macula a large chaos area may indicate that europa is actively resurfacing over a lens comparable in volume to north america s great lakes europa the innermost icy satellite of jupiter has a tortured young surface1 2 3 4 and sustains a liquid water ocean1 2 3 4 5 6 below an ice shell of highly debated thickness1 2 3 4 5 7 8 9 10 quasi circular areas of ice disruption called chaos terrains are unique to europa and both their formation and the ice shell thickness depend on europa s thermal state1 2 3 4 5 7 8 9 10 11 12 13 14 15 16 17 no model so far has been able to explain why features such as conamara chaos stand above surrounding terrain and contain matrix domes10 18 melt through of a thin few kilometre shell3 7 8 is thermodynamically improbable and cannot raise the ice10 18 the buoyancy of material rising as either plumes of warm pure ice called diapirs1 9 10 11 12 13 14 15 or convective cells16 17 in a thick 10 kilometres shell is insufficient to produce the observed chaos heights and no single plume can create matrix domes10 18 here we report an analysis of archival data from europa guided by processes observed within earth s subglacial volcanoes and ice shelves the data suggest that chaos terrains form above liquid water lenses perched within the ice shell as shallow as 3 kilometres our results suggest that ice water interactions and freeze out give rise to the diverse morphologies and topography of chaos terrains the sunken topography of thera macula indicates that europa is actively resurfacing over a lens comparable in volume to the great lakes in north america

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

simultaneous melting and compaction in deformable two phase media summary melt generation and extraction are typically modelled using the two phase equations developed by mckenzie or scott and stevenson various approximations are often made to simplify the problem which may lead to some unphysical results e g thermodynamically inconsistent conditions of melting and unrealistic porosity profiles we discuss a generalized version of the set of equations introduced by bercovici et al that allows for mass transfer between the two phases in a single component system and consider a self consistent set of equations in our description the two phases solid and melt are submitted to individual pressure fields whose difference is related to the surface tension at the interfaces changes in porosity and the melting rate a kinetic relation for the melting rate arises from the second law of thermodynamics the condition of chemical equilibrium corresponds to the usual univariant equality of the chemical potentials of each phase when the matrix and melt are motionless in the most general form phase equilibrium is influenced by both the gibbs thomson effect that arises naturally from thermodynamic considerations on surface tension and by the viscous deformation of the phases we apply these new equations to a steady state problem of pressure release melting in a univariant system we treat melting and compaction simultaneously and observe several new effects including multiple domains near the onset of melting that correspond to various force balances a consequence of matrix compaction and melt expulsion or matrix dilation and melt accumulation is a pressure difference between melt and solid that facilitates inhibits melting for parameters corresponding to mid oceanic ridge magmatism compaction permits melting to start as much as 2k m below the standard solidus numerical solutions are necessary to determine the magnitude of the melt zone shift numerical results support the boundary layer solutions obtained analytically and suggest that in most of the melting zone the movement of melt and matrix should be close to the darcy equilibrium where the buoyancy of melt is balanced by the viscous drag between the phases the darcy equilibrium follows an initial stage where the matrix viscous stresses balance darcy drag in all situations the steady state porosity profile remains a monotonic function of depth the existence of a compaction layer following a melting zone where the porosity is maximum as described in various earlier publications has never been found

implications of shear heating and fracture zones for ridge formation on europa 1 we present 2d and 3d numerical simulations of convection to test the role of shear heating and fracture zones on europan ridge formation our simulations show that a pre existing fracture zone promotes upwelling and lithospheric thinning leading to topographic uplift of 50 m shear heating also promotes lithospheric thinning and buoyant ascent producing a ridge like feature with topography up to 120 m topography remains linear along strike even under the influence of heterogeneous 3d convection within the ice shell although the central trough is not reproduced in the simulations our results support the idea that shear heating can produce ridge like structures on europa

the margins of avalonia during cambrian and earliest ordovician times avalonia was an area forming an integral part of the huge gondwanan continent probably along the northern margin of amazonia until in early ordovician late arenig or llanvirn time it split off from gondwana leaving a widening rheic ocean to its south today its southern margin with gondwana extends northeast from east of cape cod massachusetts through nova scotia north of the meguma terrane and thence below sea level to the south of newfoundland on the eastern side of the present atlantic the southern margin may separate southwest portugal from the rest of the iberian peninsula it can be traced eastwards with more certainty from the south cornwall nappes to a line separating the northern phyllite belt on the southern margin of the rhenohercynian terrane and the mid german crystalline high there is no certain evidence of avalonian crust to the northeast of the elbe line the northern margin of avalonia extends westwards from south of denmark to the british isles where it merges with the iapetus ocean suture between scotland and england traced westwards it crosses ireland and reappears in northern newfoundland to the east of new world island where it may follow the trace of the dog bay line and the cape ray fault recent work suggests that the northern margin of avalonia may clip the northern tip of cape breton island in nova scotia and then enter the north american mainland at the bay of chaleur it may then be traced from north and west of the popelogan and bronson hill arcs to long island sound near newhaven connecticut the cambrian to devonian faunas reflect the history of avalonia initially they were purely gondwanan but as ordovician time proceeded more genera crossed firstly the tornquist ocean as it narrowed between avalonia and baltica to close in latest ordovician and early silurian times and secondly the iapetus ocean so that by the early silurian most of the benthic shelly faunas apart from the ostracods were the same round the adjacent margins of all three palaeocontinents

the tidal history of iapetus spin dynamics in the light of a refined dissipation model 1 we study the tidal history of an icy moon basing our approach on a dissipation model which combines viscoelasticity with anelasticity and takes into account the microphysics of attenuation we apply this approach to iapetus the most remote large icy moon in the saturnian system different authors provide very different estimates for iapetus s despinning timescale by several orders of magnitude one reason for these differences is the choice of the dissipation model used for computing the spin evolution as laboratory data on viscoelastic properties of planetary ices are sparse many studies relied on dissipation models that turned out to be inconsistent with experiment a pure water ice composition generally assumed in the previous studies of the kind yields despinning times of the order of 3 7 gyr for most initial conditions we demonstrate that through accounting for the complexity of the material like second phase impurities one arrives at despinning times as short as 0 9 gyr a more exact estimate will remain unavailable until we learn more about the influence of impurities on ice dissipation by including the triaxial shape caused torque we encounter a chaotic behavior at the final stage of despinning with the possibility of entrapments in the intermediate resonances the duration of these entrapments turns out to be sensitive to the dissipation model no long entrapments have been found for iapetus described with our laboratory based dissipation model

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

characterization of elastic properties of near surface and subsurface deepwater hydrate bearing sediments abstracthydrate bearing sands and shallow nodular hydrate are potential energy resources and geohazards and they both need to be better understood and identified therefore it is useful to develop methodologies for modeling and simulating elastic constants of these hydrate bearing sediments a gas hydrate rock physics model based on the effective medium theory was successfully applied to dry rock water saturated rock and hydrate bearing rock the model was used to investigate the seismic interpretation capability of hydrate bearing sediments in the gulf of mexico by computing elastic constants also known as seismic attributes in terms of seismic interpretation including the normal incident reflectivity ni poisson s ratio pr p wave velocity vp s wave velocity vs and density the study of the model was concerned with the formation of gas hydrate and therefore hydrate bearing sediments were divided into hydrate bearing sands hydrate bearing sands with free gas in the pore space and sh

europa s crust and ocean origin composition and the prospects for life abstract we have considered a wide array of scenarios for europa s chemical evolution in an attempt to explain the presence of ice and hydrated materials on its surface and to understand the physical and chemical nature of any ocean that may lie below we postulate that following formation of the jovian system the europan evolutionary sequence has as its major links a initial carbonaceous chondrite rock b global primordial aqueous differentiation and formation of an impure primordial hydrous crust c brine evolution and intracrustal differentiation d degassing of europa s mantle and gas venting e hydrothermal processes and f chemical surface alteration our models were developed in the context of constraints provided by galileo imaging near infrared reflectance spectroscopy and gravity and magnetometer data low temperature aqueous differentiation from a carbonaceous ci or cm chondrite precursor without further chemical processing would result in a crust ocean enriched in magnesium sulfate and sodium sulfate consistent with galileo spectroscopy within the bounds of this simple model a wide range of possible layered structures may result the final state depends on the details of intracrustal differentiation devolatilization of the rocky mantle and hydrothermal brine reactions could have produced very different ocean crust compositions e g an ocean crust of sodium carbonate or sulfuric acid or a crust containing abundant clathrate hydrates realistic chemical physical evolution scenarios differ greatly in detailed predictions but they generally call for a highly impure and chemically layered crust some of these models could lead also to lateral chemical heterogeneities by diapiric upwellings and or cryovolcanism we describe some plausible geological consequences of the physical chemical structures predicted from these scenarios these predicted consequences and observed aspects of europa s geology may serve as a basis for further analys is and discrimination among several alternative scenarios most chemical pathways could support viable ecosystems based on analogy with the metabolic and physiological versatility of terrestrial microorganisms

constraints on the lake volume required for hydro fracture through ice sheets 1 water filled cracks are an effective mechanism to drive hydro fractures through thick ice sheets crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water filled until it reaches the bed in this study we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water filled cracks beneath supraglacial lakes we find that the cross sectional area of water filled cracks increases non linearly with ice sheet thickness using these results we place volumetric constraints on the amount of water necessary to drive cracks through 1 km of sub freezing ice for ice sheet regions under little tension lakes larger than 0 25 0 80 km in diameter contain sufficient water to rapidly drive hydro fractures through 1 1 5 km of subfreezing ice this represents 98 of the meltwater volume held in supraglacial lakes in the central western margin of the greenland ice sheet

numerical simulations of convection in europa s ice shell implications for surface features 1 europa s icy surface displays numerous small 5 to 30 km diameter pits spots and uplifts that have been suggested to result from convection in the ice shell to test this hypothesis we present numerical simulations of convection in europa s ice shell including temperature dependent viscosity and tidal heating ice shells 15 and 50 km thick are considered consistent with several estimates of the shell thickness on europa the convection produces deep pits consistent with some of the observed features when the lithospheric viscosity is 103 105 times greater than that of the underlying ice but greater viscosity contrasts lead to topography insufficient to explain the observed pits if ductile creep is the only deformation mechanism these results imply that convection cannot produce the observed pits and uplifts because in that case the predicted surface viscosity exceeds that of the warm underlying ice by at least a factor of 1010 however the strength of europa s surface may be low enough for plastic deformation to play a role in the convection opening the possibility that convection could produce some of europa s pits for plausible viscosities 1013 pa s at the melting temperature the pits are 100 300 m deep and 10 20 km in diameter greater or lesser viscosities lead to wider or narrower pits respectively none of our simulations produced isolated uplifts of any diameter however so these probably formed by another mechanism the convection can induce surface stresses 1 bar which exceeds the inferred strength of europa s crust and indicates the likelihood of surface disruption the maximum tidal heat fluxes that can be transported by convection at realistic ice viscosities are 0 05 0 07 w m 2

strike slip motion and double ridge formation on europa layer thickness of 2 km and shear velocity of 6 10 7 ms 1 such a velocity is appropriate for diurnal 85 hour tidal motion the local increase in temperature may cause 100 m uplift around the shear zone through thermal buoyancy the stresses required to produce velocities of order 10 7 ms 1 are similar to estimates for present day tidal stresses on europa 10 4 10 5 pa brittle layer thicknesses 2 km are unlikely to persist at active shear zones because of the effect of shear heating shear velocities greater than or equal to 10 6 ms 1 will give rise to melting at shallow depths the removal of material by downwards percolation of meltwater may cause surface collapse along the shear zone inward motion leading to compression may also result the combination of thermally or compression induced uplift and melt related collapse may be responsible for the pervasive double ridges seen on europa s surface index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8120 tectonophysics dynamics of lithosphere and mantle general 8160 tectonophysics rheology general keywords ice shear zone melting viscosity brittle ductile transition

europa tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting europa we show that tidal energy can be preferentially focused in rising plumes for viscosities in agreement with laboratory experiments when the plume cores reach the base of the outer cold brittle layer they spread laterally causing shallow melting disruption and formation of terrain similar to lenticulae and chaos we show that this mechanism can readily explain the major characteristics of chaos and lenticulae even if the average outer solid ice layer thickness is larger than 20 km

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

water generation and transport below europa s strike slip faults jupiter s moon europa has a very young surface with the abundance of unique terrains that indicate recent endogenic activity morphological models as well as spectral observations suggest that it might possess shallow lenses of liquid water within its outer ice shell here we investigate the generation and possible accumulation of liquid water below the tidally activated strike slip faults using a numerical model of two phase ice water mixture in two dimensional cartesian geometry our results suggest that generation of shallow partially molten regions underneath europa s active strike slip faults is possible but their lifetime is constrained by the formation of rayleigh taylor instabilities due to the negative buoyancy of the melt once formed typically within a few million years these instabilities efficiently transport the meltwater through the shell consequently the maximum water content in the partially molten regions never exceeds 10 which challenges their possible detection by future exploration mission

the stratigraphy correlation provenance and palaeogeography of the skiddaw group ordovician in the english lake district a new lithostratigraphy is presented for the skiddaw group lower ordovician of the english lake district two stratigraphical belts are described five formations are defined in the northern fells belt ranging in age from tremadoc to early llanvirn they are all mudstone or sandstone dominated of turbidite origin in ascending order they are named the bitter beck watch hill hope beck loweswater and kirk stile formations two formations are defined in the central fells belt ranging in age from late arenig to llanvirn these are the buttermere formation a major olistostrome deposit overlain by the tarn moor formation consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds a revised graptolite and new acritarch biostratigraphy for the skiddaw group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub assemblages the provenance of the group is assessed from detailed petrographical and geochemical work this suggests derivation in the early ordovician largely from an old inactive continental arc terrane lying to the south east with the appearance of juvenile volcanic material in the llanvirn comparisons and correlations of the skiddaw group are made with the isle of man and eastern ireland

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

ordovician volcano tectonics in the english lake district although the major late caledonian acadian deformation of the british slate belts south of the iapetus suture took place in early devonian time two intra ordovician unconformities are present in the lower palaeozoic succession of nw england which have been interpreted as due to regional compressional events these unconformities are at the base and top of the caradocian borrowdale volcanic group bvg and have been associated respectively with a pre bvg orogenic cycle affecting the early ordovician skiddaw group sg and with pre bala folding of the bvg a problem has been that no consistent sets of minor structures attributable to these deformations have been demonstrated in the skiddaw group recent advances in understanding of the volcanic history of the lake district lead to a radical reinterpretation of the significance of these two unconformities in the pre bala intra caradocian deformation of the bvg flexuring is now seen to be much less important than volcano tectonic faulting and block tilting associated with caldera collapse and the eruption of voluminous ash flows in the upper part of the pile foundering of the volcanic edifice accounts for the preservation of this thick subaerial sequence beneath the overstepping late ordovician marine deposits more tentatively we suggest that the unconformity beneath the borrowdale volcanic group reflects regional uplift due to buoyancy effects associated with the generation of andesitic melt by iapetus subduction and its rise through the over riding wedge of continental lithosphere several such mechanisms have been proposed to account for the topographic elevation of present day continental margin magmatic arcs the new model removes the evidence for ordovician n s regional compression from the geological record of nw england and this has geotectonic implications the evidence for late ordovician collision is weakened and the direction of iapetus subduction beneath southern britain may not have been southward

europa morphological characteristics of ridges and triple bands from galileo data e4 and e6 and assessment of a linear diapirism model galileo solid state imaging ssi images of europa provide high resolution data on the morphological characteristics of ridges and permit the development of nomenclatural schemes for their description and classification key observations are that ridges 1 are remarkably consistent in their along strike linearity width and height 2 form long linear features in which preexisting structures can sometimes be traced up the outer slopes of the ridges and in other cases appear to be buried 3 contain narrow apical zones of small scale ridge parallel faulting 4 are sometimes flanked by narrow troughs and ridge parallel fractures and 5 often display associated color variations on the basis of the characteristics and associated features of ridges we find that a process in which initial fracturing most plausibly related to tidal deformation of a brittle layer overlying a buoyant ductile substrate leading to linear diapiric upwelling provides a consistent explanation for the observed features in this process the upwelling linear diapir causes flexure bending and faulting of the region marginal to the fracture the deformation and uplift of adjacent plains material and its preexisting structures to form the apical part of the ridge the exposure of the inner walls of the crack and the mass wasting of the inner and outer walls of the ridge to modify but often not completely destroy the preexisting structure of the adjacent plains specifically this mechanism can account for many characteristics of the ridges including their linearity their consistent and regular morphology over their great lateral extent their positive topography the presence of preexisting structure on the outer ridges caused by upbowing of background ridged plains the formation of marginal troughs as diapiric rim synclines the detailed nature of their outer and inner slopes caused largely by faulting and mass wasting processes and their sequential formation with multiple orientations related to tidal deformation processes linear diapirism also provides a possible explanation for color and albedo characteristics related to thermal effects of the upwelling warm ice e g inducing volatile migration and grain size variations as the vast majority of deformation is vertical in this scenario this mechanism minimizes the necessity for complementary compressional deformation required by some other models

the geochemistry metasomatism and petrogenesis of the granites of the english lake district new geochemical data for the carrock threlkeld ennerdale shap skiddaw and eskdale granites of the lake district are presented and discussed with particular reference to the metasomatism and petrogenesis of the intrusions the caledonian granites of the lake district have more associated hydrothermal activity and mineralization than their equivalents north of the iapetus suture in scotland pervasive high temperature metasomatism which affects the shap skiddaw and eskdale intrusions is accompanied by remobilization of the large ionic lithophile elements k rb sr u and li and b although high field strength elements including the rare earth elements remain unaffected except near greisen and mineral veins the threlkeld intrusion appears to have suffered loss of a ba and sr during a low temperature event however there is no consistent relationship between granite composition hydrothermal activity and mineralization which could be used to support the granite cupola model for mineralization in the province rather the episodic mineralization which affects the lake district may be related more to the broad geothermal field consequent upon the emplacement of the deeper lake district batholith identified from geophysical data despite the variable metasomatic perturbations of their primary chemistry three distinct groups of intrusions can be recognized on geochemical grounds the carrock granophyre is a product of tholeiitic fractionation and the threlkeld and ennerdale bodies are more typical calc alkaline intrusions whereas the eskdale shap and skiddaw plutons show a progressive evolution towards more complex geochemical patterns suggesting decreasing hornblende but increasing plagioclase control of fractionating processes the culmination of this trend may have been the emplacement of a large high heat production granite beneath the lake district at the end of the caledonian orogeny the suggestion made on geophysical grounds that the eskdale intrusion is a cupola of the lake district batholith cannot be simply reconciled with geochemical modelling none of the lake district granites has geochemical or isotopic characteristics which are convincingly s type i e formed by partial melting of a sedimentary protolith and a model of subcrustal magmagenesis beneath an evolving arc or continental margin appears to be more appropriate

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

tidal dissipation in creeping ice and the thermal evolution of europa abstract the thermal and mechanical evolution of europa and comparable icy satellites the physics behind creating and sustaining a subsurface water ocean depends almost entirely on the mechanical dissipation of tidal energy in ice to produce heat the mechanism s of which remain poorly understood in deformation experiments we combine steady state creep and low frequency small strain periodic loading similar conditions in which tectonics and tidal flexing are occurring simultaneously the data reveal that the relevant power law attenuation in ice i is non linear depending on strain amplitude ii is independent of grain size and iii exceeds in absorption the prediction of the maxwell solid model by an order of magnitude the maxwell solid model is widely used to model the dynamics of planetary ice shells so this discrepancy is important the prevalent understanding of damping in the geophysical context is that it is controlled by chemical diffusion on grain boundaries which renders attenuation strongly dependent on grain size in sharp contrast our results indicate instead the importance of intracrystalline dislocations and their spatial interactions as the critical structural variable affecting dissipation these dislocation structures are controlled by stress and realized by accumulated plastic strain thus tectonics and attenuation are coupled which beyond the icy satellite subsurface ocean problem has implications also for understanding the attenuation of seismic waves in deforming regions of the earth s upper mantle

the origin of domes on europa the role of thermally induced compositional diapirism 1 the surface of jupiter s moon europa is peppered by topographic domes interpreted as sites of intrusion and extrusion diapirism is consistent with dome morphology but thermal buoyancy alone cannot produce sufficient driving pressures to create the observed dome elevations instead diapirs may initiate by thermal convection that induces compositional segregation exclusion of impurities from warm upwellings allows sufficient buoyancy for icy plumes to create the observed surface topography provided the ice shell has a small effective elastic thickness 0 2 to 0 5 km and contains low eutectic point impurities at the few percent level this model suggests that the ice shell may be depleted in impurities over time

radar attenuation in europa s ice shell obstacles and opportunities for constraining the shell thickness and its thermal structure young surface and possible recent endogenic activity make europa one of the most exciting solar system bodies and a primary target for spacecraft exploration future europa missions are expected to carry ice penetrating radar instruments designed to investigate its subsurface thermophysical structure several authors have addressed the radar sounders performance at icy moons often ignoring the complex structure of a realistic ice shell here we explore the variation in two way radar attenuation for a variety of potential thermal structures of europa s shell determined by reference viscosity activation energy tidal heating surface temperature and shell thickness as well as for low and high loss temperature dependent attenuation model we found that i for all investigated ice shell thicknesses 5 30 km the radar sounder will penetrate between 15 and 100 of the total thickness ii the maximum penetration depth varies laterally with deepest penetration possible through cold downwellings iii direct ocean detection might be possible for shells of up to 15 km thick if the signal travels through cold downwelling ice or the shell is conductive iv even if the ice ocean interface is not directly detected penetration through most of the shell could constrain the deep shell structure through returns from deep non ocean interfaces or the loss of signal itself and v for all plausible ice shells the two way attenuation to the eutectic point is 30 db which shows a robust potential for longitudinal investigation of the ice shell s shallow thermophysical structure

combined vertical incidence and wide angle seismic study of a gas hydrate zone blake ridge wide angle ocean bottom seismic data and single channel seismic reflection data were collected in june 1992 over an area where gas hydrates are thought to be extensive on blake ridge offshore south carolina wide angle reflections were observed on four woods hole oceanographic institution ocean bottom hydrophones at offsets up to 15 km results from traveltime inversion show that the bottom simulating reflector bsr that marks the base of the hydrate stability field lies 400 500 m below the seafloor and is overlain by a 200 to 300 m thick layer of average velocity 1 9 km s 1 there is no evidence for significant lateral velocity variation associated with lateral changes in bsr character ray tracing calculations show that the observation of the bsr out to large offsets 7 km constrains the maximum vertical velocity gradient to about 0 5 s 1 amplitude versus offset avo analysis was performed for ranges up to about 6 km and incidence angles of more than 70 the high amplitude of the bsr in the normal incidence data can be explained by two models both of which include a strong negative impedance contrast the hydrate wedge model and the free gas layer model synthetic seismogram calculation for the wedge model using the reflectivity method shows a strong increase in the bsr amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge this amplitude increase is clearly not observed in the data which shows only a relatively modest increase in amplitude with offset this observation agrees better with the calculated avo for the gas layer model the lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and or that the base of the gas layer is a gradational boundary

intrapermafrost gas hydrates from a deep core hole in the mackenzie delta northwest territories canada visible gas hydrate and possible pore space hydrate samples have been recovered for the first time in north america from within ice bonded permafrost in a 451 m deep core hole in the mackenzie delta northwest territories canada the visible hydrate was found as thin icelike layers that released methane gas initially upon retrieval but stabilized for up to 4 h at atmospheric pressure conditions and subfreezing temperatures the temporary stabilization of hydrate samples is attributed to the self preservation phenomenon described by others on the basis of laboratory studies high methane concentrations in well ice bonded sediments and gas releases suggest that pore space hydrate may be found at depths as shallow as 119 m geochemical and isotopic determinations suggest that the methane hydrate observed in the core hole is biogenic microbial in origin

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

mid late ordovician magmatism and metamorphism along the gander margin in central newfoundland fieldwork coupled with u pb dating has clarified the relationships between the exploits and gander derived rocks in central newfoundland during the mid ordovician opening of the exploits back arc basin reinforcing correlation across the peri gondwanan segments of the appalachian caledonian orogen we have separated a new back arc related unit the red cross group from the arc related victoria lake supergroup this group includes 466 3 ma felsic volcanic rock interlayered with alkali basalt enriched mid ocean ridge basalt 457 6 ma gabbro llanvirn caradoc normal mid ocean ridge basalt island arc tholeite limestone black shale and melange locally the red cross group is in parautochthonous contact with low grade gander psammite this contact was overrun by the meelpaeg metamorphic nappe during the devonian the nappe contains gander metapsammite meelpaeg subzone and the coticule bearing howley waters complex exploits which hosts 467 3 ma felsic porphyry the gander and exploits metasediments in the nappe are stitched by arenig llanvirn granite 467 6 ma and 468 3 5 3 ma and caradoc intrusions 458 3 ma and 454 2 ma metamorphic monazite 462 1 ma and titanite 460 3 ma from the nearby mount cormack subzone gander indicate the coeval formation of a low pressure high temperature metamorphic dome the widespread presence of caradoc magmatism suggests that the width of the back arc basin remained relatively narrow

a plate tectonic scenario for the iapetus and rheic oceans abstract the tectonics dynamics and biogeographic landscape of the early paleozoic were dominated by the opening and expansion of one large ocean the rheic and the diminution to terminal closure of another iapetus an understanding of the evolution of these oceans is thus central to an understanding of the early paleozoic but their chronicle also presents a rich temporal profile of the wilson cycle illustrating continental scale rifting microcontinent formation ocean basin development arc accretion and continent continent collision nevertheless contemporary paleogeographic models of the iapetus and rheic oceans remain mostly schematic or spatiotemporally disjointed which limits their utility and hinders their testing moreover many of the important kinematic and dynamic aspects of the evolution of these oceans are impossible to unambiguously resolve from a conceptual perspective and the existing models unsurprisingly present a host of contradictory scenarios with the specific aim to resolve some of the uncertainties in the evolution of this early paleozoic domain and a broader aim to instigate the application of quantitative kinematic models to the early paleozoic i present a new plate tectonic model for the iapetus and rheic oceans the model has realistic tectonic plates which include oceanic lithosphere that are defined by explicit and rigorously managed plate boundaries the nature and kinematics of which are derived from geological evidence and plate tectonic principles accompanying the presentation and discussion of the plate model an extensive review of the underlying geological and paleogeographic data is also presented

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

u pb chronology and duration of late ordovician magmatism in the english lake district u pb zircon ages are presented from igneous rocks in the english lake district for which a wide range of ages had been obtained previously by other methods the little stand and crinkle tuffs from the borrowdale volcanic group have ages of 451 6 1 4 and 452 8 0 7 ma 2 respectively the threlkeld microgranite 451 0 1 1 ma 2 and the carrock microgranite 452 4 4 1 ma 2 these dates are identical to the previously published dates for the ennerdale and eskdale granites and together are considered to be a good estimate for both the age and length of caradoc magmatism in the lake district this is the first of three magmatic events recognized in the lake district the other two occurring during ashgill and early devonian times regional comparisons with the leinster terrane in se ireland and the largely concealed english midlands basement emphasize the magnitude and significance of the caradoc event along the eastern avalonia margin

double ridges on europa accommodate some of the missing surface contraction crosscutting relationships of tectonic lineaments on europa record the history of surface deformation we mapped the displacement and orientation of older features crosscut by two types of lineaments bands and double ridges these measurements allow us to determine both the strike perpendicular and strike parallel displacement along investigated features double ridges record both ridge perpendicular contraction and expansion with a mean of 0 16 0 06 km needs space of contraction based on the analysis of 16 double ridges bands record expansion with a mean of 3 33 0 27 km for the six bands analyzed but with perpendicular displacement less than their apparent morphologic widths of 3 24 km the implied global surface strain for double ridges including those that expand and bands is 2 22 0 76 contraction and 7 60 3 7 expansion respectively double ridges thus may accommodate part of the surface expansion recorded by bands most current models for double ridges do not predict contraction the models that satisfy the observations for bands are slow spreading models cryovolcanism and folding

cryomagma ascent on europa abstract europa s surface exhibits morphological features which associated with a low crater density might be interpreted to have formed as a result of recent cryovolcanic activity in particular the morphology of smooth deposits covering parts of the surface and their relationship to the surrounding terrains suggest that they result from liquid extrusions furthermore recent literature suggests that the emplacement of liquid related features such as double ridges lenticulae and chaos could result from the presence of liquid reservoirs beneath the surface we model the ascent of liquid water through a fracture or a pipe like conduit from a subsurface reservoir to europa s surface and calculate the eruption time scale and the total volume extruded during the eruption as a function of the reservoir volume and depth we also estimate the freezing time of a subsurface reservoir necessary to trigger an eruption our model is derived for pure liquid water and for a briny mixture outlined by kargel 1991 81 wt h2o 16 wt mgso4 3 wt na2so4 considering compositional data for salt impurities for europa we discuss the effect of mgso4 and na2so4 on the cryomagma freezing time scale and ascent for plausible reservoir volumes and depths in the range of 106 m3 v 1010 m3 and 1 km h 10 km respectively the total extruded cryolava volume ranges from 103 m3 to 108 m3 and the duration of the eruptions varies from few minutes to few tens of hours the freezing time scale of the cryomagma reservoirs varies with cryomagma composition and the temperature gradient in the ice shell from a few days to a thousand years for pure water cryomagma and from a few months to a 104 years for briny cryomagma

a pre caldera plateau andesite field in the borrowdale volcanic group of the english lake district the 6 km thick ordovician borrowdale volcanic group is readily divisible into a lower 2 2 2 7 km thick predominantly pre caldera succession dominated by basalt andesite and dacite sheets and an upper succession of caldera related ignimbrites and volcaniclastic sedimentary rocks of c 3 km thickness the lower borrowdale volcanic group rocks here included within a single lithostratigraphical unit the birker fell formation affords a well exposed section through a pre caldera sequence the birker fell formation is dominated by andesites which comprise 60 of the stratigraphy thin sequences of reworked volcanic detritus are commonly interbedded with the andesites locally there are thicker units of volcaniclastic sedimentary rocks interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including 1 a 200 600 m thick sequence of weakly parallel bedded basaltic tuffs resting on the pre volcanic basement 2 a 200 600 m sequence of single flow unit aa basalts 3 dacite flows and ignimbrites up to 1100 m thick in the middle and upper parts of the formation and 4 aphyric to pyroxene phyric simple and compound flows of basalt 50 400 m thick locally present in the uppermost parts analysis of the facies comprising the birker fell formation indicates that it was emplaced as a sub aerial plateau andesite sequence formed by the coalescence of products erupted from a number of centres or fissures in an extensional subsiding volcano tectonic rift zone volcano tectonic faulting and eruption of thick ignimbrites locally influenced development of this field

true polar wander of enceladus from topographic data many objects in the solar system are suspected to have experienced reorientation of their spin axes as their rotation rates are slow and their shapes are nearly spherical the formation of mass anomalies by either endogenic or exogenic processes can change objects moments of inertia therefore the objects reorient to align their largest moment of inertia with their spin axis such a phenomenon is called true polar wander tpw here we report the discovery of a global series of topographic lows on saturn s satellite enceladus that we interpret to show that this synchronously locked moon has undergone tpw by 55 about the tidal axis we use improved topographic data from the spherical harmonic expansion of cassini limb and stereogrammetric measurements to characterize regional topography over the surface of enceladus we identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non equatorial circumglobal belt across enceladus surface we argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles respectively we argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell our hypothesis is consistent with a variety of geological features visible in cassini images

polythermal glacier hydrology a review 1 the manner by which meltwater drains through a glacier is critical to ice dynamics runoff characteristics and water quality however much of the contemporary knowledge relating to glacier hydrology has been based upon and conditioned by understanding gleaned from temperate valley glaciers globally a significant proportion of glaciers and ice sheets exhibit nontemperate thermal regimes the recent growing concern over the future response of polar glaciers and ice sheets to forecasts of a warming climate and lengthening summer melt season necessitates recognition of the hydrological processes in these nontemperate ice masses it is therefore timely to present an accessible review of the scientific progress in glacial hydrology where nontemperate conditions are dominant this review provides an appraisal of the glaciological literature from nontemperate glaciers examining supraglacial englacial and subglacial environments in sequence and their role in hydrological processes within glacierized catchments in particular the variability and complexity in glacier thermal regimes are discussed illustrating how a unified model of drainage architecture is likely to remain elusive due to structural controls on the presence of water cold ice near glacier surfaces may reduce meltwater flux into the glacier interior but observations suggest that the transient thermal layer of near surface ice holds a hydrological role as a depth limited aquifer englacial flowpaths may arise from the deep incision of supraglacial streams or the propagation of hydrofractures forms which are readily able to handle varied meltwater discharge or act as locations for water storage and result in spatially discrete delivery of water to the subglacial environment the influence of such drainage routes on seasonal meltwater release is explored with reference to summer season upwellings and winter icing formation moreover clear analogies emerge between nontemperate valley glacier and ice sheet hydrology the discussion of which indicates how persistent reassessment of our conceptualization of glacier drainage systems is required there is a clear emphasis that continued integrated endeavors focused on process glaciology at nontemperate glaciers are a scientific imperative to augmenting the existing body of research centered on ice mass hydrology

lithological control on gas hydrate saturation as revealed by signal classification of nmr logging data in this paper nuclear magnetic resonance nmr downhole logging data are analyzed with a new strategy to study gas hydrate bearing sediments in the mackenzie delta nw canada in nmr logging transverse relaxation time t2 distribution curves are usually used to determine single valued parameters such as apparent total porosity or hydrocarbon saturation our approach analyzes the entire t2 distribution curves as quasi continuous signals to characterize the rock formation we apply self organizing maps a neural network clustering technique to subdivide the data set of nmr curves into classes with a similar and distinctive signal shape the method includes 1 preparation of data vectors 2 unsupervised learning 3 cluster definition and 4 classification and depth mapping of all nmr signals each signal class thus represents a specific pore size distribution which can be interpreted in terms of distinct lithologies and reservoir types a key step in the interpretation strategy is to reconcile the nmr classes with other log data not considered in the clustering analysis such as gamma ray hydrate saturation and other logs our results defined six main lithologies within the target zone gas hydrate layers were recognized by their low signal amplitudes for all relaxation times most importantly two subtypes of hydrate bearing shaly sands were identified they show distinct nmr signals and differ in hydrate saturation and gamma ray values an inverse linear relationship between hydrate saturation and clay content was concluded finally we infer that the gas hydrate is not grain coating but rather pore filling with matrix support is the preferred growth habit model for the studied formation

u pb chronology of the ennerdale and eskdale intrusions supports sub volcanic relationships with the borrowdale volcanic group ordovician english lake district u pb analyses of zircons from the ennerdale granitic intrusion and eskdale granite english lake district give ages of 452 4 ma and 450 3 ma respectively these data confirm the sub volcanic association of these intrusions and show that previously published rb sr whole rock and k ar mineral ages do not correspond to the emplacement and cooling of these igneous bodies the new ages show that only two phases of acid magmatism can now be linked to the exposed parts of the lake district batholith the caradoc late ordovician phase was caused by the subductionrelated processes which gave rise to volcanism and was followed by a separate early devonian phase of granite emplacement

tectonics and water on europa the tectonics of europa one of jupiter s moons are complex this satellite probably hosts a subsurface water ocean but the thickness of the outer ice crust is poorly constrained and the episodic presence of liquid water at the surface is debated we argue that some surface features of europa are formed by soft ice that is heated by viscous dissipation of tidal motion along faults and do not depend on a shallow ocean our model suggests that transient pockets of liquid water or brine could form at shallow depths in the crust

tidally heated convection constraints on europa s ice shell thickness 1 the thickness of europa s ice shell is constrained with numerical experiments of thermal convection including heterogeneous tidal heating thermal convection occurs in the stagnant lid regime with most of the tidal heating located in the bottom half of the layer the addition of tidal heating mainly results in the increase of the temperature of the well mixed interior and in the decrease of the heat flux at the base of the ice layer in many simulations the ice in hot plumes is heated up to its melting point this induces episodic upwellings 0 5 ma of partially molten ice up to the base of the conductive lid with possible implications for the formation of lenticulae and chaos regions the thickness of the convective ice shell in equilibrium with the heat flow from the silicate core is estimated to be about 20 25 km tidal dissipation and surface temperature variations create lateral variations of the ice shell thickness of about 5 km with maxima near the equator at the jovian and anti jovian points and minima at midlatitudes surface heat flux is about 35 40 mw m 2 it is almost constant all over europa s surface even though the tidal dissipation rate is four times larger at the poles than at the equator

u pb age and genetic significance of heterogeneous zircon populations in rocks from the favourable lake area northwestern ontario the study evaluates the relationships between measured u pb ages and zircon characteristics of five morphologically texturally and isotopically complex zircon populations and compares the zircon development stages to the orogenic evolution of the favourable lake area two distinct zircon types from a hornblendite xenolith in a granitoid batholith of the sachigo subprovince of the superior province yield u pb ages of 2729 0 6 8 ma and 2714 8 6 4 7 4 ma which date specific metamorphic phases coinciding with major plutonic pulses in the batholith zircons from a metamorphosed felsic dike crosscutting the hornblendite consist of an old zircon component with a minimum age of 2788 ma possibly reflecting igneous crystallization 2950 ma ago and a younger component with an inferred age of 2725 15 ma probably reflecting metamorphism during batholith emplacement in the berens river subprovince to the south granodiorite forms both a late tectonic phase in a large batholith and a post tectonic pluton intruded into the batholith yet zircons from these granodiorites have identical ages of 2697 3 1 7 ma and 2696 2 1 2 ma respectively the late tectonic granodiorite also contains inherited zircons with a minimum age of 2767 ma which are indirect evidence for the presence of old sialic crust in this subprovince zircons from a sheared monzonite near the boundary fault between the two subprovinces yield an upper intercept age of 2769 26 63 ma which we interpret as the intrusion age of the monzonite this rock is older than most dated units in the surrounding batholiths and suggests that the boundary is a long lived archean structure a lower intercept age of about 1750 ma for zircons of the hornblendite is the result of chemical alteration of the zircons this and a similar lower intercept age shown by the sheared monzonite zircons are thought to reflect increased fluid activity and possibly shearing during the early proterozoic hudsonian orogeny which occurred in the churchill province to the northwest a later pb loss mainly from near surface domains of the zircons is indicated by lower intercept ages of about 500 100 ma

galileo magnetometer measurements a stronger case for a subsurface ocean at europa on 3 january 2000 the galileo spacecraft passed close to europa when it was located far south of jupiter s magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward jupiter this pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of europa s internal field the galileo magnetometer measured changes in the magnetic field predicted if a current carrying outer shell such as a planet scale liquid ocean is present beneath the icy surface the evidence that europa s field varies temporally strengthens the argument that a liquid ocean exists beneath the present day surface

thermo compositional convection in europa s icy shell with salinity 1 we present two dimensional numerical simulations of thermo compositional convection to test the hypothesis that the combined buoyancy from both thermal and salinity contrasts in europa s ice shell can produce the numerous uplifts and pits on europa s surface our simulations show that uplifts and pits with amplitude of 100 500 m and diameters of 10 30 km similar to some of the observed features can be produced in a 10 30 km thick ice shell with 2 10 compositional density variations if the viscosity contrast due to temperature variation does not exceed 106 the pit and uplift formation time and lifetime are approximately proportional to the surface viscosity ranging from 104 years to 107 years for viscosity contrasts of 104 106 convection cannot produce substantial surface topography if the viscosity contrast exceeds 107 108 these results imply that thermo compositional convection can only produce europa s pits and uplifts if europa s surface is weak

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses a global subsurface ocean of liquid water galileo gravity data verify that the satellite is differentiated into an outer h2o layer about 100 km thick but cannot determine the current physical state of this layer liquid or solid here we summarize the geological evidence regarding an extant subsurface ocean concentrating on galileo imaging data we describe and assess nine pertinent lines of geological evidence impact morphologies lenticulae cryovolcanic features pull apart bands chaos ridges surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates an average age of 50 myr searches for evidence of current geological activity on europa such as plumes or surface changes have yielded negative results to date the current existence of a global subsurface ocean while attractive in explaining the observations remains inconclusive future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within europa today

europa s differentiated internal structure inferences from four galileo encounters radio doppler data from four encounters of the galileo spacecraft with the jovian moon europa have been used to refine models of europa s interior europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice liquid outer shell but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice liquid shell the size of a metallic core is uncertain because of its unknown composition but it could be as large as about 50 percent of europa s radius the thickness of europa s outer shell of water ice liquid must lie in the range of about 80 to 170 kilometers

domes pits and small chaos on europa produced by water sills pits domes and small chaos on europa s surface are quasi circular features a few to a few tens of kilometers in diameter we examine if injection of water sills into europa s ice shell and their subsequent evolution can induce successive surface deformations similar to the morphologies of these features we study the dynamics of water spreading within the elastic part of the ice shell and show that the mechanical properties of ice exert a strong control on the lateral extent of the sill at shallow depths water makes room for itself by lifting the overlying ice layer and water weight promotes lateral spreading of the sill in contrast a deep sill bends the underlying elastic layer and its weight does not affect its spreading in that case the sill lateral extent is limited by the fracture toughness of ice and the sill can thicken substantially after emplacement cooling of the sill warms the surrounding ice and thins the overlying elastic ice layer as a result preexisting stresses in the elastic part of the ice shell increase locally to the point that they may disrupt the ice above the sill small chaos disruption of the surface also allows for partial isostatic compensation of water weight leading to a topographic depression at the surface pit of the order of 10 2 m complete water solidification finally causes expansion of the initial sill volume and results in an uplifted topography dome of 10 2 m

subduction related magmatism of late ordovician age in eastern england deep boreholes show that plutonic and volcanic igneous rocks comprise an important component of the caledonian basement in eastern england the isotopic compositions of these rocks reveal that many of them are of late ordovician age 440 460 ma and their geochemical compositions suggest calc alkaline affinities the intermediate diorite tonalite plutonic rocks are associated with a prominent northwest southeast trending belt of aeromagnetic anomalies extending from derby to st ives hunts which is interpreted to work the plutonic core of a calc alkaline magmatic arc it is inferred that this arc was generated by the subduction of oceanic lithosphere possibly from the tornquist sea in a south or southwest direction beneath the midlands microcraton in late ordovician times the age and geochemical composition of concealed ordovician volcanic rocks in eastern england and hypabyssal intrusions of the midlands minor intrusive suite in central england is compatible with such a hypothesis

the significance of isotopic dates from the english lake district for the ordovician saurian time scale the lower palaeozoic time scale is discussed in the light of a new date for the threlkeld microgranite and a reassessment of other published dates from the lake district which suggest ages of c 410 ma for the base of the devonian c 421 ma for the ashgill c 429 ma for the late caradoc c 439 ma for the early llandeilo and c 468 ma for the early llanvirn

a model for the temperature dependence of tidal dissipation in convective plumes on icy satellites implications for europa and enceladus abstract to explain the formation of surface features on europa enceladus and other satellites many authors have postulated the spatial localization of tidal heating within convective plumes however the concept that enhanced tidal heating can occur within a convective plume has not been rigorously tested most models of this phenomenon adopt a tidal heating with a temperature dependence derived for an incompressible homogeneous zero dimensional maxwell material but it is unclear whether this formulation is relevant to the heterogeneous situation of a warm plume surrounded by cold ice to determine whether concentrated dissipation can occur in convective plumes we develop a two dimensional model to compute the volumetric dissipation rate for an idealized vertically oriented isolated convective plume obeying a maxwellian viscoelastic compressible rheology we apply the model to the europa and enceladus ice shells and we investigate the consequences for partial melting and resurfacing processes on these bodies we find that the tidal heating is strongly temperature dependent in a convective ice plume and could produce elevated temperatures and local partial melting in the ice shells of europa and enceladus our calculation provides the first quantitative verification of the hypothesis by sotin et al sotin c head j w tobie g 2002 geophys res lett 29 74 1 and others that the tidal dissipation rate is a strong function of temperature inside a convective plume on europa such localized heating could help allow the formation of domes and chaos terrains by convection on enceladus localized tidal heating in a thermal plume could explain the concentrated activity at the south pole and its associated heat transport of 2 7 gw