effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

fluid volcanism on miranda and ariel flow morphology and composition several types of volcanic units have been recognized on the icy uranian satellites miranda and ariel on miranda ridges characterized by crest grooves are up to 10 km wide and 500 m high a continuous flat topped flow band also 10 km wide and 500 m high forms the outer southern margin of elsinore corona elsinore coronae at least appears to comprise coalesced flow bands and ridges on ariel in addition to at least one ridge unit similar to these on miranda flood plains material has covered the floors of deep chasmata grabens and an irregular depression flows on both satellites are characterized by linear vent geometries and distinct topographic margins which indicate extrusion of a relatively viscous material the topography of the flows can be used to estimate flow viscosity or yield strength using a bingham plastic model extrusion viscosity estimates incorporating plausible volcanologically based emplacement time scales and a rigid crust correction range from 107 to 109 p 1013 p in the unlikely absence of a chilled crust viscosity estimates are dependent on the assumed emplacement time scale however and could be as high as 1016 p if a solid stale based time scale is assumed bingham models indicate that it is yield strength that controls the final flow width and thickness that we see yield strength estimates are probably more reliable because of uncertainties in the viscous flow time scale yield strength estimates range from 10 1 to 10 2 bars consistent with yield strengths of partially crystallized silicate melts on other planets nevertheless both the viscosities and yield strengths are well within the range of and in some cases lower than silicate flows silicate compositions are not possible for these flows because of the ice rich composition of these satellites measured properties of ammonia water liquid crystal mixtures are consistent with the observed and estimated flow rheologic properties however and extrusion of such material s is a viable alternative to solid state extrusion for the origin of these flow features

constraints on the expansion of ganymede and the thickness of the lithosphere grooves and furrows have been interpreted as grabens resulting from extension of ganymede s lithosphere geologically reasonable estimates of the dip 90 60 and displacement less than 1 km to a few hundred meters of faults bounding grooves on ganymede allow calculation of the extension and surface area increase 0 04 2 required for the formation of all grooved terrain the implied radius increase is thus limited between 0 02 and 1 1 planetary expansion is the maximum possible since formation of the grooved terrain began similar estimates for faults bounding furrows the oldest preserved tectonic features on ganymede indicate a maximum surface area increase of 0 5 for the best developed furrow system which is located in galileo regio if equivalent furrow systems existed on all of the cratered terrain then a maximum of 0 5 increase in surface area of ganymede is possible after the formation of the lithosphere and prior to the formation of grooved terrain faults bounding simple graben originate at a subsurface mechanical discontinuity and propagate upwards resulting in grabens with similar widths and similar spacings between members of a set the depth to this mechanical discontinuity for furrows and grooves on ganymede must give at least a minimum lithosphere thickness because it is impossible for faults to originate and propagate upwards from a point below the lithosphere therefore the minimum lithosphere thickness on ganymede is 5 km and 9 km in marius and galileo regiones respectively at the time of furrow formation and 4 km at the time of groove formation

landslides on charon abstract we investigated five large landslides identified in the serenity chasma region of charon the identification of these landslides involved a search for these features in images taken by cameras onboard the new horizons spacecraft various landslide properties were analyzed based on their morphologies using a digital terrain model of the region we found that landslides are confined to the walls of the large normal fault scarps that make up serenity chasma based on extensive landslide runout lengths l relative to their drop heights h we classified these features as long runout landslides by analyzing their geometries we estimated the friction coefficients of the landslide material h l to be between 0 15 to 0 31 and the runout efficiencies l h to be between 3 2 and 6 8 we also estimated that the specific energy released during landslide motion ranged from 0 8 to 1 3 kj kg 1 these amounts of energy were too low to have generated significant melt around landslide particles

radii shapes and topography of the satellites of uranus from limb coordinates abstract limb coordinates are used to find the radii shapes and local topography of the five large satellites of uranus the technique provides a direct measure of ellipsoidal shapes of satellites and fixes radii to subpixel accuracy umbriel titania and oberon are best fit by spheres miranda and ariel are ellipsoids whose equatorial bulges are consistent with published mean densities limb topography on miranda shows substantial deformation of both old cratered terrain and the younger coronae and complex faulting and uplift at the margins of the coronae the maximum deformation is about 10 km umbriel s limb shows a basin of undetermined origin about 500 km across and 6 km deep

solid state ice volcanism on the satellites of uranus voyager images of the uranian satellites ariel and miranda show flow features with morphologies indicating that ice has been extruded to the satellites9 surfaces in the solid state these images provide the first observational evidence for solid state ice volcanism in the solar system topographic profiles have been measured across a number of flow features on ariel with a simple model of extrusion spreading and cooling of a viscous flow the initial viscosity of the flow material is found to have been no more than about 10 16 poise far lower than expected for h 2 o ice at the ambient surface temperatures in the uranian system sharply reduced viscosities may have resulted from incorporation of ices like nh 3 or ch 4 in the uranian satellites

structural mapping of enceladus and implications for formation of tectonized regions global structural mapping of high resolution cassini images of enceladus reveals a richly varied surface most notable are three main regions of deformation each containing multiple structural units in addition to the well known south polar terrain spt there are two other large regions of deformation that we term leading hemisphere terrain lht and trailing hemisphere terrain tht each of these three terrains includes a circumferential belt that encloses one or more other structurally deformed units areal extents range from about 80 000 km2 spt to 195 000 km2 lht or 160 to 250 km equivalent circular radius based on relative crater densities the tht is inferred to be older than the lht the geologically active spt is the youngest the overall similarities in shape and dimension of the three tectonized terrains suggest similar formational processes plausibly related to broad loading of a thin elastic shell a viable scenario is that each tectonized terrain formed above a large scale region of warm upwelling ice with subsequent downwarping triggered by cooling and or subsurface melting however differences in morphological detail suggest that the specific evolution of each tectonized terrain has been different

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

the flexural isostatic response of the lithosphere to extensional tectonics abstract the tectonic loading resulting from the extension of the lithosphere is examined loads resulting from crustal thinning thermal perturbations and basin infill are recognised and the resulting flexural isostatic deflection of the lithosphere is modelled in response to each of them in the presence of a listric fault during rifting regional uplift or rebound occurs as a result of both crustal thinning due to normal faulting simple shear and the elevation of the geothermal gradient while subsidence is generated by the downward acting loads created by both basin infill and thinning of the lower crust by stretching pure shear following rifting further flexural subsidence occurs in response to the cooling of the temperature field and associated sedimentary infill one of the major tectonic effects resulting dominantly from crustal thinning by normal faulting is the generation of footwall uplift the magnitude of this uplift along with basin depth and underlying crustal structure are strongly determined by the flexural rigidity of the lithosphere the amount of extension and the dip of the major basement fault along which extension is taking place depending upon the values of these parameters model calculations predict between a few tens of metres and 1 5 km of footwall uplift these values correlate closely with the magnitude of footwall uplift estimated from examples within the geological record the erosion of this rift induced topography is also proposed as a possible driving mechanism for the generation of major basin unconformities

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

voyager 2 in the uranian system imaging science results voyager 2 images of the southern hemisphere of uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere the alignment of the cloud bands is similar to that of bands on jupiter and saturn but the zonal winds are nearly opposite at mid latitudes 70 degrees to 27 degrees where winds were measured the atmosphere rotates faster than the magnetic field however the rotation rate of the atmosphere decreases toward the equator so that the two probably corotate at about 20 degrees voyager images confirm the extremely low albedo of the ring particles high phase angle images reveal on the order of 10 2 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings as well as a broad diffuse low optical depth ring just inside the main rings system nine of the newly discovered small satellites 40 to 165 kilometers in diameter orbit between the rings and miranda the tenth is within the ring system two of these small objects may gravitationally confine the e ring oberon and umbriel have heavily cratered surfaces resembling the ancient cratered highlands of earth s moon although umbriel is almost completely covered with uniform dark material which perhaps indicates some ongoing process titania and ariel show crater populations different from those on oberon and umbriel these were probably generated by collisions with debris confined to their orbits titania and ariel also show many extensional fault systems ariel shows strong evidence for the presence of extrusive material about halfof miranda s surface is relatively bland old cratered terrain the remainder comprises three large regions of younger terrain each rectangular to ovoid in plan that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials perhaps suggesting some exotic composition

extensional tilt blocks on miranda evidence for an upwelling origin of arden corona subparallel ridges and troughs in the outer belt of arden corona on the uranian satellite miranda are interpreted as tilt blocks formed by extension and normal faulting fault scarps generally face outward from the corona exposing dark material in the subsurface reconstruction of faults along a deep rift zone bounding the corona suggests initial dips of 50 local extension reaches 70 extremely high in comparison to previous estimates of strain on icy satellites a rise adjacent to the rift zone is modeled as flexural and indicates an effective elastic lithospheric thickness of 2 km at the time of flexure the assumption that faulting has significantly weakened the lithosphere suggests a mechanical lithosphere thickness of 5 to 10 km corresponding thermal gradients in a frictionally controlled ice lithosphere are 8 to 20 k km 1 and lithospheric tensional strength is 0 4 to 1 8 mpa normal faulting in arden corona indicates that internal upwelling likely formed the corona and the outward facing direction of faults is consistent with such a model an upwelling origin of miranda s coronae eliminates the need to invoke catastrophic breakup and reaccretion of the satellite as an explanation for its surface geology

sinker tectonics an approach to the surface of miranda images of the uranian moon miranda show three areas of complex terrain trapezoidal to ovoid in shape with no previously known counterpart these features called coronae are characterized by an inner core of intersecting ridges and troughs surrounded by a series of subparallel and concentric bands two of the proposed explanations for these tectonic features involve mantle convection driven by density anomalies the present study determines the surface stresses induced by such density anomalies and their expected surface expression miranda is modeled as having a rigid core surrounded by a mantle capable of viscous flow which is overlain by a thin elastic lithosphere the density anomaly is modeled as a point mass representing the sinker or riser the model is developed in a spherical coordinate system having axial symmetry about a 0 axis passing through the sinker the flow field set up by the sinking or rising mass is expressed in terms of a stream function using legendre sums streamlines are functions of the position of the mass anomaly and the radii of the core mantle and mantle lithosphere boundaries only magnitudes of the stream function depend upon the size of the mass anomaly its local gravitational field and the mantle lithosphere radius the stresses on the base of the lithosphere are then determined from the equations of motion the viscosity of the mantle material has no effect on the stresses produced at any given time but does control the velocity of the anomaly expressions for displacements and stresses in the elastic shell are developed as legendre sums and coupled to the viscous mantle solution with appropriate boundary conditions the stresses derived yield a pattern of expected tectonic expression this tectonic style varies with the radius of the body for a small body surface stresses are dominant in the area above the mass anomaly while farther away stresses at the base of the lithosphere control the tectonics directly over a sinker and extending outward 15 is a zone of both latitudinal and longitudinal surface compression which produces folding and thrust faults with no preferred orientation beyond 15 longitudinal stresses at the base of the lithosphere are extensional while latitudinal stresses are compressional producing a zone of folding and thrusts concentric about the sinker stresses induced by a riser are opposite in sign and predict extensional features with the same orientation as the compressional features predicted for a sinker stress magnitudes generally decrease with distance from the sinker and also decrease with both increasing sinker depth and lithosphere thickness the predictions of the sinker model are in good agreement with the tectonic styles of the banded and ridged terrains of miranda as determined from the topography as imaged by voyager 2 this agreement supports the idea that the coronae on miranda were produced by dense masses originally located near the surface that sank into miranda s interior for larger bodies the stresses at the surface and base are nearly equal longitudinal stresses are greater than latitudinal stresses resulting in compressional features with a preferred orientation radial to the sinker location the changeover in tectonic styles occurs at a planetary radius of approximately 1500 km

red material on the large moons of uranus dust from the irregular satellites abstract the large and tidally locked classical moons of uranus display longitudinal and planetocentric trends in their surface compositions spectrally red material has been detected primarily on the leading hemispheres of the outer moons titania and oberon furthermore detected h2o ice bands are stronger on the leading hemispheres of the classical satellites and the leading trailing asymmetry in h2o ice band strengths decreases with distance from uranus we hypothesize that the observed distribution of red material and trends in h2o ice band strengths results from infalling dust from uranus irregular satellites these dust particles migrate inward on slowly decaying orbits eventually reaching the classical satellite zone where they collide primarily with the outer moons the latitudinal distribution of dust swept up by these moons should be fairly even across their southern and northern hemispheres however red material has only been detected over the southern hemispheres of these moons during the voyager 2 flyby of the uranian system subsolar latitude 81 s consequently to test whether irregular satellite dust impacts drive the observed enhancement in reddening we have gathered new ground based data of the now observable northern hemispheres of these satellites sub observer latitudes 17 35 n our results and analyses indicate that longitudinal and planetocentric trends in reddening and h2o ice band strengths are broadly consistent across both southern and northern latitudes of these moons thereby supporting our hypothesis utilizing a suite of numerical best fit models we investigate the composition of the reddening agent finding that both complex organics and amorphous pyroxene match the spectral slopes of our data we also present spectra that span l l bands 2 9 4 1 m a previously unexplored wavelength range in terms of spectroscopy for the uranian satellites and we compare the shape and albedo of the spectral continua in these l l band data to other icy moons in the jovian and saturnian systems additionally we discuss possible localized enhancement of reddening on titania subtle differences in h2o ice band strengths between the southern and northern hemispheres of the classical satellites the distribution of constituents on miranda and the possible presence of nh3 hydrates on these moons in closing we explore potential directions for future observational and numerical modeling work in the uranian system

miranda color and albedo variations from voyager photometry abstract we have constructed albedo color maps of miranda based on minnaert parameters b 0 k derived from voyager clear filter 0 48 m images covering phase angles of 13 to 39 and ultraviolet 0 35 m violet 0 41 m and green 0 56 m data at 16 phase angle for miranda s bright cratered terrain bct k 0 474 0 0036 while for global miranda a slightly higher value of k obtains given by k 0 485 0 008 mean normal reflectances for geologic subunits within the coronae range from highs near 0 32 0 33 to lows around 0 25 0 26 locally the widest spread of reflectances is found within inverness corona 0 20 to at least 0 40 the bright cratered terrain is remarkably homogeneous photometrically and has a mean normal reflectance of 0 29 compared with the globally averaged value for miranda of 0 28 the above normal reflectance estimates correspond to the voyager clear filter and involve a conservative extrapolation of the phase curve from 16 to opposition 0 if miranda s opposition surge turns out to be similar to that of titania the values must be increased by a factor of 1 2 our data do not reveal any significant wavelength dependence for the minnaert parameter k between 0 35 and 0 56 m color ratios v cl v g g cl are essentially constant over all terrain types and albedo ranges on the satellite although there is an indication that darker areas of miranda may be very slightly redder at the level of a few percent spectrally over the wavelength range of the voyager cameras miranda is gray the color data are consistent with a model in which differing amounts of a material spectrally similar to asteroidal f material are mixed in with an icy component

mobilization of cryogenic ice in outer solar system satellites voyager images of the uranian satellites show a diversity of geological features including clear evidence for the softening and mobilization of ice on miranda and ariel some of these features are similar to those seen on jovian and saturnian satellites where the mobile material is believed to be water or a water ammonia mixture however the extremely low temperatures and probable unavailability of large energy sources within the uranian satellites lead us to consider flow mechanisms that operate at very low temperature t 100 k we propose here a form of pressure solution creep in which very fine grained water ice or clathrate hydrate is mobilized by a small amount of intergranular cryogenic fluid ch 4 co or n 2 viscosities as low as 10 12 p are possible for a limited time sufficient to allow flooding of rift valleys and perhaps even substantial lateral flows glaciers

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

relaxation of impact basins on icy satellites the extent and style of viscous relaxation of the largest crater basins on the icy satellites of saturn can be used as a probe of the viscosity of the deep interiors of these bodies in particular some constraints on the size of a silicate core and the magnitude of the internal viscosity gradient can be determined if the relaxation process is sufficiently well understood we discuss the application of a unite element model incorporating spherical symmetry to the general problem of relaxation of large craters on spherical bodies we examine the influence of core size geothermal gradient and rheologic law on relaxation flow and consider the implications for the internal structures of mimas and tethys our model results show that relaxation of crater topography on a spherical satellite is qualitatively and quantitatively different from relaxation on a planar surface once the crater diameter becomes a significant fraction of the satellite radius for small to moderate sized basins relaxation flow is bidirectional with flow both from rim to bowl and rim to surrounds this is the same style of flow observed for craters on planar surfaces for large enough basins however the spherical geometry of the problem leads to flow that is unidirectional toward the crater bowl throughout the satellite the transition from bidirectional to unidirectional flow appears to be very sensitive to the basin diameter the critical basin diameter for transition increases with core size thermal gradient rim height and rheologie law stress exponent unidirectional and bidirectional flow produce fundamentally different stress fields in the regions surrounding the relaxing crater relaxation velocities for large craters on spherical bodies can be appreciably more rapid than those for equivalent craters on planar surfaces again because of the geometric effects of relaxation on a sphere the difference in degree of relaxation between the large basins odysseus on tethys which has undergone significant modification and herschel on mimas which appears essentially unmodified can be explained by different thermal histories for tethys and mimas by the effects of stress dependent rheology or by a combination of the two the influence of a core at sizes consistent with the observed densities of the satellites on relaxation time scales is unimportant if the ithaca chasma graben system on tethys is the result of stresses generated by relaxation of the odysseus basin then tethys s core must be small enough that it allows unidirectional relaxation flow for a basin of odysseus s size if this assumption it correct we estimate an upper limit of roughly 20 of tethys s radius for the radius of the satellite s core

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

dynamics of rifting and modes of extension on icy satellites 1 a simple numerical model of extension in icy satellite shells is developed thinning of the ice weakens the shell promoting further extension if lateral flow in the lower part of the shell is unimportant extension is opposed and wide rifts are generated if lateral flow is rapid localized extension is favored and narrow rifts are produced thick shells or high strain rates favor the development of narrow rifts low strain rates favor wide rifting it is proposed that bands extensional features on europa are narrow rifts while groove lanes on ganymede are wide rifts the existence of wide rifting on ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4 8 km and a strain rate of 10 15 s 1 strain rates at the time of rifting whether this shell thickness applies to present day europa depends on the age of band formation which is poorly known the difference between rifting behavior on ganymede and europa is due to either higher strain rates or higher shell thicknesses on europa during rifting the mean stresses required to cause the observed rifting are 0 2 mpa for ganymede and 0 3 mpa for europa these values are comparable to estimates derived from flexural features index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8109 tectonophysics continental tectonics extensional 0905 8160 tectonophysics rheology general keywords europa ganymede extension faulting ice

sr ca ratios as indicators of varying modes of pelagic carbonate diagenesis in the ooze chalk and limestone realms abstract there has been an interest in the application of sr ca ratios in pelagic carbonates to reconstruct paleoceanographic conditions such as changes in the oceanic ca and sr budgets or calcareous nannoplankton productivity but the effect of burial diagenesis on primary sr ca records has not been sufficiently understood in order to clearly document sr ca records of pelagic carbonates relative to original paleoceanography or secondary diagenetic processes sr ca analyses were performed on bulk sediments from an 822 5 m thick ooze chalk limestone sequence at the mid pacific mountains deep sea drilling project site 463 the sr ca ratios exponentially decrease from 2 3 to 1 1 mmol mol with increasing burial depth in the ooze chalk interval 0 452 0 m below seafloor mbsf the sr ca ratios further decline to as low as 0 4 mmol mol in the limestone interval 452 0 mbsf with relatively high values of 1 1 mmol mol at carbonate poor lithology around 620 mbsf the stratigraphic sr ca trends appear to be consistent with porosity profile and similar sr ca porosity covariation in association with diagnetic changes in lithologies is also common in ontong java plateau sections by adopting a recent diagenetic model from backscattered electron image analysis of ontong java plateau sections it is postulated for site 463 that exponential sr ca decrease in the ooze chalk interval is due to the release of sr into interstitial water via recrystallization i e purification effect and that further decrease in sr ca ratios in the limestone interval is most likely due to acquisition of sr depleted calcite cement at the chalk limestone transition newly proposed as the dilution effect specifically sr ca ratios of deeply buried pelagic limestones are sensitive to the amount of calcite cement that is acquired in proportion to caco3 content i e carbonate rich lithology facilitates precipitation of secondary calcite at the chalk limestone transition therefore paleoceanographic interpretation by means of sr ca ratios in pelagic carbonates will become valid only when these diagenetic effects are appropriately constrained