a new magma chamber model based on structural studies in the oman ophiolite abstract structural mapping in the mafic ultramafic transition zone and in the overlying crustal section of the oman ophiolite has revealed the importance of magmatic flow in the formation of layering in the gabbros and the mechanical coupling between this flow and the asthenospheric flow of the underlying peridotites the crustal section near the spreading axis and in particular the magma chamber are fed by two independent melts a dominant gabbroic melt episodically injected from depth and a subordinate wehrlitic crystal mixture produced by compaction of the transition zone layering in the gabbros can be partly produced by tectonic transposition of the gabbroic and wehrlitic mixtures during magmatic flow whatever its origin the layering orientation in the newly created crust does not necessarily reflect its original attitude the final orientation of the layering is determined by the attitude of the magmatic flow plane at the time it was frozen a new magma chamber model is proposed which solves some of the physical difficulties of previous models and accounts for the seismic imaging of magma chambers beneath oceanic ridges

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

effect of state of compensation on the relaxation of crustal plateaus on venus 1 building on nunes et al 2004 which examined the end member case of viscous relaxation of fully compensated crustal plateaus on venus we now investigate viscous relaxation for the other end member case of uncompensated topography the approach consists of modeling viscous flow in a crust mantle system by means of analytic and finite element methods differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle loss of topography occurs via fast subsidence due to isostatic adjustment and a slower lateral flow due to spreading of the thickened crust rims arise from the isostatic adjustment and rim dimensions depend on the thickness of the competent layer controlled by the thermal state of the system rim dimensions after 1 myr are 1 km high and 300 km wide for a surface temperature of 740 k and thermal gradient of 10 k km and 600 m high and 100 km wide for hotter crustal conditions although models predict the marginal circumferential hoop folding observed at crustal plateaus they do not account for the large radial grabens also observed adding a subcrustal zone of time dependent buoyancy causes a delay in the isostatic adjustment during this period of delay viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on venus consequently phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support lateral variation in heat flow and thickness of the competent layer may help address the latter and needs to be explored through both lithospheric and mantle convection models

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

delayed formation of the equatorial ridge on iapetus from a subsatellite created in a giant impact 1 the great equatorial ridge on saturn s moon iapetus is arguably the most perplexing landform in the solar system the ridge is a mountain range up to 20 km tall and sitting on the equator of iapetus and explaining its creation is an unresolved challenge models of its formation must satisfy three critical observations why the ridge 1 sits exactly on the equator 2 is found only on the equator and 3 is thus far found only on iapetus we argue that all previously proposed models fail to satisfy these observations and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around iapetus the orbit of this subsatellite would then decay once iapetus itself had despun due to tides raised by saturn until tidal forces from iapetus tore the subsatellite apart the resultant debris formed a transient ring around iapetus the material of which rained down on the surface to build the ridge by sequestering the material in a subsatellite with a tidally evolving orbit formation of the ridge is delayed which increases the likelihood of preservation against the high impact flux early in the solar system s history and allows the ridge to form on thick stiff lithosphere heat flow likely 1 mw m 2 required to support this massive load without apparent flexure this mechanism thus explains the three critical observations

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

parameters of subsurface brines and hydrothermal processes 12 15 months after the 1999 magmatic event at the main endeavor field as inferred from in situ time series measurements of chloride and temperature properties of subsurface brines at mef

crater relaxation as a probe of europa s interior viscous relaxation rates of craters are examined to discriminate among models of europa s interior it is shown that the presence of an insulating surface regolith is required for ice lithospheres that do not contain a liquid h2o layer without the presence of such an insulating regolith relaxation times for craters 100 km in diameter are excessively long 1 b y and thus inconsistent with the absence of such features in voyager images finite element calculations indicate that a thick insulating surface frost layer raising surface temperatures 40 k above the solar radiation ambient temperature of 92 k allows a 25 km thick surface viscous layer to relax 100 km diameter craters on the short timescales 100 m y required if the icy lithosphere is 100 km thick and without a liquid h2o layer but containing a subsolidus convecting region surface temperatures need to be raised 20 k by insulation to allow relaxation of craters in this size range to occur within the required time

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

petrology and geochemistry of the lower ocean crust formed at the east pacific rise and exposed at hess deep a synthesis and new results 1 the geochemistry and petrology of the lower oceanic crust record information about the compositions of melts extracted from the mantle how these melts mix and crystallize and the role of hydrothermal circulation in this portion of the crust unfortunately lower oceanic crust formed at fast spreading ridges is rarely exposed at the seafloor making it difficult to study these processes at hess deep crust formed at the east pacific rise epr is exposed due to the propagation of the cocos nazca spreading center westward here we review our state of knowledge of the petrology of lower crustal material from hess deep and document new mineral major and trace element compositions amphibole plagioclase thermometry and plagioclase crystal size distributions samples from the deeper parts of the gabbroic sequence contain clinopyroxene that is close to being in trace element equilibrium with erupted basalts but which can contain primitive moderate cr high mg orthopyroxene and very calcic plagioclase because primitive mid ocean ridge basalts morbs are not saturated with orthopyroxene or very calcic plagioclase this suggests that melts added to the crust have variable compositions and that some may be in major but not trace element equilibrium with shallow depleted mantle these apparently conflicting data are most readily explained if some of the melt extracted from the mantle is fully aggregated within the mantle but reacts with the shallow mantle during melt extraction the occurrence of cumulates with these characteristics suggests that melts added to the crust do not all get mixed with normal morb in the axial magma chamber amc but rather that some melts partially crystallize in isolation within the lower crust however evidence that primitive melts fed the amc along with steep fabrics in shallow gabbros from near the base of the dyke complex provides support for models in which crystallization within the amc followed by crystal subsidence is also an important process in lower crustal accretion more evolved bulk compositions of gabbros from the upper than lower parts of the plutonic section are due to greater amounts of reaction with interstitial melt and not because their parental melt had become highly fractionated through the formation of large volumes of cumulates deeper in the crust amphibole plagioclase thermometry confirms previous reports that the initial ingress of fluid occurs at high temperatures in the shallow gabbros tave 713 c and show that the temperature of amphibole formation was similar in deeper gabbros tave 722 c this thermometry also suggests that fracture and grain boundary permeability for seawater derived fluids was open over the same temperature interval

magma chambers at oceanic ridges how large seismic models of magma chambers at fast spreading oceanic ridges predict small perched magma chambers above a wide low velocity zone seismic attenuation in a low velocity zone allows for 10 melt in hot gabbros in order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the oman ophiolite we propose that the low velocity zone belongs to the magma chamber in such a dynamic magma chamber gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension this process results in a drastic reduction of the melt fraction that is compatible with magmatic flow as confirmed by preliminary experiments we propose here that dynamic magma chambers below ridges may contain as little as 10 melt

thermally induced lineations on the asteroid eros evidence of orbit transfer 1 several systems of tectonic lineations have been observed on the surface of the asteroid 433 eros we suggest that many of these lineations may have resulted from thermal stresses associated with a secular change in surface temperature due to orbit migration we demonstrate how thermal diffusion can lead to tectonic features and how such features should be oriented with respect to eros complex geometry comparison to observed lineations suggests that some may be explained by thermal stresses such stresses represent a largely unexplored tectonic process on asteroids and could be used to gain rare insight into chaotic orbital dynamics if correct our results imply a gross orbital history over the last 100 myr involving slow several myr inward migration followed by more rapid outward migration to its present orbit

viscous flow rates of icy topography on the north polar layered deposits of mars we investigate the importance of viscous flow in shaping topography at the north polar layered deposits npld of mars by using finite element modeling to calculate the distribution of stresses and flow velocities present day impact craters on thenpldare too small and cold for viscous relaxation tohavebeen an important mechanism in controlling their current dimensions this effect may be ignored when analyzing crater size frequency distributions scarps at the npldmargins where avalanches of dust and carbon dioxide frost occur are sufficiently steep high andwarm to experience significant viscous flow we find flow velocities at the base of these steep scarps on the order of tens to hundreds of cm yr which are fast enough to significantly affect their slopeover kiloyear timescales alternatively the scarps could be close to steady state in which observed block falls provide a competing effect to viscous flow

ganymede and callisto complex crater formation and planetary crusts new depth diameter d d rim and peak height and width measurements of fresh craters on ganymede and callisto are utilized to explore not only crater mechanics on icy satellites but intersatellite crater scaling and crustal properties and composition significant systematic differences in crater morphology on icy and rocky planets are confirmed simple to complex transition diameters on icy satellites including ganymede and callisto are much lower than those on rocky planets also complex crater depths and rim heights on ganymede and callisto are inherently 60 70 shallower than lunar complex craters despite similar surface gravity hence viscous relaxation is not as important as is generally assumed central peaks on most icy satellites are 1 2 km higher and considerably wider with respect to diameter than for similar sized lunar craters terracing and rim slumping is also rare in these craters central peaks in craters on ganymede and callisto larger than 15 km however are relatively small in both width and height however correlating with the occurrence of rim slumping thus central peak dimensions are an indirect indicator of the degree of rim slumping transition diameters for the occurrence of central peaks and rim slumps and for d d curve inflections for icy satellites scale approximately with the inverse of surface gravity except possibly at very low g the transition diameters for central peaks and for rim slumping differ however by 5 15 km on ganymede and callisto but by 50 or more km on the smaller icy satellites these large differences suggest that d d curves on icy satellites are comprised of three segments of decreasing slope one for smaller simple craters a second for central peak craters and a third for larger true complex craters with both central peaks and rim slumping the shallow slopes of d d curves on ganymede and callisto resemble that of the moon consistent with the occurrence of both central peaks and rim failure the high slopes of d d curves on the middle sized icy satellites indicate that they are middle segments consistent with the dominance of central peaks in those craters complex crater depths on icy satellites in general scale with the inverse of surface gravity the depths of all the observed large basin scale craters on these small satellites when scaled to ganymede gravity are very similar to those of complex craters on ganymede and callisto these large craters including odysseus 425 km diameter on tethys are probably true complex craters that have not undergone significant viscous relaxation ithaca chasma on tethys probably formed during the prompt collapse of the transient odysseus crater crater morphology is clearly controlled by both gravity and by large variations in material properties i e composition and should be useful in constraining bulk crustal composition i e ice rock ratios the similarity of various morphological transition diameters and complex crater depths on ganymede and callisto two geophysically very similar but geologically divergent large icy satellites indicates that the crusts of both bodies are dominated by water ice with only a minor rocky component

impact cratering on titan ii global melt escaping ejecta and aqueous alteration of surface organics abstract new three dimensional hydrodynamic simulations of hypervelocity impacts into the crust of titan were undertaken to determine the fraction of liquid water generated on the surface of saturn s largest moon over its history and hence the potential for surface modification of hydrocarbons and nitriles by exposure to liquid water we model in detail an individual impact event in terms of ejecta produced and melt generated and use this to estimate melt production over titan s history taking into account the total flux of the impactors and its decay over time our estimates show that a global melt layer at any time after the very beginning of titan s history is improbable but transient melting local to newly formed craters has occurred over large parts of the surface local maxima of the melt are connected with the largest impact events we also calculate the amount of volatiles delivered at the impact with various impact velocities from 3 km s for possible hyperion fragments to 11 km s for jupiter family comets and their retention as a possible source of titan s atmosphere we find the probability of impact ejecta escaping titan with its modern dense and thick atmosphere is rather low and dispersal of titan organics throughout the rest of the solar system requires impactors tens of kilometers in diameter water ice melting and exposure of organics to liquid water has been widespread because of impacts but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation the largest impactors may breach an ammonia water mantle layer creating a circular albedo contrast rather than a crater

evolution of heat flow hydrothermal circulation and permeability on the young southern flank of the costa rica rift we analyze 67 new conductive heat flow measurements on the southern flank of the costa rica rift crr heat flow measurements cover five sites ranging in oceanic crustal age between approximately 1 6 and 5 7 ma and are co located with a high resolution multi channel seismic line that extends from slightly north of the first heat flow site 1 6 ma to beyond odp hole 504b in 6 9 ma crust for the five heat flow sites the mean observed conductive heat flow is 85 mwm 2 this value is approximately 30 per cent of the mean lithospheric heat flux expected from a half space conductive cooling model indicating that hydrothermal processes account for about 70 per cent of the heat loss the advective heat loss fraction varies from site to site and is explained by a combination of outcrop to outcrop circulation through exposed basement outcrops and discharge through faults super critical convection in layer 2a extrusives occurs between 1 6 and 3 5 ma and flow through a thinly sedimented basement high occurs at 4 6 ma advective heat loss diminishes rapidly between 4 5 and 5 7 ma which contrasts with plate cooling reference models that predict a significant deficit in conductive heat flow up to ages 65 10 ma at 5 7 ma the crr topography is buried under sediment with an average thickness 150 m and hydrothermal circulation in the basement becomes sub critical or perhaps marginally critical the absence of significant advective heat loss at 5 7 ma at the crr is thus a function of both burial of basement exposure under the sediment load and a reduction in basement permeability that possibly occurs as result of mineral precipitation and original permeability at the time of formation permeability is a non monotonic function of age along the southern flank of the crr in general agreement with seismic velocity tomography interpretations that reflect variations in the degree of ridge axis magma supply and tectonic extension hydrothermal circulation in the young oceanic crust at southern flank of crr is affected by the interplay and complex interconnectedness of variations in permeability sediment thickness topographical structure and tectonic and magmatic activities with age

high temperature alteration of oceanic gabbros by seawater hess deep ocean drilling program leg 147 evidence from oxygen isotopes and elemental fluxes two major stages of high temperature water rock interactions have been identified in gabbros of oceanic layer 3 exposed in the hess deep rift valley at the east pacific rise 2 n 101 w the 154 m of plutonic rocks cored at site 894g show mineralogies and textures that suggest that they represent the roof of an approximate 1 myr magma chamber a late stage of hydrothermal alteration produced mineral assemblages typical of lower amphibolite upper greenschist facies conditions the hydration of the gabbros occurred locally through the development of well crystallized green amphiboles significant chemical fluxes were associated with the fluid flow and precipitation dissolution mechanisms oxygen isotope data suggest an earlier cryptic stage of hydrothermal alteration of the hess deep gabbros at temperatures above 500 c hydration was limited with only minute amphibole lamellae in pyroxenes that trapped cl na and minor k from seawater ca plagioclases remained stable and preserved their magmatic cationic compositions and zoning patterns we propose that this high temperature event lowered the 18o of the gabbros by isotopic exchange with a hydrothermal fluid through solid state oxygen diffusion although it is unclear whether the 18o depletion of hole 894g gabbros is representative of the entire lower oceanic crust it is nevertheless thought to contribute to the oxygen isotope buffering of the oceans during this stage water rock ratios calculated using sr isotopes are underestimated 0 1 0 5 because of insignificant diffusion of this cation in plagioclases better estimates of water rock ratios are provided by oxygen isotopes 0 2 1 which are important in unraveling the cooling history of the oceanic crust

a mid ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux the depth to the axial magma chamber at three oceanic spreading centers as indicated by seismic reflection surveys is greater than that predicted by conductive cooling thermal models of ridge crests the additional cooling at the ridge axis is owed to the circulation of seawater through the shallow crust a theoretical model for the temperature distribution at spreading centers which includes distributed heat sources and sinks is presented by representing the hydrothermal heat loss as a series of heat sinks the depths to the axial magma chamber at the east pacific rise at latitude 9 n the southern juan de fuca ridge and the lau basin spreading center are modeled heat sinks are added to the input model to increase the solidus depth until the computed magma chamber depth matches the magma chamber depth determined by seismic reflection surveys the thermal modeling indicates that the axial hydrothermal heat flux is between 7 1 106 and 13 8 106 mj m2 or 10 to 20 of the total missing heat at the ridge axis requiring extensive low temperature circulation off axis high temperature vents that are spaced 1 km apart and are active between 4 and 10 of the time would account for the axial hydrothermal heat loss

impact crater relaxation on dione and tethys and relation to past heat flow abstract relating relaxation of impact crater topography to past heat flow through the crusts of icy satellites is a technique that has been applied to satellites around jupiter and saturn we use global digital elevation models of the surfaces of dione and tethys generated from cassini data to obtain crater depth diameter d d data relaxation is found to affect craters down to smaller diameters on these satellites compared to rhea we perform relaxation simulations in order to assess the heat flow necessary to relax craters on dione and tethys to their present morphologies heat flows exceeding 60 mw m 2 are required to relax several craters on both satellites and relaxation appears to be subject to geographical controls on dione we define a relaxation dichotomy that separates the more relaxed craters in sparsely cratered plains from the less relaxed craters in heavily cratered terrain the configuration of this dichotomy resembles that of the structural geological dichotomy on enceladus implying that a similar resonance induced tidal heating mechanism concentrated in the southern hemisphere may have affected both satellites defining geographical distribution of relaxation on tethys is hindered by the presence of the young odysseus impact and its associated ejecta

thickness constraints on the icy shells of the galilean satellites from a comparison of crater shapes a thin outer ice shell on jupiter s large moon europa would imply easy exchange between the surface and any organic or biotic material in its putative subsurface ocean1 2 3 4 the thickness of the outer ice shell is poorly constrained however with model dependent estimates ranging from a few kilometres5 6 to ten or more kilometres7 here i present measurements of depths of impact craters on europa ganymede and callisto that reveal two anomalous transitions in crater shape with diameter the first transition is probably related to temperature dependent ductility of the crust at shallow depths 7 8 km on europa the second transition is attributed to the influence of subsurface oceans on all three satellites3 8 9 which constrains europa s icy shell to be at least 19 km thick the icy lithospheres of ganymede and callisto are equally ice rich but europa s icy shell has a thermal structure about 0 25 0 5 times the thicknesses of ganymede s or callisto s shells depending on epoch the appearances of the craters on europa are inconsistent with thin ice shell models1 and indicate that exchange of oceanic and surface material could be difficult

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

formation of fast spreading lower oceanic crust as revealed by a new mg ree coupled geospeedometer abstract a new geospeedometer is developed based on the differential closures of mg and rare earth element ree bulk diffusion between coexisting plagioclase and clinopyroxene by coupling the two elements with distinct bulk closure temperatures this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples as the existing mg exchange thermometer was calibrated for a narrow temperature range and strongly relies on model dependent silica activities a new thermometer is developed using literature experimental data when the bulk closure temperatures of mg and ree are determined respectively using this new mg exchange thermometer and the existing ree exchange thermometer this speedometer can be implemented for a wide range of compositions mineral modes and grain sizes applications of this new geospeedometer to oceanic gabbros from the fast spreading east pacific rise at hess deep reveal that the lower oceanic crust crystallized at temperatures of 998 1353 c with cooling rates of 0 003 10 2 c yr stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies together with existing petrological geochemical and geophysical evidence results from this new speedometry suggest that the lower crust formation at fast spreading mid ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal scale hydrothermal circulations the replenished melts become chemically and thermally evolved accumulate as small magma bodies at various depths feed the shallow axial magma chamber and may also escape from the mush zone to generate off axial magma lenses

a thermomechanical numerical model for crustal accretion of medium to fast spreading mid ocean ridges we propose a new thermomechanical numerical model of mid ocean ridge accretion aimed at investigating asymmetric spreading rates diverse configurations of lens and sill magma injections crystallization and depth and on and off axis patterns of hydrothermal cooling the numerical algorithm iteratively resolves temperature and motion equations until it reaches a stationary solution the motion equation was written in a vorticity stream function formalism with boundary and internal conditions applied to the stream function to impose the style of magma injection unlike in previous models our model does not assume an a priori shape for the temperature field which is initiated by an initial half space cooling according to the left and right spreading rates complex patterns of hydrothermal cooling are simulated by enhanced thermal diffusivity the model succeeds in describing the dynamic and thermal effects of spreading rates the style of magma intrusion and the hydrothermal cooling accurate descriptions of these are essential to study the cooling histories of crustal rocks and geophysical observables

salinity variations in submarine hydrothermal systems by layered double diffusive convection various mechanisms have been proposed to explain the salinity variations in vent fluids of seafloor geothermal systems new experiments reacting diabase and evolved seawater were carried out to reproduce earlier published observations of cl depletions attributed to formation of an ephemeral cl bearing mineral the absence of any cl depletions in the present study suggests that the formation of cl bearing minerals is not sufficiently widespread to account for the observed salinity variations in the vent fluids a re evaluation of both field and laboratory evidence has led to a new model for subseafloor circulation that accounts for salinity variations as well as other chemical and mineralogic observations in place of a simple single pass convection system we propose that the seafloor systems consist of two vertically nested convection cells in which a brine layer at depth heats and drives an overlying seawater cell such layering of salinities a process known in fluid mechanics as double diffusive conve

fluid rock interactions in the lower oceanic crust thermodynamic models of hydrothermal alteration rocks from the lower oceanic crust show both petrologic and isotopic evidence for extensive alteration by circulating hydrothermal fluids derived from seawater deeply circulating fluids may have a substantial but as yet largely undetermined impact on the thermal regime of spreading centers magmatic processes at mid ocean ridges and elemental fluxes at ridge hydrothermal systems in the present study the consequences of water rock interactions during the penetration of seawater into hot lower oceanic crust were investigated using thermodynamic reaction path models alteration assemblages predicted for reaction of an evolving seawater with olivine gabbro gabbronorite and troctolite during heating from 300 to 900 c closely resemble alteration assemblages observed in rock samples from the lower oceanic crust these assemblages include sodic plagioclase actinolite diopsidic clinopyroxene epidote chlorite prehnite analcime quartz at lower temperatures 300 500 c and plagioclase hornblende diopside chlorite trace magnetite at intermediate temperatures 500 700 c the relative abundances of these alteration products show much greater variation with temperature than with water rock w r ratio indicating that variations in the amounts of hydrated minerals such as amphiboles may not be an accurate indication of the w r ratio experienced by rocks in the lower crust since the same amount of fluid may produce widely differing amounts of amphibole at slightly different temperatures depending on the temperature of interaction some rocks in the lower oceanic crust may have experienced much higher w r ratios than has previously been proposed based on the amount of amphibole present at higher temperatures 750 900 c the models predict that very little alteration occurs before the fluid equilibrates with the rock producing only trace amounts of amphibole as an alteration phase with the remainder of the rock retaining its igneous mineral composition essentially unchanged this result suggests that extensive penetration of rocks by aqueous fluids may occur in the lower ocean crust at temperatures 700 c and leave very little conspicuous petrologic evidence for their presence such cryptic alteration is consistent with isotopic evidence for hydrothermal alteration of oxygen and sr isotopes by seawater in lower ocean crust rock samples which otherwise retain their igneous compositions and textures taken together the model results allow for much more extensive circulation of fluids in the lower oceanic crust and at much higher temperatures than has been proposed by previous studies based on petrologic interpretations

time dependent flexure of the lithospheres on the icy satellites of jupiter and saturn abstract previous analyses into flexural deformation on the icy satellites of jupiter and saturn have assumed static elastic lithospheres viscous creep within the lithosphere however can cause evolution over time here we apply a finite element model that employs a time dependent elastic viscous plastic rheology in order to investigate flexure on icy satellites factors that affect this time dependent response are those that control creep rates surface temperature heat flow and grain size our results show that surface temperature is by far the dominant factor at higher surface temperatures 100 130 k the evolution of the deformation is such that the thickness of a modeled elastic lithosphere could vary by up to an order of magnitude depending on the time scale over which the deformation occurred because the flexure observed on icy satellites generally indicates transient high heat flow events our results indicate that the duration of the heat pulse is an important factor for the icy worlds of jupiter and saturn static models of lithospheric flexure should be used with caution

parameters of subsurface brines and hydrothermal processes 12 15 months after the 1999 magmatic event at the main endeavor field as inferred from in situ time series measurements of chloride and temperature 1 phase separation at mid ocean ridge hydrothermal systems is a widely distributed frequently occurring process it is highly sensitive to dynamic subsurface pressure and temperature conditions and has a significant impact on chloride concentrations of hydrothermal fluid although vapor phases produced by phase separation are observed in a variety of hydrothermal settings observation of conjugate brines that must be produced concurrently are more rare and the fate of subsurface brines is still not well understood here we use an array of in situ chloride sensors deployed at the main endeavor field on the juan de fuca ridge in the wake of a magmatic event to monitor the behavior of hydrothermal vent effluent with high temporal resolution our results provide evidence of near critical chloride depleted fluids within the crust up to 486 m beneath the seafloor we estimate that a brine reservoir stored beneath the main endeavor field is constrained to temperatures between 131 c and 326 c for chloride concentrations ranging from 6 to 25 wt nacl respectively our data suggest that subsurface fluids and circulation pathways vary widely over spatial scales on the order of meters under these circumstances simultaneous measurements of multiple fluid parameters at a high temporal resolution are essential to understanding subsurface hydrothermal processes associated with magmatic events

along axis dynamic topography constrained by major element chemistry abstract variations in thickness and density of both the crust and the associated uper mantle have been derived from a compilation of zero age major element composition along the mid atlantic ridge the east pacific rise and the southeast indian ridge assuming isostatic compensation the axial depth computed from major element data correctly agrees with observed axial depth discrepancies are essentially located near hotspots such as iceland and azores the residual topography expressed as the difference between observed and compensated axial depth has a root mean square of 426 m along the three spreading axes which is below the resolution power of the method this insignificant topography which is assumed to contain the dynamic surface topography associated with mantle convection bears an important constraint on the relative variations of the dynamic topography predicted by models of mantle circulation

constraints on the accretion of the gabbroic lower oceanic crust from plagioclase lattice preferred orientation in the samail ophiolite abstract oceanic crust represents more than 60 of the earth s surface and despite a large body of knowledge regarding the formation and chemistry of the extrusive upper oceanic crust there still remains significant debate over how the intrusive gabbroic lower oceanic crust is accreted at the ridge axis the two proposed end member models the gabbro glacier and the sheeted sills predict radically different strain accumulation in the lower crust during accretion in order to determine which of these two hypotheses is most applicable to a well studied lower crustal section we present data on plagioclase lattice preferred orientations lpo in the wadi khafifah section of the samail ophiolite we observe no systematic change in the strength of the plagioclase lpo with height above the crust mantle transition no dominant orientation of the plagioclase a axis lineation and no systematic change in the obliquity of the plagioclase lpo with respect to the modal layering and macroscopic foliation evident in outcrop these observations are most consistent with the sheeted sills hypothesis in which gabbros are crystallized in situ and fabrics are dominated by compaction and localized extension rather than by systematically increasing shear strain with increasing depth in a gabbro glacier our data support the hypothesis of macleod and yaouancq 2000 that the rotation of the outcrop scale layering from sub horizontal in the layered gabbros to sub vertical near the sheeted dikes is due to rapid vertical melt migration through upper gabbros close to the axial magma chamber additionally our results support the hypothesis that the majority of extensional strain in fast spreading ridges is accommodated in partially molten regions at the ridge axis whereas in slow and ultra slow ridges large shear strains are accommodated by plastic deformation

paragenesis of magma chamber internal wall discovered in oman ophiolite gabbros a detailed study in wadi farah reveals a singular point in jebel dihm southern oman ophiolite here we investigated some primitive features of accretion at a fast spreading ridge that has exceptionally been exempt from hydrous alteration while crossing the tbl thermal boundary layer the conjunction of two factors has made this possible in the farah area its location at the point of sharp foliation rotation from the upper to the transitional gabbros and a locally twisted tbl these factors may have favoured the intrusion within the tbl of hydrous wedges transverse to the ridge deeper into the tbl near the active ridge the wedges merge and are covered by veneers of totally fresh gabbros bearing glittering acicular clinopyroxene these gabbros are interpreted as relics of the crystallizing internal wall of the magma chamber that have interacted with hydrous fluids this internal tbl has anisotropic thermal properties and acts as a thermal blanket maintaining high magma chamber temperatures

numerical models of magma chambers in the oman ophiolite long standing studies conducted in the oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the east pacific rise epr this is true in particular for the shape size and internal magmatic flow of magma chambers thus justifying numerical modeling our modeling founded on the equations of fluid mechanics is based on a two dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber boundary conditions deduced from the geophysical data of the epr and geological observations from the oman ophiolite the model predicts a magma chamber shape consistent with field studies whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills it has only a weak influence on the shape of the chamber the viscosity of the magma plays a crucial role for the setting of convection in the chamber according to geological observations which argue against the existence of convective flow patterns the viscosity of the gabbroic mush within the chamber must exceed 1014 1015 pa s depending on the oceanic plate velocity the flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive foliations oriented inward or active foliations oriented outward the active structural analysis in the oman model is supported by ophiolite data

formation of the bulge of iapetus through long wavelength folding of the lithosphere abstract previous models that attempted to explain the formation of the pronounced oblate shape of iapetus suggested that it was a preserved rotational bulge these models found that heating was provided by short lived radioactive isotopes that decayed rapidly and allowed the excess flattening of the lithosphere to be locked in by a thickening lithosphere but placed severe timing constraints on the formation of iapetus and its bulge here we show that finite element simulations with an elastic viscous plastic rheology indicate it is possible to form the bulge through long wavelength folding of the lithosphere of iapetus during an epoch of contraction combined with a latitudinal surface temperature gradient in contrast to models of a frozen rotational bulge heat generated by long lived radioactive isotopes warms the interior which causes porosity loss and forces iapetus to compact by 10 our simulations are most successful when there is a 30 k temperature difference between the pole and the equator tectonic growth of the bulge is not sensitive to the time scale over which the moon contracts and lithospheric thickness primarily controls whether a fold can form not fold wavelength in addition long term simulations show that when no stress is applied the mechanical lithosphere is strong enough to support the bulge with negligible relaxation over billion year time scales

coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges the genesis of oceanic crust at intermediate to fast spreading ridges occurs by the crystallization of mantle melts accumulated in at least one shallow melt lens situated below the ridge axis seismic reflection data suggest that the depth of this melt lens is inversely correlated with spreading rate and thereby magma supply the heat released in it by crystallization and melt injection is removed by a combination of hydrothermal cooling and diffusion due to the different time scales of hydrothermal cooling and crustal accretion numerical models have so far focused on only one of the two processes here we present the results from a coupled model that solves simultaneously for crustal accretion and hydrothermal cooling our approach resolves both processes within one 2d finite element model that self consistently solves for crustal mantle and hydrothermal flow the formation of new oceanic crust is approximated as a gabbro glacier in which the entire lower crust crystallizes in one shallow melt lens we find that the depth of the melt lens and the shape of hot potentially molten lower crust are highly dependent on the ridge permeability structure the predicted depth of the melt lens is primarily controlled by the permeability at the ridge axis whereas the off axis permeability determines the width of hot lower crust a detailed comparison of the modeling results with observed locations of the melt lens at intermediate to fast spreading ridges shows that only a relatively narrow range of crustal permeabilities is consistent with observations in addition we find significant deviations between models that resolve or parameterize hydrothermal cooling the predicted crustal thermal structures show major differences for models that predict the same melt lens location this illustrates the importance of resolving hydrothermal flow in simulations of crustal accretion

crater relaxation on ganymede implications for ice rheology controversy has existed over whether or not viscous relaxation is an important process on the icy satellites previous models involved large extrapolations of newtonian flow laws for ice whereas ice is known to exhibit non newtonian behavior recently the flow law parameters for ice at the appropriate temperatures and stresses have been measured numerical modelling of the viscous relaxation of basins on ganymede using these parameters has given implausibly short relaxation times however this model treated ice as a purely viscous substance so that no elastic lithosphere could develop near the surface here we treat ice as a maxwell visco elastic material and numerically model the relaxation of basins on ganymede we find that realistic young s moduli lead to little relaxation occurring in basins even 4 0 ga after their formation further we demonstrate that within broad limits the near surface temperature gradient has little effect on this result finally through examination of the distribution of maxwell times in the vicinity of the crater we show that most viscous relaxation occurs early in the basin s history when stresses are high and thus maxwell time is short as stresses are relieved the maxwell time becomes long and relaxation essentially ceases

impact basin relaxation on rhea and iapetus and relation to past heat flow abstract evidence for relaxation of impact crater topography has been observed on many icy satellites including those of saturn and the magnitude of relaxation can be related to past heat flow e g moore j m schenk p m bruesch l s asphaug e mckinnon w b 2004 icarus 171 421 443 dombard a j mckinnon w b 2006 j geophys res 111 e01001 http dx doi org 10 1029 2005je002445 we use new global digital elevation models of the surfaces of rhea and iapetus generated from cassini data to obtain crater depth diameter data for both satellites and topographic profiles of large basins on each in addition to the factor of three lower amplitude of global topography on rhea compared to iapetus we show that basins on iapetus 100 km in diameter show little relaxation compared to similar sized basins on rhea because of the similar gravities of rhea and iapetus we show that iapetus basin morphologies can be used to represent the initial unrelaxed morphologies of the rhea basins and we use topographic profiles taken across selected basins to model heat flow on both satellites we find that iapetus has only experienced radiogenic heat flow since formation whereas rhea must have experienced heat flow reaching a few tens of mw m 2 although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes rhea experienced a different thermal history from iapetus which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system a recent model for the formation of saturn s mid sized icy satellites interior to and including rhea charnoz s et al 2011 icarus 216 535 550 describes how rhea s orbit would have expanded outwards after its accretion from a giant primordial ring which would have instigated early heating through rapid despinning and tidal interaction with saturn and other satellites rhea s basins would therefore be required to have formed within the first few tens of myr of rhea s formation in order to relax due of this heating and if so may provide an important anchor point for saturn system chronology none of these heating mechanisms are viable for iapetus in its isolated position far from saturn and as such it has remained dynamically inert since formation confirming conclusions based on thermal modeling of iapetus interior rapid and complete relaxation and subsequent erosion by bombardment of a first generation of large basins on rhea is regarded as an explanation for the lower counts of large basins on rhea relative to iapetus and the overall lower amplitude of topography on rhea compared to iapetus

800 yr long records of annual air temperature and precipitation over southern siberia inferred from teletskoye lake sediments a unique 800 yr long record of annual temperatures and precipitation over the south of western siberia has been reconstructed from the bottom sediments of teletskoye lake altai mountains using an x ray fluorescence scanner xrf providing 0 1 mm resolution timeseries of elemental composition and x ray density xrd br content appears to be broadly correlative with mean annual temperature variations because of changes in catchment vegetation productivity sr rb ratio reflects the proportion of the unweathered terrestrial fraction xrd appears to reflect water yield regime and sediment flux sedimentation is rather continuous because annual clastic supply and deposited mass are the same the artificial neural networks method was applied to convert annual sedimentary time series of xrd br content and sr rb ratio to annual records of temperature and precipitation using a transfer function comparison of these reconstructed siberian records with the annual record of air temperature for the northern hemisphere shows similar trends in climatic variability over the past 800 yr estimated harmonic oscillations of temperature and precipitation values for both historical and reconstructed periods reveal subdecadal cyclicity

rapid high temperature metamorphism of east pacific rise gabbros from hess deep abstract metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid ocean ridges but the temperature range rate and location of subsolidus events are poorly known we combine hornblende plagioclase thermometry statistical analysis and thermal models to estimate precisely the temperature time distance from axis and duration of metamorphism in east pacific rise gabbros from hess deep odp hole 894g metamorphic hornblende and plagioclase which formed during microfracturing and sea water penetration equilibrated at a mean temperature of 716 8 c 90 confidence level comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned 60 c when combined with thermal models of fast spreading centers this implies that metamorphism was rapid 6000 yr and occurred 1 4 km off axis application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting

controls of bathymetric relief on hydrothermal fluid flow at mid ocean ridges we present quantitative modeling results for the effects of surface relief on hydrothermal convection at ocean spreading centers investigating how vent site locations and subsurface flow patterns are affected by bathymetry induced sub seafloor pressure variations the model is based on a 2 d fem solver for fluid flow in porous media and is used to simulate hydrothermal convection systematically in 375 synthetic studies the results of these studies show that bathymetric relief has a profound effect on hydrothermal flow bathymetric highs induce subsurface pressure variations that can deviate upwelling zones and favor venting at structural highs the deviation angle from vertical upwelling can be expressed by a single linear dependence relating deviation angle to bathymetric slope and depth of the heat source these findings are confirmed in two case studies for the east pacific rise at 9 30 n and lucky strike hydrothermal fields in both cases it is possible to predict the observed vent field locations only if bathymetry is taken into account our results thereby show that bathymetric relief should be considered in simulations of submarine hydrothermal systems and plays a key role especially in focusing venting of across axis hydrothermal flow onto the ridge axis of fast spreading ridges

testing the viability of topographic relaxation as a mechanism for the formation of lunar floor fractured craters historically two modes of origin for lunar floor fractured craters have been proposed subcrater laccolith emplacement and topographic relaxation to test the viability of the topographic relaxation model we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase an analog to lunar crustal materials trials were run under two situations 1 under extreme conditions e g high heat flow to determine the maximum degree of relaxation that the model can accommodate and 2 under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions we find that in the extreme case topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor fractured craters 100 km in diameter results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor fractured crater population these results indicate that topographic relaxation is not the primary mechanism for the formation of floor fractured craters because the topographic relaxation model does not apply for the vast majority of floor fractured craters i e those with diameters less than 60 km we conclude that the laccolith intrusion model is a more viable alternative

comparison of central pit craters on mars mercury ganymede and the saturnian satellites we report on the first results of a large scale comparison study of central pit craters throughout the solar system focused on mars mercury ganymede rhea dione and tethys we have identified 10 more central pit craters on rhea dione and tethys than have previously been reported we see a general trend that the median ratio of the pit to crater diameter dp dc decreases with increasing gravity and decreasing volatile content of the crust floor pits are more common on volatile rich bodies while summit pits become more common as crustal volatile content decreases uplifted bedrock from below the crater floor occurs in the central peak upon which summit pits are found and in rims around floor pits which may or may not break the surface peaks on which summit pits are found on mars and mercury share similar characteristics to those of nonpitted central peaks indicating that some normal central peaks undergo an additional process to create summit pits martian floor pits do not appear to be the result of a central peak collapse as the median ratio of the peak to crater diameter dpk dc is about twice as high for central peaks summit pits than dp dc values for floor pits median dpk dc is twice as high for mars as for mercury reflecting differing crustal strength between the two bodies results indicate that a complicated interplay of crustal volatiles target strength surface gravity and impactor energy along with both uplift and collapse are involved in central pit formation multiple formation models may be required to explain the range of central pits seen throughout the solar system

the evolution of impact basins viscous relaxation of topographic relief we evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon we adopt a representative topographic profile of the orientale basin the youngest large impact basin on the moon as an estimate of the initial topography of older basins of similar horizontal dimensions and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography at wavelengths greater than the thickness of a high viscosity lithosphere both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins the effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths while the rate of relaxation decreases substantially for the fraction of long wavelength topography which is isostatically compensated as an initial condition these models are applied to two pre nectarian basins on the moon the tranquillitatis basin on the lunar nearside and the much larger and older south pole aitken basin on the farside the topographic profile of tranquillitatis after correcting for the effect of mare basalt fill is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit the large topographic relief of the larger farside basin on the contrary is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as tranquillitatis probably occurred a difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference such a nearside farside asymmetry in near surface temperature may date from the time of crust mantle differentiation and may have persisted because of the different histories of late stage impact basin formation and mare volcanism on the two hemispheres

volatile lines of descent in submarine plutonic environments insights from stable isotope and fluid inclusion analyses microthermometric and isotopic analyses of fluid inclusions in primitive olivine gabbros oxide gabbros and evolved granitic material recovered from ocean drilling program hole 735b at the southwest indian ridge provide new insights into the evolution of c o h nacl fluids in the plutonic foundation of the oceanic crust the variably altered and deformed plutonic rocks span a crustal section of over 1500 m and record a remarkably complex magma hydrothermal history magmatic fluids within this suite followed two chemically distinct paths during cooling through the subsolidus regime the first path included formation of co21ch41h2o1c fluids with up to 43 mole ch 4 the second path produced hypersaline brines that contain up to 50 nacl equivalent salinities subsequent to devolatilization respeciation of magmatic co2 attendant graphite precipitation and cooling from 800 c to 500 c promoted formation of ch 4 enriched fluids these fluids are characterized by average d 13 c ch4 values of 227 1 6 4 3 n5 45 with associated d 13 c co2 compositions ranging from 224 9 to 21 9 n539 and average dd values of exsolved vapor of 241 6 12 n523 in pods veins and lenses of highly fractionated residual material hypersaline brines formed during condensation and by direct exsolution in the absence of a conjugate vapor phase entrapped co21ch41h2o rich fluids within many oxide bearing rocks and felsic zones are significantly depleted in 13 c with d 13 c co2 values down to about 225 and contain co 2 concentrations higher than those predicted by equilibrium devolatilization models we hypothesize that lower effective pressures in high temperature shear zones promoted infiltration of highly fractionated melts and compositionally evolved volatiles into focused zones of deformation significantly weakening the rock strength in felsic rich zones volatile build up may have driven hydraulic fracturing of gabbroic wall rocks resulting in the formation of magmatic breccias comparison of isotopic compositions of fluids in plutonic rocks from 735b the mark area of the mid atlantic ridge and the mid cayman rise indicate 1 that the carbon isotope composition of the lower oceanic crust may be far more heterogeneous than previously believed and 2 that carbon bearing species in the oceanic crust and their distribution at depth are highly variable copyright 2001 elsevier science ltd

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

power law rheology of ice and the relaxation style and retention of craters on ganymede numerical models of viscous relaxation of craters in ice using rheological laws determined from laboratory data previously indicated excessively short relaxation times te the time required for crater depth to become 1 e of its initial value for craters on the icy satellites a typical value of te was 104 years for a 100 km diameter crater these time scales are totally incompatible with the large regions of heavily cratered terrain on the icy satellites we discuss the application of a numerical unite element model incorporating the most recent rheological data for ice at temperatures and pressures appropriate to the near surface regions of ganymede the model permits a full representation of a temperature and stress dependent power law viscosity for ice for temperature gradients in reasonable agreement with those obtained from thermal and structural models of ganymede we obtain relaxation times te 10 years for craters with diameters less than 100 km for all craters with diameters greater than 10 km the strong dependence of viscosity on stress was found to significantly shorten the relaxation time an attempt was also made to determine whether newtonian or non newtonian rheological laws dominated relaxation for craters of various sizes newtonian rheologies appear to dominate crater relaxation only for situations where the effective temperature is several tens of degrees warmer than the surface is observed to be perhaps because of the effect of an insulating regolith and where temperature gradients are small in this case viscous relaxation of craters with diameters smaller than 25 km is expected to be dominated by newtonian flow laws provided that the currently accepted parameters gross extrapolations from near melting temperature measurements are valid for the low temperatures of the icy satellites on the other hand larger diameter craters undergo at least initially viscous relaxation determined by a strongly non newtonian rheological law and have geologically short relaxation times te 107 years for a 300 km crater although these times may be extended by silicate suspensions and by dispersion hardening this presumably accounts for the absence of such craters in unmodified form on ganymede finally the large stress exponent n 4 7 of the current rheological data for ice indicates a strong role for non newtonian flow in determining the topography of viscously relaxing craterforms of this size in principle a threshold crater diameter and hence stress for the onset of non newtonian viscous behavior could be obtained from galileo images allowing estimates to be made of near surface temperatures and thermal gradients in ganymede s past in practice however any morphological variability is almost certainly too subtle to be detected from imagery the calculated relaxation times from this work are still much shorter than those indicated by the topographic relief of ganymede s surface this remains an unresolved problem possible ways in which the observations can be resolved with current laboratory data on ice deformation are discussed

relaxation of compensated topography and the evolution of crustal plateaus on venus 1 crustal plateaus dominant physiographic features on venus likely originate through dynamic mantle processes although a debate exists on whether they formed by mantle upwellings or downwellings regardless of the mode of formation several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high standing plateau to a low standing plateau with elevated margins we apply analytic and finite element models to test this hypothesis for isostatically compensated topography as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots geotherm values 5 k km 1 combined with a surface temperature of 740 k preclude relaxation within 109 years while geotherm values 20 k km 1 can yield relaxation times of 108 years or less hence significant relaxation requires hot conditions in order to occur within the appropriate 1 gyr timescale set by crustal plateau ages we also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance mantle temperatures 1400 k allow strong crust mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength thus preserving the original topographic shape during relaxation in either case relaxation of compensated plateaus does not yield elevated rims we propose that the state of compensation must be considered variable

extensional and compressional instabilities in icy satellite lithospheres a simple mechanical model of planetary lithospheres and asthenospheres is combined with the most recent experimental results concerning the brittle and ductile deformation of ice i h to determine the plausibility of forming the grooved terrains on ganymede enceladus and miranda by means of lithospheric instabilities the model assumes a lithosphere of uniform strength atop a substrate whose strength decreases exponentially with depth the thickness of the lithosphere is defined by the intersection of a near surface byerlee s law relationship for brittle failure with a standard exponential ductile flow law at depth for ganymede we find that the relatively high surface gravity warm temperatures render virtually unachievable an instability strong enough to account for the observed topographic relief however a sufficiently strong instability capable of producing features with the proper spacing could develop on a smaller colder body such as enceladus or miranda the requirements in this case would be a strain rate greater than about 10 14 sec 1 and a near surface thermal gradient of a few tens of degrees kelvin per kilometer the most reasonable way to accomplish these requirements is by the intrusive emplacement of a thick layer of warm ice beneath a preexisting lithosphere of about a kilometer or so in thickness accompanied almost contemporaneously by a strain event lasting on the order of 10 5 years for extension or 10 3 years for compression however observed properties of grooved terrains on enceladus and miranda such as nonuniform feature spacing and low crater counts force us to conclude that unstable deformation must be more complicated than this simple model would suggest if it is to explain these features

creep of water ices at planetary conditions a compilation many constitutive laws for the flow of ice have been published since the advent of the voyager explorations of the outer solar system conflicting data have occasionally come from different laboratories and refinement of experimental techniques has led to the publication of laws that supersede earlier ones in addition there are unpublished data from ongoing research that also amend the constitutive laws here we compile the most current laboratory derived flow laws for water ice phases i ii iii v and vi and ice i mixtures with hard particulates the rheology of interest is mainly that of steady state and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system advances in grain size dependent creep in ices i and ii as well as in phase transformations and metastability under differential stress are also included in this compilation at laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress temperature and pressure dependencies but with the exception of ice iii have fairly similar strengths hard particulates strengthen ice i significantly only at high particulate volume fractions ice iii has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability which may extend metastably well into what is nominally the ice ii field is located near likely geotherms of large icy moons

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

reconciling temperatures of metamorphism fluid fluxes and heat transport in the upper crust at intermediate to fast spreading mid ocean ridges 1 current models of the mineralogical and chemical modification of the crust within mid ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes in particular the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust this distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single pass convection cell simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high temperature metamorphism of the upper dikes to occur in the downflow zone in fact temperatures throughout most of the sheeted dike complex are too high three alternative models are considered fluid rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes reaction rates required to grow metamorphic minerals and the relatively constant composition of many hydrothermal vents metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism based on the sr isotopic composition of the sheeted dike complex and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas in contrast fluid rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data in this model downwelling seawater cools the recharge zone and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber metamorphism of the sheeted dike complex then largely occurs in the discharge zone

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

the seismic attenuation structure of a fast spreading mid ocean ridge the two dimensional p wave attenuation structure of the axial crust of the east pacific rise was obtained from an inversion of waveform spectra collected during an active source seismic tomography experiment the structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis the near axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation high attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers less than 1 kilometer beneath the axial magma lens to the base of the crust the levels of attenuation in this deeper region require at most only a small fraction of partial melt

folding of europa s icy lithosphere an analysis of viscous plastic buckling and subsequent topographic relaxation abstract potential contractional folds on jupiter s icy moon europa have been identified the best example is at the extensional band astypalaea linea where a series of subtle topographic undulations 25 km in wavelength possess parasitic tectonic features that support a folding origin a scenario has been qualitatively proposed whereby folds form via unstable contraction of the icy lithosphere compensate for extension elsewhere on europa and then subsequently relax here we quantitatively address this scenario applying a model for viscous plastic buckling of planetary lithospheres and finite element simulations of topographic relaxation our results suggest that the lithosphere of europa could indeed be unstable but the low required surface temperatures limit fold formation to higher latitudes and the high required driving stresses 9 10 mpa are difficult to achieve on the satellite the depth to the brittle ductile transition is well constrained and high thermal gradients are indicated implying heat flows near 100 mw m 2 in addition topographic relaxation progresses so slowly even at these heat flows that it is not a viable mechanism to eliminate such features over the age of europa s surface given the paucity of identified folds we conclude that the necessary conditions for their formation are rare and that lithospheric folding is a minor mechanism for compensating the large amounts of extension seen elsewhere on europa

a fossil melt lens in the oman ophiolite implications for magma chamber processes at fast spreading ridges from seismic evidence we know that a small body of magma is present at mid crustal levels beneath most fast spreading ridges and that this small sill or melt lens overlies a much broader area of hot but largely solid material a crystal mush that constitutes the lower ocean crust beyond this however we have little direct knowledge of the physical and chemical processes that operate in and below the melt lens nor the role the melt lens plays in the storage and aggregation of mid ocean ridge basalts morb we here offer constraints on the processes of melt transport modification and residence beneath fast spreading ridges from a coupled structural and geochemical study through the crustal section of the oman ophiolite concentrating on the uppermost plutonic rocks and transition into sheeted dykes we find that almost the entire oman plutonic suite the layered gabbro and most of the high level gabbro is dominated by cumulate rocks containing very low proportions of trapped interstitial melt the exception to this is the very uppermost part of the section within 150 metres of the base of the sheeted dyke complex at which level the gabbros instead display a marked heterogeneity of structure texture and composition and plutonic rocks of generally basaltic composition occur for the first time associated pegmatitic ferrogabbros formed by in situ fractionation of these liquids this horizon passes up into a more homogeneous microgabbroic facies also of basaltic composition within which isolated asymmetric doleritic chilled margins are found and thence into doleritic sheeted dykes we argue that the thin heterogeneous gabbro horizon is a fossilised melt lens containing pooled liquids of net basaltic composition mg of 65 expelled from the crystal mush beneath existing models have suggested that steep fabrics observed in homogeneous gabbros beneath this horizon formed by subsidence of originally horizontal cumulus layers formed at the base of the melt lens but this is not supported by the field relationships instead we propose that the fabrics record the buoyant ascent of magma through the mush pile probably in part by porous flow and reflect crystallisation essentially in situ without significant vertical transport of the mush low residual melt porosities in the sub melt lens region imply efficient extraction of interstitial liquid and at least episodically high permeabilities in the upper part of the lower crust

formation of mountains on io variable volcanism and thermal stresses abstract thermal stresses are potentially important drivers of io s tectonics and mountain building it has been hypothesized that sustained local or regional shut down of heat pipe volcanism on io could lead to deep crustal heating and large compressive stresses mckinnon w b schenk p m dombard a j 2001 geology 29 103 106 such large stresses would then be relieved by thrust faulting and uplifting of crustal blocks producing mountains like those observed on io here we analyze the tectonic consequences of the heat pipe model in detail considering both the initial thermal stress state of a basalt or peridotite crust created by heat pipe volcanism and relative roles of subsidence stresses due to burial of preexisting layers and thermal stresses arising from variable volcanism and changes in crustal lithosphere thickness we limit the magnitude of the potential subsidence stresses in our study because the magnitude of subsidence stresses can be quite large if not dominant results indicate that for a fixed crustal thickness the region of failure and faulting moves closer to the surface as eruption rate decreases and time increases when the crust melts at its base as volcanism decreases as might occur under steady state tidal heating resulting in crustal thinning the region of failure is brought even closer to the surface naturally when compressive subsidence stresses are included the vertical extent of crust in brittle failure thickens to include most of the lithosphere in contrast increases in eruption rate cause the extent of the region in compressional failure to decrease and be driven very deep in the crust in the absence of sufficient subsidence stress therefore regions of declining volcanism are more likely to produce mountains whereas regions of extensive or increasing volcanism are less likely to do so this is consistent with the observation of a global anticorrelation between mountains and volcanic centers on io finally we find that the choice of crustal composition rheology dry basalt vs dry peridotite has little effect on our results implying that basalt peridotite and komatiite are all similarly stiff in the io environment

can high temperature high heat flux hydrothermal vent fields be explained by thermal convection in the lower crust along fast spreading mid ocean ridges we present numerical models to explore possible couplings along the axis of fast spreading ridges between hydrothermal convection in the upper crust and magmatic flow in the lower crust in an end member category of models corresponding to effective viscosities m lower than 1013 pa s in a melt rich lower crustal along axis corridor and permeability k not exceeding 10 16 m2 in the upper crust the hot melt rich gabbroic lower crust convects as a viscous fluid with convection rolls parallel to the ridge axis in these models we show that the magmatic hydrothermal interface settles at realistic depths for fast ridges i e 1 2 km below seafloor convection cells in both horizons are strongly coupled and kilometer wide hydrothermal upflows plumes spaced by 8 10 km arise on top of the magmatic upflows such magmatic hydrothermal convective couplings may explain the distribution of vent fields along the east epr and south east pacific rise sepr the lower crustal plumes deliver melt locally at the top of the magmatic horizon possibly explaining the observed distribution of melt rich regions pockets in the axial melt lenses of epr and sepr crystallization of this melt provides the necessary latent heat to sustain permanent 100 mw vents fields our models also contribute to current discussions on how the lower crust forms at fast ridges they provide a possible mechanism for focused transport of melt rich crystal mushes from moho level to the axial melt lens where they further crystallize feed eruptions and are transported both along and off axis to produce the lower crust

thermal effects of gabbro accretion from a deeper second melt lens at the fast spreading east pacific rise motivated by recent observations a ridge model has been developed which includes both a shallow melt lens and a second deeper melt lens above the moho to assess the thermal effect of the gabbro accretion from the lower melt lens at the fast spreading east pacific rise modeling results argue for a limited role of the lower melt lens in constructing the layer 3 gabbros if significant 10 amount of the layer 3 gabbros is emplaced near the moho then the liberated latent heat during crystallization would generate a large molten section of the lower crust due to the inability of cold seawater to penetrate into this hot ductile region a large molten section of the lower crust is not consistent with recent seismic tomographic results of the east pacific rise thus it is likely that the amount of the gabbro being emplaced from the lower melt lens at the epr is significantly less than that suggested by oman ophiolite studies such a lower melt lens is short lived or a large portion of the sill intrusions observed at oman was emplaced at a few kilometers off axis 3 4 km

hydrocode simulation of ganymede and europa cratering trends how thick is europa s crust abstract one of the continuing debates of outer solar system research centers on the thickness of europa s ice crust as it affects both the habitability and accessibility of its sub surface ocean here we use hydrocode modeling of the impact process in layered ice and water targets and comparison to europan cratering trends and galileo derived topographic profiles to investigate the crustal thickness full or partial penetration of the ice crust by an impactor occurred in simulations in which the ice thickness was less than 14 times the projectile radius craters produced in these thin shell simulations were consistently smaller than for larger ice thicknesses which will complicate inference of large impactor population sizes simulations in which the resultant crater was 3 times the ice layer thickness resulted in summit pit morphology this work supports that summit pit craters noted on both rocky and icy bodies can be created by the presence of a weaker layer at depth we suggest that floor pits seen only on ice rich bodies require a different formation mechanism to summit pits pristine craters formed in a target with high heat flow were shallower than for the same impact into a target of lesser heat flow suggesting that the starting crater morphology for viscous relaxation isostatic readjustments and erosion rate studies is different for craters formed in times of different heat flow we find that the crater depth diameter trend of europa can only be recreated when simulating impact into an upper brittle ice layer of 7 km depth with a corresponding geothermal gradient of 0 025 k m as this ice thickness estimate is below 10 km results from this work suggest that convective overturn of the surface ice may occur or have occurred on europa making the development of indigenous life a possibility

variation of cooling rate with depth in lower crust formed at an oceanic spreading ridge plagioclase crystal size distributions in gabbros from the oman ophiolite 1 analysis of crystal size distributions csd of plagioclase in gabbros from the oman ophiolite indicates that cooling rates in the lower crust beneath a medium to fast spreading oceanic ridge did not vary smoothly with depth cooling rates in the upper half of the gabbro section were approximately 1 5 2 times faster than in the lower half of the gabbro section during igneous crystallization and or high temperature grain growth processes our results are inconsistent with thermal models for oceanic spreading ridges that approximate the effect of hydrothermal convection by using an enhanced thermal conductivity together with a conductive heat transfer mechanism instead the oman csd data are consistent with recent seismic and modeling studies that suggest that hydrothermal convection leads to closely spaced near vertical isotherms in the upper half of the plutonic section within a few kilometers of the ridge axis

a numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid ocean ridge 1 there are two competing models for lower crustal accretion at fast spreading ridges in the gabbro glacier model all the lower crust solidifies in a magma lens at the dike gabbro boundary and then flows to its final level in the sheeted sill model the lower crust is formed by sills emplaced at a variety of depths we present a steady state numerical representation of the sill model that explicitly includes hydrothermal circulation the crust is accreted uniformly at the ridge axis at all depths and hydrothermal circulation occurs in crustal rocks that have cooled below a threshold temperature the results show that when the permeability exceeds a threshold value 4 10 14 m2 for a cracking temperature of 800 c and a uniform permeability hydrothermal circulation can cool the entire crust near the ridge axis producing vertical isotherms in the lower crust at lower permeabilities the properties of seawater lead to bimodal cooling with the lowermost crust cooling more slowly than shallower depths these results are reasonably consistent with studies of the seismic structure of the east pacific rise and of plagioclase crystal size distributions in gabbros from oman we infer that the sheeted sill model cannot be discarded on thermal grounds

central pit craters on ganymede central pit craters are common on mars ganymede and callisto and thus are generally believed to require target volatiles in their formation the purpose of this study is to identify the environmental conditions under which central pit craters form on ganymede we have conducted a study of 471 central pit craters with diameters between 5 and 150 km on ganymede and compared the results to 1604 central pit craters on mars diameter range 5 160 km both floor and summit pits occur on mars whereas floor pits dominate on ganymede central peak craters are found in similar locations and diameter ranges as central pit craters on mars and overlap in location and at diameters 60 km on ganymede central pit craters show no regional variations on either ganymede or mars and are not concentrated on specific geologic units central pit craters show a range of preservation states indicating that conditions favoring central pit formation have existed since crater retaining surfaces have existed on ganymede and mars central pit craters on ganymede are generally about three times larger than those on mars probably due to gravity scaling although target characteristics and resolution also may play a role central pits tend to be larger relative to their parent crater on ganymede than on mars probably because of ganymede s purer ice crust a transition to different characteristics occurs in ganymede s icy crust at depths of 4 7 km based on the larger pit to crater diameter relationship for craters in the 70 130 km diameter range and lack of central peaks in craters larger than 60 km diameter we use our results to constrain the proposed formation models for central pits on these two bodies our results are most consistent with the melt drainage model for central pit formation

impact basin relaxation at iapetus abstract we investigate impact basin relaxation on iapetus by combining a 3d thermal evolution model robuchon g choblet g tobie g cadek o sotin c grasset o 2010 icarus 207 959 971 with a spherical axisymmetric viscoelastic relaxation code zhong s paulson a wahr j 2003 geophys j int 155 679 695 due to the progressive cooling of iapetus younger basins relax less than older basins for an ice reference viscosity of 10 14 pa s an 800 km diameter basin relaxes by 30 if it formed in the first 50 myr but by 10 if it formed at 1 2 gyr bigger basins relax more rapidly than smaller ones because the inferred thickness of the ice shell exceeds the diameter of all but the largest basins considered stereo topography shows that all basins 600 km in diameter or smaller are relaxed by 25 or less our model can match the relaxation of all the basins considered within error by assuming a single basin formation age 4 36 ga for our nominal viscosity this result is consistent with crater counts which show no detectable age variation between the basins examined

cooling rates in the lower crust of the oman ophiolite ca in olivine revisited debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models the thermal profile of the crust specifically the roles of hydrothermal circulation and cooling rate is a key component in distinguishing between these two models results of this study show no systematic variation of cooling rate with depth in the lower crust of the khafifah section in the wadi tayin massif of the oman ophiolite additionally very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust while the conclusions presented here do not rule out the possibility of accretion by gabbro glacier they do contradict the initial tenet of the gabbro glacier model convective heat loss by hydrothermal circulation can penetrate to moho depths and crystallization is not by necessity constrained to the upper crust on the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust

modeling the morphological diversity of impact craters on icy satellites abstract impact craters on icy satellites display a wide range of morphologies some of which have no counterpart on rocky bodies numerical simulation studies have struggled to reproduce the diversity of features such as central pits and transitions in crater depth with increasing diameter observed on the icy galilean satellites the transitions in crater depth at diameters of about 26 and 150 km on ganymede and callisto have been interpreted as reflecting subsurface structure using the cth shock physics code we model the formation of craters with diameters between 400 m and about 200 km on ganymede using different subsurface temperature profiles our calculations include recent improvements in the model equation of state for h 2 o and quasi static strength parameters for ice we find that the shock induced formation of dense high pressure polymorphs ices vi and vii creates a gap in the crater excavation flow which we call discontinuous excavation for craters larger than about 20 km discontinuous excavation concentrates a hot plug of material 270 k and mostly on the melting curve in the center of the crater floor the size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters and we propose that a genetic link exists between them we also derive depth versus diameter curves for different internal temperature profiles in a 120 k isothermal crust calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior hence the depth to diameter transition at about 26 km reflects thermal weakening of ice finally simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean

viscous relaxation of impact craters on icy planetary surfaces determination of viscosity variation with depth spacecraft images show that the icy galilean satellites have surfaces with very low topographic relief impact craters on ganymede and callisto are anomalously shallow and are characterized by sharp well defined rims and domed floors these morphological characteristics can be explained by viscous relaxation of topography on an icy crust in which the viscosity is uniform or decreases with depth under these conditions large craters relax more rapidly than small craters therefore explaining a possible underabundance of large craters viscous relaxation on an icy crust that is thin compared to the crater diameter or on a thick icy crust in which viscosity increases with depth could not produce this crater morphology and would result in the more rapid relaxation of small craters rather than large craters the results of this study suggest that more detailed analysis of relaxing impact crater morphology may resolve the rate of viscosity decrease with depth and so provide evidence on the interior thermal evolution of icy planetary bodies

on the resurfacing of ganymede by liquid water volcanism abstract a long popular model for producing ganymede s bright terrain involves flooding of low lying graben with liquid water slush or warm soft ice the model suffers from major problems however including the absence of obvious near surface heat sources the negative buoyancy of liquid water and the lack of a mechanism for confining the flows to graben floors we present new models for cryovolcanic resurfacing to overcome these difficulties tidal heating within an ancient laplace like orbital resonance showman and malhotra 1997 icarus 127 93 showman et al 1997 icarus 129 367 provides a plausible heat source and could allow partial melting to occur as shallow as 5 10 km depth our favored mechanism for delivering this water to the surface invokes the fact that topography such as a global set of graben causes subsurface pressure gradients that can pump water or slush upward onto the floors of topographic lows graben despite the negative buoyancy of the liquid these eruptions can occur only within the topographic lows furthermore as the low areas become full the pressure gradients disappear and the resurfacing ceases this provides an explanation for the observed straight dark bright terrain boundaries water cannot overflow the graben so resurfacing rarely embays craters or other rough topography pure liquid water can be pumped to the surface from only 5 10 km depth but macroscopic bodies of slush ascending within fractures can reach the surface from much greater depths due to the smaller negative buoyancy of slush a challenge for these models is the short predicted gravitational relaxation timescale of topographic features at high heat flows the resurfacing must occur before the graben topography disappears we also evaluate alternate resurfacing mechanisms such as pumping of liquid water to the surface by thermal expansion stresses and buoyant rise of water through a silicate contaminated crust that is denser than liquid water and conclude that they are unlikely to explain ganymede s bright terrain

viscous relaxation of craters within the martian south polar layered deposits abstract the south polar layered deposits pld are of fundamental importance to martian climatology as they may comprise the largest reservoir of near surface water on mars the south pld exhibit relatively young crater retention surface ages which are widely attributed to recent resurfacing however we show that both constructional and destructional resurfacing mechanisms such as dust deposition and water ice sublimation respectively are inconsistent with the size depth and spatial distributions of south pld craters we demonstrate that another process viscous creep relaxation of dusty water ice is more compatible with the observed cratering of south pld surfaces the results of our finite element relaxation simulations suggest that despite their apparent youthfulness the pld have been stable for at least several hundred million and perhaps even over a billion years consequently our modeling implies that the time scales for the formation and preservation of the layers characteristic of the south and possibly north pld are much longer than generally assumed

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

relaxed impact craters on ganymede regional variation and high heat flows abstract viscously relaxed craters provide a window into the thermal history of ganymede a satellite with copious geologic signs of past high heat flows here we present measurements of relaxed craters in four regions for which suitable imaging exists near anshar sulcus tiamat sulcus northern marius regio and ganymede s south pole we describe a technique to measure apparent depth or depth of the crater with respect to the surrounding terrain elevation measured relaxation states are compared with results from finite element modeling to constrain heat flow scenarios see companion paper bland et al 2017 the presence of numerous substantially relaxed craters indicates high heat flows in excess of 30 40 mw m 2 over 2 gyr with many small

effects of the legacy of axial cooling on partitioning of hydrothermal heat extraction from oceanic lithosphere the predicted and observed heat flux across the seafloor these methods have assumed a dynamic steady state thermal system we show that this assumption is not warranted and leads to an incorrect partitioning of hydrothermal circulation between ridge axes and flanks to more accurately estimate hydrothermal heat loss on ridge axes and flanks we consider the spatial and temporal extent and vigor of axial hydrothermal circulation in calculating hydrothermal heat extraction axial fluid circulation perturbs the thermal state of oceanic lithosphere for 5 ma after that circulation ceases reducing the hydrothermal heat extraction on ridge flanks we find 30 of hydrothermal heat extracted on axis 10 extractednearaxis fromendofaxialhydrothermalcirculationto1ma and 60 extracted from lithosphere 1 ma

finite element modeling of short wavelength folding on venus implications for the plume hypothesis for crustal plateau formation 1 the key geological observations and structural interpretations associated with the current plume hypothesis for venusian crustal plateau formation are that 1 short wavelength spatially periodic tectonic structures originated as mechanical instabilities in a regionally extensive surface brittle layer whose basal boundary was thermally generated and controlled 2 characteristic wavelengths of spatially periodic structures record the brittle layer thickness at the time of deformation and 3 structural wavelengths increased with time driven by brittle layer thickening in response to cooling the plume hypothesis accommodates these constraints by proposing that crustal plateaus formed above mantle plumes impinging on thin lithosphere it further suggests that surface temperatures temporarily elevated to 1000 k may have been required to maintain a sufficiently thin brittle layer for formation of the shortest wavelength structures we report here on finite element simulations designed to test the feasibility of the proposed thermal conditions specifically we model formation of short wavelength folds thought to have initiated as contractional layer instabilities early in the plateau formation process under high surface temperatures the finite element meshes develop semibrittle zones in which short wavelength folds can form but development of even modest structural relief requires unrealistically high total mesh shortening thus elevated surface temperatures inhibit development of short wavelength folds because the models effective integrated mechanical strength under such hot conditions is excessively low decreasing the surface temperature increases structural relief but produces tectonic wavelengths that are larger than those observed we conclude that a model with solely thermal control of mechanical properties cannot explain the observed structures