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

slip system domains 2 kinematic aspects of fabric development in polycrystalline aggregates abstract in the first paper of the present series cobbold and gapais 1986 we develop theoretical models of plane strain domainal deformation by slip along inextensible and indestructible fibres for such extreme mechanical conditions we show that deformation is largely controlled by kinematic factors we use our models to explain domainal patterns of natural slip systems including bedding and foliation planes faults ductile shear zones and lattice planes the present paper focusses on kinematic aspects of fabric development in polycrystalline aggregates a three dimensional kinematic model is developed it reveals that common quartz fabrics may develop domainally by reorientation and selection of inextensible slip directions a axes or slip planes with initially random orientation stable orientations of slip directions are close to both surfaces of no finite extension and orientations of large amount of shear we explain 1 1 common patterns of quartz a and c axis fabrics e g small circles and crossed girdles 2 2 characteristic differences between fabric patterns according to the shape of the finite strain ellipsoid 3 3 development of asymmetric fabrics during non coaxial deformation histories this last process involves selective destruction or replacement of certain orientation domains so that a preferred orientation of slip lines develops close to the bulk shear direction rotation recrystallization is shown to be an efficient natural selection mechanism the models also account for many microstructural features found in oriented polycrystalline rocks especially 1 1 alternating band like or kink like domains of twinned preferred orientations 2 2 individual grains or clusters of grains with retort like shapes and extensive recrystallisation at boundaries 3 3 small scale shear bands formed by strain partitioning and shear localization

the shape of mars and the topographic signature of the hemispheric dichotomy reanalysis of occultation data from the mariner 9 and viking orbiter spacecraft to determine the shape of mars indicated that the hemispheric dichotomy is not a fundamental feature of the shape of the planet it is a consequence of an approximately 3 kilometer offset between mars9s center of mass and center of figure and the boundary along most of its length consists of broad gradual surface slopes over distances of thousands of kilometers this result was supported by analysis of high spatial resolution earth based radar topographic profiles any successful model for the origin of the dichotomy must explain a planet with an ellipsoidal shape and a long wavelength gradual topographic transition between the northern and southern hemispheres

development of single crystal σ shaped quartz porphyroclasts by dissolution precipitation creep in a calcite marble shear zone abstract within a greenschist facies calcite marble shear zone isolated quartz grains change shape across the shear zone profile whereas quartz grains have a spherical to elongated shape in the coarse grained marble protolith and protomylonite they are asymmetric shaped porphyroclasts with wedge shaped appendages wings in ultramylonites in all cases quartz grains are single crystals they are sometimes twinned but never recrystallized stress induced dissolution precipitation creep is the favourable shape controlling process during deformation providing a relatively undeformed core truncation of pre existing cathodoluminescence cl patterns and syntaxial precipitation of wings the necessary fluids may have been released from fluid inclusions during dynamic recrystallization of the calcite matrix the development of shape is not related to the crystallographic orientation of the quartz porphyroclasts crystallographic orientation analysis by electron backscatter diffraction ebsd and cl analyses exclude crystal plasticity as a shape controlling mechanism of quartz grains during mylonitisation however in ultramylonites quartz clasts have a strong crystallographic preferred orientation cpo with c axes sub parallel to the shear direction this fabric is uncommon for quartz under greenschist facies conditions it might be explained by a strengthening of a pre existing weak cpo during mylonitisation by rigid particle rotation of elongated quartz grains

lineated valley fill lvf and lobate debris aprons lda in the deuteronilus mensae northern dichotomy boundary region mars constraints on the extent age and episodicity of amazonian glacial events abstract in order to assess the nature degradational processes and history of the dichotomy boundary on mars we conducted a detailed morphological analysis of a 70 000 km 2 region of its northern portion north central deuteronilus mensae south of lyot in the vicinity of sinton crater this region is characterized by the distinctive sinuous 2 km high plateau scarp boundary outlying massifs to the north and extensive fretted valleys dissecting the plateau to the south these features represent the first order modification and retreat of the dichotomy boundary and are further modified by processes that form lineated valley fill lvf in the fretted valleys and lobate debris aprons lda along the dichotomy scarp and surrounding the outlying massifs we use new high resolution image and topography data to examine the nature and origin of lvf and lda and to investigate the climatic and accompanying degradational history of the escarpment on the basis of our analysis we conclude that 1 lvf and lda deposits within the study region are comprised of the same material show integrated flow patterns and originate as debris covered valley glaciers a significant amount of ice hundreds of meters is likely to remain today beneath a thin cover of sublimation till 2 there is depositional evidence to suggest glacial highstands at least 800 m above the present level implying previous conditions in which the distribution of ice was much more widespread this is supported by similar deposits within many other areas across the dichotomy boundary 3 the timing of the most recent large scale activity of the lda lvf in this area is about 100 500 million years ago similar to ages reported elsewhere along the dichotomy boundary 4 there is evidence for a secondary but significantly limited phase of glaciation the deposits of which are limited to the vicinity of the alcoves similar later phases have also been reported elsewhere along the dichotomy boundary 5 modification of the fretted valleys of the dichotomy boundary has been substantial locally but we find no evidence that the amazonian glacial epochs caused retreat of the dichotomy boundary of the scale of tens to hundreds of kilometers our findings support the results of an analysis just to the east of the study region and of studies carried out elsewhere along the dichotomy boundary that find further evidence for the remnants of debris covered glaciers and extensive valley glacial land systems

rotational bulge and one plume convection pattern influence on martian true polar wander abstract motion of the entire solid planet with respect to its spin axis have been proposed on mars this movement is known as true polar wander tpw according to the conservation of angular momentum with no external torque on geological time scales the axis of maximum inertia of a planet is aligned with the rotation axis then rearrangement of masses within the mantle disturbs the planet s inertia and induces tpw the convection pattern on mars is possibly controlled by a sequence of single plumes originating from the core mantle boundary using a homogeneous model of the martian mantle and modelling the plume as a sphere we calculate the inertial tensor perturbations caused by the plume mass anomaly we investigate the stabilizing influence of the remnant rotational bulge due to the lithosphere elasticity on these perturbations it appears that during early martian history the elastic lithosphere was thin enough to allow its fractures under the inertia perturbations induced by a hot plume consequently the lithosphere s behaviour became effectively viscoelastic and the plume could induce large tpw we conclude that one plume convection pattern should have greatly influenced the rotation pole behaviour during early mars history around 4 gyr ago mars already could have experienced two tpw events lasting possibly only a few million years each we then compare our scenario with others already published in the literature

the evolution of perpendicular linear fabrics in synkinematically emplaced tourmaline granite central moravia bohemian massif abstract sheets of leucocratic peraluminous tourmaline granite were synkinematically emplaced into metasediments of the svratka crystalline unit during the variscan orogeny the granite was deformed under hightemperature conditions close to the solidus as indicated by quartz microstructures and the presence of prism c glide in quartz decrease in the temperature of deformation causing orthogneissification of the granite is documented by transition from prism c to prism a and basal a glide operating in quartz grains two mutually perpendicular stretching lineations originated during high temperature deformation of the granite detailed three dimensional analysis of tourmaline preferred orientations underlining both lineations has been performed and the distributions have been compared with theoretical predictions of jeffery and march for rotation of rigid linear markers the origin of mutually perpendicular linear fabrics is discussed on the basis of numerical models transpression considered as a combination of northwestward shearing accompanied by shortening across and lengthening along the shear plane is proposed to explain the perpendicular linear fabrics observed

fabric transitions in the saxony granulite terrain abstract quartz fabric transitions reported from the saxony granulite terrain and from the nearby ore mountains are interesting because of the changes in the pattern types and the variation in the opening angles of the fabric skeletons a short summary is presented of salient features in research documented in the east german literature between 1961 and 1978 the changes in fabric skeleton with decreasing metamorphic grade are proposed as mainly consequent to the deactivation of prism c glide systems as temperature decreases fabrics indicative of the appearance of the c burgers vector might be diagnostic for deformation in regimes of comparatively high temperatures or low strainrates or for deformation under conditions such that there is a relatively high kinetically effective hydroxyl content in the quartz structure

relaxation of the martian dichotomy boundary faulting in the ismenius region and constraints on the early evolution of mars 1 the martian dichotomy is a global feature separating the cratered southern highlands and smooth northern lowlands parts of the boundary are defined by steep slopes with elevation differences of 2 6 km here we use constraints from the well preserved section of the dichotomy in the ismenius region we study the relaxation of the boundary using both finite element and semianalytical techniques to better constrain the thermomechanical conditions in early mars we argue that the steep slopes are the product of faulting resulting from relaxation of the edge of the boundary by lower crustal flow this relaxation requires that the lower crust must be at least 10 km thick and have a viscosity of 1021 pa s during the first 100 300 myr of relaxation we show that a wet crustal rheology and a thermal gradient of 15 20 k km at 4 ga allows for both the partial relaxation of the martian dichotomy and preservation of long wavelength topography as observed on mars this result is generally consistent with a stagnant lid model of cooling of martian interior the thermal gradient at 4 ga is also consistent with the estimated elastic thickness on early mars our modeling constrains the original slope of the boundary to approximately 2 we cannot constrain the crustal thickness more tightly than the current estimates of 50 100 km on the basis of our semianalytical modeling the relaxation of the boundary can contribute to the general north south slope observed on mars

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

deformation and geochronology of syntectonic granitoids emplaced in the major gercino shear zone southeastern south america abstract the major gercino shear zone is one of the ne sw lineaments that separate the neoproterozoic dom feliciano belt of brazil and uruguay into two different domains a northwestern supracrustal domain from a southeastern granitoid domain the shear zone striking ne is composed of protomylonites to ultramylonites with mainly dextral kinematic indicators in santa catarina state southern brazil the shear zone is composed of two mylonite belts the mylonites have mineral orientations produced under greenschist facies conditions at a high strain rate strong flattening and coaxial deformation indicate the transpressive character while the role of pure shear is emphasized by the orientation of the mylonite belts in relation to the inferred stress field component the quartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity were the dominant mechanisms of deformation during the mylonitization process additionally the fabrics suggest that the glide systems are activated for deformation conditions compatible with the metamorphism in the middle greenschist facies elongated granitoid intrusions belonging to two petrographically geochemically and isotopically distinct rock associations occur between the two mylonite belts the structures observed in the granites result from a deformation range from magmatic to solid state conditions points to a continuum of magma straining during and just after its crystallization conventional u pb analysis of multi crystal zircon fractions yielded essentially identical ages of 609 16 ma and 614 2 ma for the two granitic associations and constrain the transpressive phase of the shear zone k ar ages of biotites between 585 and 560 ma record the slow cooling and uplift of the intrusions some k ar ages of micas in regional mylonites are similar suggesting that thermo tectonic activity was intense up to this time probably related to the agglutination of the granite belt to the supracrustal belt nw of the mgsz

quartz microstructures and c axis preferred orientations in high grade gneisses and mylonites around the morin anorthosite grenville province quartz in deformed rocks from two large high grade shear zones around the morin anorthosite morin terrane grenville province displays distinctive microstructures as well as c axis preferred orientations in the west dipping morin shear zone east of the morin anorthosite four distinct quartz microstructures types 1 4 are identified based on deformation features and grain size the c axis orientations are characterized by a single maximum near the stretching lineation and two maxima in type 1 microstructure and by an asymmetrical single girdle in type 2 microstructure quartz c axes show crossed girdle pattern in type 3 microstructure both quartz microstructures and c axis preferred orientations suggest that crystal plastic slip and dynamic recrystallization are the dominant deformation mechanisms the asymmetry of c axis orientations with respect to the mylonitic foliation as well as the substructures developed in quartz indicates a dextral sense of shear in the morin shear zone type 4 micr

microfabrics indicating granulite facies metamorphism in the low pressure central damara orogen namibia abstract the reconstruction of the fluid evolution during progressive metamorphism in the central damara orogen by hoernes and hoffer indicated that fluid present conditions were only periodically attained among these rocks and that the highgrade metamorphism along the west coast of namibia occurred under essentially water absent conditions the characteristic mineral assemblage in the metapelites is plagioclase quartz alkali feldspar biotite cordierite garnet sillimanite apatite zircon ore orthopyroxene bearing parageneses in the metapelites are missing and granulite facies metamorphism may not be detected on the basis of mineral assemblages a detailed study along a 100 km traverse across the migmatite area in the central damara orogen namibia reveals several microfabric features which indicate in analogy to many other well known granulite facies terrains where similar microfabrics have been described that the pan african regional metamorphism in this area culminated at conditions of low pressure granulite facies the observed features are 1 a the activation of basal a and prism c glide systems in quartz under conditions of plastic deformation and the development of equivalent subgrain boundaries parallel prism faces and parallel basal planes 2 b the orientated exsolution of rutile needles in quartz 3 c the exsolution of string perthites mesoperthitic string perthites and antiperthites in feldspars 4 d in addition the homogeneous distribution of major elements in garnet crystals and garnet biotite thermometry suggest granulite facies conditions

lithospheric flexure and the evolution of the dichotomy boundary on mars 1 the boundary of the martian crustal dichotomy in the eastern hemisphere is one of the most striking topographic features on the planet the long wavelength topography of much of the boundary is expressed by a broad rise and an arched ramp that slopes downward from the southern highlands into the northern lowlands and often ends in a steep scarp lithospheric flexure of the southern highlands for both a continuous and broken plate boundary with the northern lowlands lithosphere is modeled we find that the long wavelength topography of the boundary is best fit by a lithospheric deflection profile of a broken lithosphere for reasonable values of the flexural parameter the lithosphere near the dichotomy boundary may have been weakened by tectonic stress associated with the formation of the crustal dichotomy by subcrustal transport caused by overturn of an early magma ocean

garnet plasticity in the lower continental crust implications for deformation mechanisms based on microstructures and sem electron channeling pattern analysis abstract elongated garnets which preserved deformational microstructures boudinage pinch and swell structures occur in granulite facies quartzites in the highland complex of sri lanka in spite of these microstructures and in contrast with previous reports on garnet plasticity there are no or only few signs of intracrystalline deformation like subgrains or lattice distortion this can be explained by annealing and slow static cooling from high temperatures sem electron channeling pattern analysis reveals that the garnets have a significant crystallographic preferred orientation aspect ratio grain size analysis shows that the deformation mechanism is grain size sensitive these features indicate a diffusion assisted dislocation glide mechanism with dominant 1 2 110 slip system a comparison of the deformation behavior between garnet quartz and feldspar shows that differences in flow strength are low under the high grade conditions 850 50 c garnet is still the phase with the highest flow strength but dry quartz is slightly stronger than feldspar

asymmetrical quartz crystallographic fabrics formed during constrictional deformation numerical simulations predict unique quartz crystallographic fabric patterns for plane strain flattening and constriction multiple studies support the predictions for plane strain and flattening to test predictions for constriction this paper analyzes five examples of quartz crystallographic fabrics from a 1km wide domain of l tectonites in the pigeon point high strain zone klamath mountains california u s a these samples were deformed under greenschist to amphibolite facies conditions quartz c axis fabrics are similar to the predicted double girdle fabrics except that amphibolite facies samples exhibit caxis maxima and are distinctly asymmetrical about the elongation lineations activation of different slip systems combined with small deviations from pure constriction account for the c axis maxima and noncoaxial flow accounts for the fabric asymmetry the simple shear component is randomly oriented in geographic coordinates throughout the domain of l tectonites these data confirm that numerical simulations predict the quartz c axis fabric geometry developed during constriction for some deformation conditions and they confirm the quartz a axis patterns predicted for constriction for the first time these data also demonstrate that the relationship between quartz crystallographic fabrics and strain geometry is not straightforward and they indicate that a axis fabrics may be more useful indicators of strain geometry variations

deformation temperatures and flow vorticities near the base of the greater himalayan series sutlej valley and shimla klippe nw india abstract we report new deformation temperature and flow vorticity data from the base of the greater himalayan series ghs exposed in the sutlej valley and shimla klippe of nw india we focus on three groups of transects across the hanging wall of the main central thrust mct in order of relative foreland hinterland positions they are the shimla klippe western and eastern sutlej transects deformation temperatures indicated by quartz c axis fabric opening angles increase both from foreland to hinterland at a given structural distance above the mct and up structural section from the mct within individual transects deformation temperatures in the immediate hanging wall to the mct are estimated at 510 535 535 550 and 610 c on the shimla western sutlej and eastern sutlej transects respectively the steepest inferred field gradients in deformation temperatures are recorded adjacent to the mct and progressively decrease up structural section following a power law relationship comparison with temperature estimates based on multi mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions vorticity analyses indicate that shearing along the base of the ghs occurred under sub simple shear conditions wm values of 0 9 1 0 with a minor component of pure shear

the martian hemispheric dichotomy may be due to a giant impact mars is divided into two fundamentally different geological provinces of approximately hemispherical extent1 3 the more southerly province is heavily cratered contains relatively old geological units and superficially resembles the lunar and mercurian highlands the northern province is relatively lightly cratered and contains younger geological units including extensive plains volcanic edifices and volcanic calderas except for the tharsis and elysium regions and other large volcanoes most of the younger northern province consists of lowlands which lie an average of 3 km below the highlands lowlands occupy about one third of mars they are separated from the highlands by a distinct scarp or by a sloping transitional zone as much as 700km wide in which highland materials have been disrupted and partly replaced by lowland deposits fig 1 the transition is characterized by a variety of landforms unknown on other planets the highlands and lowlands are in isostatic equilibrium across the transitional zone4 no generally accepted explanation for the cause of the highland lowland dichotomy has been proposed although thinning of the lithosph re in the northern hemisphere by internal processes has been suggested5 large impacts which were a feature of early solar system history have also been suggested as an important cause of such early planetary surface heterogeneities3 6 7 subcircular lowlands such as argyre chryse hellas and isidis planitia and the plains south of tharsis occupy previously recognized impact basins8 10 we propose here that the largest expanse of lowlands also results from formation of a large impact basin early in mars history that has profoundly shaped the planet s surface

a critical assessment of the tectono thermal memory of rocks and definition of tectono metamorphic units evidence from fabric and degree of metamorphic transformations

the effect of the basal prism mechanism switch on fabric development during plastic deformation of quartzite abstract blacic described a transition from basal a to prism c dislocation glide systems as temperature increased or strain rate decreased in sequences of experiments involving deformation of single crystals of quartz in this paper theoretical aspects of competition between these systems during plastic deformation are discussed it is concluded that the basal prism mechanism switch should have important consequences for the development of crystallographic fabrics during plastic deformation of quartzite and the taylor bishop hill analysis is used in an attempt to predict expected fabric transitions

elastic thickness estimates for the northern lowlands of mars abstract the northern lowlands cover approximately one third of mars surface and are a fundamental part of the geologic evolution of mars we examine the admittance signature the ratio of gravity to topography data in the spectral domain for four regions in the northern lowlands that are both well resolved in the gravity data and have well constrained lithospheric parameters these regions also have topographic power spectra similar to many highlands regions a cartesian multitaper method is used to calculate admittance we compare the observed admittance signatures to those predicted from models of lithospheric flexure on the basis of these comparisons we estimate the thickness of the martian elastic lithosphere t e required to support the observed topographic load since the time of loading we use both top and bottom loading models to derive values of elastic thickness and crustal thickness or apparent depth of compensation although all four regions are best fit by bottom loading models we obtain best fit elastic thickness estimates between 10 and 25 km with an acceptable error range of 0 to 45 km these low elastic thickness estimates are consistent with the formation in the noachian when heat flow was high the consistency in t e estimates between the noachian highlands and lowlands basement suggests that both regions of the crust formed at similar times the paucity of crustal magnetization in the lowlands is thus more likely a result of demagnetization than formation following shut down of the dynamo

new martian paradigms several interrelated paradigms concerning the nature and the evolution of mars are discussed these include an impact origin of the northern lowland mars as a water rich planet former oceans in the northern lowlands a warm wet climate on early mars and the possibility of ancient life on mars locations on mars suggested for the search of possible ancient life include the heavily cratered terrain dissected by valley networks possible lacustrine deposits in valles marineris the oxidizing layer below the surface sites along the fringes of the hypothetical northern circumpolar ocean and in evaporite deposits

hemispheres apart the crustal dichotomy on mars the hemispheric dichotomy is a fundamental feature of mars expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands the origin of the dichotomy which may have set the course for most of the subsequent geologic evolution of mars remains unclear internally driven models for the dichotomy form the lowlands by mantle convection plate tectonics or early mantle overturn externally driven models invoke one giant impact or multiple impacts areal densities of buried basins expressed by quasicircular depressions and subsurface echoes in radar sounding data suggest that the dichotomy formed early in the geologic evolution of mars tectonic features along the dichotomy boundary suggest late stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands subsequent deposition and erosion by fluvial aeolian and glacial processes shaped the present day dichotomy boundary

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

consequences of giant impacts in early mars core merging and martian dynamo evolution a giant impact is an increasingly popular explanation for the formation of the northern lowland on mars it is plausible that at the impact time both mars and the impactor were differentiated with solid silicate mantles and liquid iron cores such a large impact likely resulted in merging of the cores of both bodies a process which will have implications on the thermal state of the planet we model the evolution of the martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor s core as a single diapir the impact heating and the viscous heating induced during the core merging may affect the early thermal state of mars during several tens of million years our results show that large viscosity contrasts between the impactor s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature when the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000 the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core mantle boundary finally the direct impact heating of martian core leads to thermal stratification of the core and kills the core dynamo it takes on the order of 150 200 myr to reinitiate a strong dynamo anew the merging of the impactor s core with the martian core only delays the reinitiation of the dynamo for a very short time

marsis radar sounder evidence of buried basins in the northern lowlands of mars a hemispheric dichotomy on mars is marked by the sharp contrast between the sparsely cratered northern lowland plains and the heavily cratered southern highlands mechanisms proposed to remove ancient crust or form younger lowland crust include one or more giant impacts subcrustal transport by mantle convection the generation of thinner crust by plate tectonics and mantle overturn following solidification of an early magma ocean the age of the northern lowland crust is a significant constraint on these models the mars advanced radar for subsurface and ionospheric sounding marsis instrument on the european space agency s mars express spacecraft is providing new constraints on the martian subsurface here we show evidence of buried impact basins ranging in diameter from about 130 km to 470 km found over approximately 14 per cent of the northern lowlands the number of detected buried basins 200 km in diameter indicates that the lowland crust is ancient dating back to the early noachian epoch this crater density is a lower limit because of the likelihood that not all buried basins in the area surveyed by marsis have been detected an early noachian age for the lowland crust has been previously suggested on the basis of a large number of quasi circular topographic depressions interpreted to be evidence of buried basins only a few of these depressions in the area surveyed by marsis however correlate with the detected subsurface echoes on the basis of the marsis data we conclude that the northern lowland crust is at least as old as the oldest exposed highland crust this suggests that the crustal dichotomy formed early in the geologic evolution of mars

new constraints on the thermal and volatile evolution of mars abstract the thermal and volatile evolution of mars has not been studied from the perspective of consistency with the preservation of the martian global dichotomy and with estimates of the elastic thickness over time we use three thermal evolution models for mars 1 stagnant lid 2 early plate tectonics followed by stagnant lid and 3 mantle overturn to calculate the amount of relaxation of the dichotomy boundary and elastic thickness values for noachian and hesperian aged terrains to explore a wide range of parameters we evaluate two different initial mantle temperatures and wet and dry rheologies our model results show that the relative water content of the crust has an effect roughly equal to 500 k variations of initial mantle temperature for all three thermal models a lower crust viscosity of 10 20 10 21 pa s during the first 0 1 ga after formation of dichotomy would allow for the preservation of the long wavelength topography of mars and fitting of the elastic thickness this viscosity range implies either wet cold 1500 k lower crust or dry hot 2000 k lower crust in noachian additional constraints are necessary to distinguish between the individual thermal models for the stagnant lid model neither the cold wet crust nor the hot dry crust agree with timing and amount of the crustal production hauck ii s a phillips r j 2002 thermal and crustal evolution of mars j geophys res 107 5052 moreover drying of the crust is required for this model in order to match the admittance elastic thickness at the hesperian amazonian boundary implying remelting of the crust the hot dry crust in the early plate tectonics model limits a plate tectonic epoch to only 100 200 myr and implies dry mantle which is in disagreement with water found in meteorites the cold wet crustal rheology implies the formation of crust during the plate tectonics regime because of the low crustal production during the stagnant lid regime for mantle overturn the temperature required for wet crust does not fit the original mantle profile while the dry crust does however in order to explain the initially hot thermal profile the crust must have been emplaced very fast generally dry crustal rheology does not fit low elastic values in the hesperian and implies either that rheology may differ between the southern and northern hemispheres wet in northern hemisphere and dry in southern hemisphere or that local weakening occurred wet crustal rheology fits well all elastic data except s hellas rim which may be anomalous mantle rheology is unconstrained by our modeling and can be either dry or wet

magellan and comparative planetology magellan data have been widely used to provide explanations for the tectonics and volcanism on venus however it is also important to compare our understanding of these processes on venus with the earth moon and mars one essential question concerning the behavior of these bodies is how does the heat get out internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism on the earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells the subduction of these cold plates at ocean trenches contributes about 75 of the mantle heat flux it is now quite clear that active plate tectonics is not occurring on venus the surface of venus is a single rigid plate as is the case for the moon mars and mercury prior to magellan the general consensus was that venus was in a near uniformatarian state with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet however crater studies from magellan images provide direct evidence for a catastrophic or episodic evolution of venus one explanation is that venus died tectonically 500 m y ago plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased an alternative explanation is that venus experiences episodic global subduction events two major questions concerning the earth remain largely unanswered particularly when compared with venus 1 why does the earth have continents 2 why does the earth have plate tectonics a major difference between the earth and venus is the presence of liquid water oceans during much of the evolution of the earth it has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the earth and the absence of well defined continental masses on venus supports this hypothesis it has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates plate evolution in time results in geometrical incompatibilities that require overlaps or gaps on the earth these incompatibilities lead to broad zones of continental deformation i e the western united states and china the absence of plate tectonics on venus suggests that rheologically soft continents are necessary for plate tectonics a reasonable conclusion is that the earth is the remarkable planet in terms of tectonics and volcanism not venus

natural annealing of dynamically recrystallised quartzite fabrics example from the cevennes se french massif central abstract quartzite samples from the sw cevennes french massif central have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric the studied quartzite is interlayered with turbiditic micaschist series and after regional deformation the whole sequence was intruded by the st guiral granodiorite at 1 3 kbar and up to 650 700 c recorded crystallographic preferred orientation cpo patterns of the dynamically recrystallised quartzite are classified as type i with a monoclinic symmetry and type ii with an orthorhombic symmetry type i is related to simple shear dominated deformation and type ii to apparent constriction these two fabric types are consistent with structural observation that a constrictive and coaxial deformation regime controlled the formation of non cylindrical and refolded folds in the metamorphic aureole which produced essentially static annealing cpo patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low to high quartz is triggered this inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content this study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems reactivate slip systems and amplify older cpo fabrics this conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occured

degree 1 mantle convection and the crustal dichotomy on mars the surface of mars consists of an old heavily cratered and elevated southern hemisphere and younger resurfaced and depressed northern hemisphere a feature often termed the crustal dichotomy the global crustal structure zuber et al 2000 revealed by topography and gravity data from the mars global surveyor spacecraft and the possible late formation of the boundary zone between the hemispheres mcgill and dimitriou 1990 have been proposed to indicate an endogenic origin for the dichotomy however degree 1 mantle convection that is required for any endogenic process to be viable cannot be produced with conventional mantle convection models schubert et al 1990 we have studied the role of radially stratified viscosity on mantle deformation by using rayleigh taylor instability analyses in a spherical shell geometry our analyses reveal that when mantle viscosity is stratified with a weak asthenosphere deformation at long wavelengths is more efficient than that at short wavelengths the weaker the asthenosphere the longer the wavelength at which the deformation is the most efficient a thicker asthenosphere also favors the deformation at long wavelengths both the rayleigh taylor instability analyses and numerical modeling of mantle convection show that degree 1 convection can be produced within the martian mantle provided that the mantle had a weak asthenosphere v500 km thick and v10 2 times weaker than the underlying mantle early in planetary history the degree 1 convection causes preferential heating of one hemisphere that may explain the primary features associated with the dichotomy in crustal structure fl 2001 elsevier science b v all rights reserved

a horseshoe crab arthropoda chelicerata xiphosura from the lower devonian lochkovian of yunnan china a single specimen of a new species of the synziphosurine kasibelinurus pickett 1993 is described from the lower devonian lochkovian xiaxishancun formation of yunnan province china the new species k yueya sp nov extends the geographic extent of the family kasibelinuridae from the australian palaeocontinent to the south china palaeocontinent and the stratigraphic range back some 50 ma from late to early devonian

the giant polygons of utopia northern martian plains much of utopia planitia is underlain by polygonal terrane most of the troughs defining the polygons are 250 m to 1 km wide typical diameters of polygons are 4 5 km these dimensions are orders of magnitude greater than the corresponding dimensions for familiar polygonal structures on earth consequently the earth structures are not valid mechanical analogues crater and superposition relationships indicate that trough formation was coeval with deposition of the polygonal terrane material based on crater age correlations with elysium volcanism and outflow channel activity either a volcanic or a sedimentary source for this material is possible tensile stresses responsible for the troughs were produced by cooling or dessication shrinkage plus differential compaction over a rough buried surface that is the structurally disrupted remnant of the cratered highlands shrinkage alone will produce polygons orders of magnitude smaller than those present but differential compaction will reflect the scale and pattern of the buried topography and thus control the scale and pattern of the giant polygons of utopia

origin of the martian global dichotomy by crustal thinning in the late noachian or early hesperian the marked dichotomy in topography surface age and crustal thickness between the northern lowland and southern upland of mars has been explained as due to an initially inhomogeneous crust a single mega impact event several overlapping large basin impacts and first order convective overturn of the martian mantle all of the published hypotheses propose that the dichotomy was formed early in martian history before the end of the primordial heavy bombardment a primordial origin is inherent in the initial crustal inhomogeneity hypothesis and required for both impact hypotheses endogenic hypotheses are not so constrained geological data indicate episodes of fracturing and faulting in the late noachian and the early hesperian this fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland igneous activity also peaked in the late noachian and early hesperian these data suggest a tectonic event near the noachian hesperian boundary characterized by enhanced heat loss and extensive fracturing including formation of the faults that define much of the highland lowland boundary we argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume

impact constraints on the age and origin of the lowlands of mars 1 visible and buried impact basins seen as quasi circular depressions qcds in mola data provide important new constraints on the age of the martian lowlands the buried lowlands are no younger than early noachian at least as old as the oldest exposed visible surface units in the highlands a model absolute age for these buried lowlands is 4 04 4 11 gy or earlier but similar model ages for the largest lowland basins are older yet 4 08 4 18 gy the lowland crust both formed and became low no later than 500 million years after mars formed and likely even earlier this constrains models for the origin of the fundamental crustal topographic dichotomy on mars mechanisms which operated both early and quickly during the earliest history of mars e g large impacts may be more likely than those requiring extended periods of time i e endogenic models

buried impact craters a topographic analysis of quasi circular depressions utopia basin mars 1 many quasi circular depressions qcds revealed in the mars orbiter laser altimeter data set have no obvious structural representation in visible images of mars it has been suggested that these qcds are surface representations of buried impact craters and on the basis of this assumption cumulative count versus diameter plots of qcds in the northern lowlands have been constructed to date the buried lowland floor if the qcds are in fact buried impact craters their surface relief should scale directly with their diameter this relationship holds true for 115 qcds with diameters ranging from 7 to 100 km within roughly 1500 km of the center of the utopia basin the coefficient of determination r2 of the relationship more closely approaches 1 when qcds in zones of comparable cover thickness are evaluated the surface relief versus diameter relationship of 55 randomly selected pseudo qcds was also determined their poor correlation indicates that the direct relationship of surface relief to diameter of actual qcds is due to differential compaction of a cover material over buried impact craters and is not just a random effect of qcd diameter the slope of the trend of the qcds surface relief versus diameter when divided into zones of comparable cover thickness can be used to determine which zones have thicker or thinner cover

the utopia basin revisited regional slope and shorelines from mola profiles mola profiles indicate that utopia planitia including the utopia basin has been tilted northward by 0 025 since formation of the basin but no later than mid late hesperian this tilt could be a relic of the endogenic formation of the global dichotomy in late noachian to early hesperian by a process involving subcrustal erosion in the lowland the utopia basin survives as a deep circular depression within utopia planitia along its south flank are several horizontal or near horizontal terraces and benches that may be shorelines of an extensive northern lowland ocean and a more restricted lake within the utopia basin the large size up to 200 km wide of these putative shorelines represents a major geomorphic problem to be solved

late mesozoic strike slip movement on the border ranges fault system in the eastern chugach mountains southern alaska preliminary studies of mylonitic rocks from the haley creek metamorphic assemblage in south central alaska suggest that the border ranges fault system was a strike slip ductile shear zone during part of its late mesozoic history evidence for this strike slip event is limited to the hanging wall of the border ranges fault system where deformed crystalline rocks were apparently subjected to a retrograde mylonitic deformation prior to subduction related underthrusting of the latest cretaceous valdez group but subsequent to a late jurassic plutonic event the strike slip interpretation contrasts with previous interpretations which related mylonitic fabric development to multiple deformational events during underthrusting of the chugach terrane a conflict resulting from the interpretation of lineation as an intersection lineation parallel to the y axis of strain versus an extension lineation evidence for horizontal stretching on a vertical flattening plane includes 1 a parallelism between all lineations including intersection lineations and unequivocal extension lineations developed within systems of strike slip ductile shear zones 2 extensional fragmentation of feldspars parallel to lineation 3 local rodding and development of l tectonites 4 boudin axes at a high angle to lineation 5 crystallographic preferred orientations of quartz and 6 syn kinematic crack seal veins with fibers parallel to lineation the local parallelism between intersection lineations and extension lineation is interpreted as a consequence of sheath folding in a high strain zone and we suggest that it is this complication that is largely responsible for confusion on the kinematic history despite evidence for a strain pattern consistent with a strike slip shear zone evidence for noncoaxial strain and a clear sense of shear is elusive nonetheless coaxial deformation seems unlikely given the structural position of the mylonitic rocks along an ancient plate boundary regional correlation of rock types is suggestive of a dextral offset of at least 50 km although this offset could be due to younger faulting thus further work will be needed to clearly distinguish a dextral versus sinistral sense of shear important absolute age problems still remain and thus the tectonic significance of the strike slip event is uncertain however the general late mesozoic age places the event within an important time period in the history of the northern cordillera thus the strike slip event may have 1 played a role in a transform history preceding early cretaceous thrusting along the border ranges fault system 2 been a complication from collision of wrangellia with north america and 3 been important in the northward transport of outer cordilleran terranes that is implied by paleomagnetism

effects of the borealis impact on the mantle dynamics of mars abstract we investigate the effects of a huge impact called borealis impact possibly occurred at 4 5 ga and created the northern lowland of mars on the thermal evolution of mars using 2d axi symmetric convection in a spherical shell of temperature and pressure dependent viscosity a suite of models are calculated to study effects of different physical parameters on the impact induced mantle dynamics we use the conventional scaling laws of basin diameter and impactor size the impact shock pressure in the interior of mars and the foundering mechanism of impact heating to calculate the temperature increase in the mantle the mantle is allowed to melt as its temperature surpasses the solidus to assess the effects of the impact on the thermal evolution of mars we compare the thermal evolution of the models with impact to the equivalent models having no impact it is shown that the impact increases the rate of crustal production for a period of about 50 m y after the impact the initial impact induced melt volume is about 3 44 108 km3 whereas the impact induced mantle upwelling beneath the impact site results in the production of an extra 8 8 108 km3 of melt the models with no impact also produce melt in due time and the total volume of melt at 4 6 gyr is not significantly sensitive to the impact this observation suggests that large impacts can significantly alter the rate of crustal production of mars but have minor effects on the total volume of crust or melt production

a new estimate of the conditions for transition from basal a to prism c slip in naturally deformed quartz abstract quartz c axis fabrics of naturally deformed quartz in metacherts from the ryoke metamorphic belt in the yanai district southwestern japan suggest that the prism c prism a and basal a slip systems were activated during the plastic deformation under high temperature metamorphic conditions with increasing temperature quartz c axis fabric patterns change from girdle type through x maximum with a faint crossed girdle to x maximum the dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature closely correlated with the change in quartz c axis fabric the fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size moreover observations with transmission electron microscopy tem clearly identify the dominant activation of c slip through the presence of basal subgrain boundaries and free dislocations with the 0001 burgers vector in the quartz from the metachert samples exhibiting x maximum c axis fabric based on these observations it is concluded that the mechanism switch from basal a to prism c slip systems occurred with increasing temperature in the metacherts the transition temperature is petrologically estimated to be ca 550 600 c at the natural strain rate

subduction origin on early earth a hypothesis i propose a hypothesis for the origin of subduction on early earth that is directly coupled with mantle upwelling and the formation of mafi c crust the hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes broad quasi cylindrical mantle upwelling and large bolide impact respectively leading to subduction and spreading two signature processes of modern plate tectonics the spatial and temporal intersection of these processes could have occurred variably across early earth and thus subduction could have begun at different locations and times globally the hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive both factors to a fi rst order are a function of planet size

mapping and modeling of magnetic anomalies in the northern polar region of mars vector crustal magnetic field maps of the northern polar zone 60 n to 90 n are constructed from selected mars global surveyor magnetometer data obtained during the period from may 28 to september 13 1998 two medium anomalies amplitudes 50 nt at 170 km altitude are mapped in locations consistent with earlier studies no visible surface features correlate with the anomalies suggesting that the sources lie beneath the visible veneer of polar deposits and volcanic lava flows if so then they formed prior to the immediate end of the heavy bombardment upper hesperian period modeling of anomaly vector field components combined with independent constraints on the depth to the curie isotherm yields lower limits on bulk magnetization intensities 0 4 0 9 a m that are significantly greater than those measured for martian snc meteorite samples rocks that contain substantially more titanomagnetite than snc meteorites or that contain magnetic phases in addition to titanomagnetite possibly resulting from hydrothermal alteration are therefore suggested alternatively remanence acquisition in a field of earthlike intensity 50 t rather than in the relatively weak inferred paleointensities for snc meteorites 1 10 t would also help to explain the relatively strong inferred remanent magnetizations the approximate south paleomagnetic pole positions corresponding to these two anomaly sources are located in a region between olympus mons and the present north rotational pole this region is adjacent to the approximate location predicted by melosh 1980 for the paleopole prior to the formation of the tharsis gravity anomaly

quartz fabric evolution within the adra nappe betic cordilleras spain abstract the adra nappe alpujarride complex betic cordilleras is an imbricated thrust sheet of metamorphic rocks at the regional scale changes in c axis fabrics of deformed quartzites in the nappe are fully consistent with the strain path inferred from the microstructures in the upper part of each imbricate where the reference frame s 2 l 2 is weakly deformed by open f 3 folds quartz c axis fabrics are characterized by symmetrical patterns of small circle girdles around z increasing strain towards the basal thrust faults is marked by new quartz fabrics developed during a pervasive d 3 mylonitization these fabrics correspond to asymmetrical single girdles or asymmetrical type ii cross girdles these changes in quartz fabric can be correlated with the progressive replacement of an initial coaxial flow d 2 by a non coaxial flow d 3 near the thrust fault as a result of the movement of the adra nappe

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

origin of the martian crustal dichotomy evaluating hypotheses abstract any hypothesis for the origin of the martian global dichotomy should survive three elementary tests 1 it must account for the observed plan shape and apparent depth of the martian northern lowland 2 it must be physically consistent and 3 it must be compatible with available geological and geophysical data at present there are three contending types of hypotheses for the origin of the dichotomy creation by some endogenic process or processes creation by a single mega impact or creation by several overlapping large impacts none of these hypotheses can survive all three tests without the incorporation of additional processes the endogenic and mega impact hypotheses require the presence of additional impact basins to explain many of the topographic details of the martian northern lowland and of the dichotomy boundary and the mega impact hypothesis probably also requires extensive primordial erosion the multiple impact hypothesis requires an additional process or processes to account for the large portion of the northern lowland that is external to the rims of the basins inferred to be the cause of the lowland while we believe that aspects of the multiple impact hypothesis are required to account for some topographic details of the boundary and of the lowland other processes appear better able to account for the dichotomy as a whole these could include an early mega impact endogenic processes or some combination of these

the effects of a deep mantle endothermic phase change on the structure of thermal convection in silicate planets previous studies of the effects of silicate phase transitions on mantle convection have tended to use numerical models in which the phase transitions have the same depths as found in the earth s mantle or in which a single transition is placed at a depth equal to half the layer thickness this study presents a systematic investigation designed to examine the effects of an endothermic phase transition near the core mantle boundary of terrestrial type planets the results indicate that a deep mantle endothermic phase change may trap the lower thermal boundary layer allowing only one upwelling to penetrate the phase transition in these situations the deep endothermic phase change excites a dipolar convection pattern this mechanism may have a role in the degree one power observed in the distribution of the earth s hotspots and the development of the martian crustal dichotomy

tectonic interpretations of systematic variations in quartz c axis fabrics across the thompson belt the thompson belt is a linear ne trending belt of archean and early proterozoic rocks forming the boundary between the archean superior and early proterozoic churchill provinces the gneisses in the belt record deformation which took place during the early proterozoic hudsonian orogeny quartz c axis fabrics were measured from gneisses collected along a transect across the western portion of the thompson belt well developed quartz c axis fabrics were found in virtually all gneisses even in coarse grained quartz poor and weakly lineated rocks several distinctive c axis fabric types were found to occur throughout the area and can be related to specific deformation events different c axis fabric types can be linked to the activation of separate glide systems within quartz at different stages of the orogeny polar asymmetry within fabrics that should be symmetric if they had formed from a population of grains with random orientations are evidence that the present c axis fabric has overprinted an earlier preferred crystallographic orientation c axis fabric variations in the thompson belt show similarities to variations observed in the saxony granulite region results of this study indicate that quartz c axis fabric analysis can contribute to the knowledge of the regional deformation in the thompson belt regional quartz c axis variations may prove to be useful for studying the deformation history of other high grade gneissic terrains

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

strain localization and large scale block rotation in the lower continental crust kataragama area sri lanka in sri lanka a suite of rocks is regionally exposed which once formed a part of the middle to lower crust of a collisional orogen with about doubled crustal thickness a subhorizontal to gently west dipping major thrust zone highland basal thrust crosses sri lanka from nne to ssw it separates a granulite facies upper unit the highland complex from an amphibolite facies lower unit the vijayan complex with a displacement of more than 300 km the basal highland complex was migmatized during thrusting rheological contrasts between dry granulites and migmatites were high in the presence of partial melts granulite facies fragments within the migmatites vary from the decimetre to kilometre scale granulites in the area of kataragama formerly interpreted as the kataragama klippe are now identified as a large scale raft within the migmatites which is completely separated from the overriding highland complex during thrusting strain was concentrated in the migmatized basal parts of the highland complex after crystallization of the migmatites thrusting continued and strain was still localized within the migmatites therefore granulite facies parageneses structures and textures are preserved in the granulite rafts the deviation of the granulite facies structures in the kataragama raft from the general trend in the highland complex is interpreted to indicate counterclockwise rotation of the whole raft by about 90 around a subvertical rotation axis

estimates of martian crustal thickness from viscous relaxation of topography isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients which may cause the lower crust to flow and reduce the relief areas of thicker crust are generally associated with more rapid relaxation of topography on mars topographic features such as impact basins and the hemispheric dichotomy have survived for 4 gyr we use a finite difference representation of depth dependent non newtonian lower crustal flow to investigate how topography decays with time for a dry diabase rheology total radiogenic concentrations 80 of terrestrial values and crustal radiogenic concentrations similar to terrestrial basalts we find that an upper bound on the mean planetary crustal thickness is 100 km in the probably unrealistic case where all the radiogenic elements are in the crust this maximum crustal thickness can be increased to 115 km the main uncertainty in these results is the total radiogenic abundances on mars comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow both hellas and the dichotomy are isostatically compensated if the mechanism is airy isostasy then the lower bound on mean crustal thickness is 30 km crustal thicknesses of 30 100 km on mars can be produced by mid ocean ridge spreading at potential temperatures of 1350 1600 c however for such crustal thicknesses the lithosphere is likely to be positively buoyant making subduction difficult

the structure and evolution of ancient impact basins on mars the number of martian impact basins once thought deficient owing to a low number of impactors has been significantly increased through the identification of structural imprints that remain even after nearly complete erasure of basin massifs five ancient multiringed impact basins were selected for detailed study and exhibit a systematic pattern of rejuvenation of the multiring plan through endogenic modification concentric zones of unstable terrain are believed to reflect deep seated impact related fractures that have localized regional igneous processes in a manner analogous with the basin controlled distribution of basalts on the moon most martian outflow channels were found to originate along the unstable ring zones hydrothermal melting and temporary confinement of channel source materials at sites analogous to lunar sinuous rille source regions are proposed to play an important role in the distribution of martian outflow channels ancient basins also can be delineated by the concentric pattern of topographically and structurally controlled narrow valley networks that are particularly abundant along the outer rings where basin ejecta deposits occur remnant massifs and scarps commonly exhibit extensive furrowing by narrow valley networks perhaps due to a different paleoclimate or sapping during a period of geothermal melting of trapped ices the systematic patterns of valley networks stable and unstable annuli scarps and massifs provide bases for interpreting impact structure in general and for recognizing multiringed structures elsewhere the chryse basin in particular is proposed to represent a major multiringed structure that controlled the formation and distribution of eastern valles marineris and the chaotic terrains of margaritifer sinus in response to volcanism tectonism related to tharsis we conclude that 1 mars is probably not deficient in large body impactors 2 the multiring patterns of basins represent deep seated fracture zones that can be reexposed through subsequent volcanism hydrothermal activity sapping and differential erosion 3 many martian terrains and features e g chaotic terrains and outflow channels are controlled by ancient basins and 4 the resurrection and destruction of impact basins reveal an important third dimension depth for interpreting impact basin formation

sedimentary deposits in the northern lowland plains mars the lowland plains on mars have surfaces marked by large polygonal fracture patterns it was recently proposed that the fracture patterns were developed on sedimentary deposits from outflow channels we support this hypothesis because of the following observations 1 polygonal fracture patterns tend to occur in low areas on mars that apparently received influx of sediments the area of northern plains in which the fractures occur also forms a regional low 2 the fracture patterns are concentrated near reentrants of the plains projecting into the southern highlands not only are these reentrants associated with outflow channels but channels and polygonally fractured deposits have similar ages 3 sinuous ridges at the mouths of martian outflow channels resemble ridges in antarctic ice streams and ice shelves the similarity suggests that the martian material was transported from the southern highlands toward the northern plains as the antarctic material is transported from the continent toward the adjacent ocean overall it appears that a massive transfer of material took place midway in martian history either through the outflow channels or elsewhere along the northern highland scarp and that this material probably formed the deposits now characterized by polygonal fracture patterns

progressive development of lattice preferred orientations lpos of naturally deformed quartz within a transpressional collision zone panafrican orogen in the eastern desert of egypt abstract lattice preferred orientations lpos of quartz were used to establish differences in deformation geometry finite strain and temperature within a transpressional collision zone within the panafrican orogen in the eastern desert of egypt metamorphic and or magmatic core complexes in the area are bordered in the nw and sw by ductile sinistral nw trending strike slip zones and low angle normal faults lanfs simultaneous activity of both fault systems suggests bulk w e shortening coeval with orogen parallel extension displacement partitioned into orogen parallel sinistral strike slip faults and lanfs this study compares both quartz lpos in shear zones and normal faults from south to north quartz c axis data show a continuous evolution along orogen parallel strike slip faults from maxima in y with a slight tendency to oblique single girdles at the margins of the wadi beitan and hafafit complexes to asymmetric crossed girdles and oblique single girdles along the margins of the sibai and meatiq complexes the nw directed lanfs to the nw of the hafafit and the se directed lanfs to the se of the sibai show maxima in y the se directed lanf at the se margin of the meatiq complex shows symmetric crossed girdles indicating coaxial deformation geometry oblique single girdles and maxima in y occur in the southern part of the orogen whereas crossed girdle distributions dominate in the northern part the variation in quartz c axis patterns is explained in terms of decreasing metamorphic grade during deformation from the s medium to high grade to the n low grade and decreasing finite strain this is in accordance with the general progression of transpressional tectonics and exhumation of core complexes from s to n

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

tectonic consequences of martian dichotomy modification by lower crustal flow and erosion the martian hemispheric dichotomy shows compressional features a few hundred kilometers south of the dichotomy boundary and contemporaneous extensional features along the boundary two processes that may have contributed to the observed deformation are lateral lower crustal flow and erosion crustal flow causes a decrease in crustal thickness in the south and an increase in the north the result is compressional surface stresses in the highlands and extensional stresses around the dichotomy boundary the magnitude location and timing of the predicted stresses are consistent with the observed tectonic features and strongly suggest that moderate lateral flow occurred for the nominal heat flux and rheology sufficient lateral flow is generated for a mean crustal thickness of 70 km erosion generates extension in the south and compression in the north opposite to what is observed either erosion only resulted in local as opposed to regional distribution of highland material or the rigidity of the lithosphere was sufficiently high at the time of erosion that significant stresses did not develop

the kinematic and geodynamic significance of the atacama fault zone northern chile abstract the atacama fault zone afz is the dominant feature in the structure of the north chilean coastal cordillera left lateral displacement took place along its system of longitudinal faults during the jurassic and early cretaceous this development was contemporaneous with arc magmatism and was later reactivated resulting in a steep normal fault strike slip movements along the afz consist of two sets of ductile shear zones of different ages one jurassic formed under amphibolite facies conditions the other early cretaceous with greenschist facies mylonites structural asymmetries point to a sinistral sense of shear in both sets the afz can be interpreted as a magmatic arc structure which accommodated the oblique subduction of an oceanic plate trench linked strike slip fault the sinistral sense of shear is consistent with reconstructions of late jurassic to early cretaceous plate configurations in the se pacific

inverted fluvial features in the aeolis zephyria plana western medusae fossae formation mars evidence for post formation modification 1 the aeolis and zephyria plana contain the western most portion of the medusae fossae formation mff an enigmatic and extensive light toned deposit located in the martian equatorial region and dated from the hesperian to amazonian epochs this area hosts a large population of sinuous ridges srs interpreted as inverted fluvial features formed by precipitation indurated by chemical cementation buried by subsequent deposition and finally exhumed this interpretation of srs as uniformly fluvial represents a modification to an earlier hypothesis for one particular sr of possible glaciofluvial i e esker formation these srs provide a tool to investigate the degree and character of post fluvial modification processes in this region we combined digital terrain models made from context camera ctx and high resolution imaging science experiment hirise stereo image pairs with individual data points from the mars orbiter laser altimeter mola to estimate relief cross sectional profiles longitudinal profiles and slope directions of selected srs longitudinal profiles of several srs display undulations with amplitudes of up to order 100 m while some of the lower amplitude undulations may be due to differential erosion undulations having amplitudes in excess of sr relief require alternative explanations our combined morphologic and topographic analysis suggests that multiple post flow processes including compaction of the deposits and tectonic displacements have modified the original sr profiles specification of the type s and magnitudes of these modification processes will contribute to understanding both the potential of post flow modification of fluvial profiles elsewhere on mars as well as the nature and properties of the mff

using the viscoelastic relaxation of large impact craters to study the thermal history of mars we simulate the long term deformation of martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography using the finite element method and a viscoelastic rheological model we model the deformation of more than 30 large craters and quasi circular depressions qcds in the diameter range of 200 500 km in both the northern lowlands and southern highlands we determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust mantle boundary ranges from 40 to 90 mw m 2 our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface by applying various viscous creep parameters for hydrous and anhydrous rheologies we demonstrate that mars s interior is wet to a certain degree which is consistent with other estimates since craters and qcds are distributed fairly equally on the surface of the red planet this study provides a less regionally biased picture of the thermal history of early mars than in previous studies based on our results the ancient average background heat flux in the northern lowlands was higher than that of the southern highlands which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle noachian

lobate scarps and the martian crustal dichotomy landforms reflecting crustal shortening are found in the ancient highlands of the eastern hemisphere of mars these structures referred to as lobate scarps are interpreted to be thrust faults lobate scarps occur near and are oriented roughly parallel to the martian crustal dichotomy a major geologic and topographic boundary that divides the heavily cratered highlands from the relatively smooth featureless northern lowlands the long and short wavelength topography of lobate scarps in the northern terra cimmeria amenthes region have been analyzed using photoclinometry earth based radar altimetry and mars orbiter laser altimeter data the measured relief of lobate scarps in this region ranges from 110 to 1230 m and they occur on gentle regional slopes that dip both toward and away from the dichotomy estimates of the horizontal shortening across the lobate scarps studied range from roughly 0 24 to 2 6 km n 9 assuming fault plane dips of 25 the displacement length d l relationships of thrust faults associated with the lobate scarps are consistent with those observed for terrestrial fault populations the compressional strain in the heavily cratered highlands near the dichotomy determined using the d l data for the lobate scarps is estimated to be 0 17 topographic data indicate that the dichotomy in the northern terra cimmeria amenthes region has a distinct topographic signature the spatial and temporal relationship of the lobate scarps to the boundary suggests that they are related to its formation supporting models for a tectonic origin of the crustal dichotomy

convection underneath a crust inhomogeneously enriched in heat sources application to martian mantle dynamics abstract two and three dimensional spherical convection models are presented which investigate the convection structure beneath a crust inhomogeneously enriched in radioactive elements the resulting convection patterns show a focusing of upwellings under the enriched part of the crust and downwellings under the not enriched part that maintain large scale horizontal motion from the warmer to the cooler region beneath the lithosphere and vice versa above the core mantle boundary this flow pattern is observed in all examined models independent of the mantle heating mode it is however more pronounced for models with a considerable amount of internal heating than for models with almost total bottom heating thermal blanketing of the mantle beneath the enriched part of the crust plays only a minor role in the lateral heat flux distribution within the lithospheric mantle and is almost insignificant for the surface heat flux distribution which is dominated by crustal heat generation implications of the results for mars are discussed

degree 1 convection in the martian mantle and the origin of the hemispheric dichotomy 1 the surface of mars appears dramatically different between the northern and southern hemispheres any endogenic origin for this hemispheric dichotomy must involve a pattern of mantle convection that reflects the shape of the dichotomy primarily spherical harmonic degree 1 we investigated two mechanisms by which degree 1 convection may be initiated in the martian mantle 1 an endothermic phase change near the cmb and 2 viscosity layering in the mid mantle using two dimensional 2 d and 3 d spherical finite element convection models we explored the conditions under which each mechanism can produce degree 1 structures the phase transition is only effective at generating degree 1 structures when the mantle viscosity is constant or weakly temperature dependent activation energy 100 kj mol but the degree 1 pattern requires several billion years to develop increasing convective vigor in phase change models leads to reduced wavelengths for convective structures degree 1 convection can also develop in a layered viscosity mantle with temperature and depth dependent viscosity an overall sublithospheric radial viscosity variation of a factor of 100 including a factor of 8 25 jump in the midmantle can lead to formation of degree 1 structure in a timescale ranging from 100 my to several hundred my consistent with the timescale for the formation of the dichotomy neither convective vigor nor the internal heating rate greatly affects the formation of degree 1 structures we propose that degree 1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy

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

the mola topographic signature at the crustal dichotomy boundary zone on mars mola data collected during the aerobraking hiatus provide good longitudinal sampling of the crustal dichotomy boundary zone in eastern mars and reveal a significant topographic signature associated with the transition from cratered highlands to lowland plains total relief between the two major units is always 2 5 km and sometimes 6 km median elevation differences are about 4 km regional slopes are very low 0 02 in both cratered terrain south and in lowland plains north of the boundary within the transition zone where cratered terrain changes to detached plateaus mesas and knobby terrain slopes are 50 100 times steeper and over a few hundred kilometers average about 1 the topographic signature of the boundary zone is best described as a 2 4 km step function between two nearly flat surfaces similar in slope and amplitude to some terrestrial passive margins this supports previous conclusions of a significant difference in crustal properties thickness composition or both between the lowland plains and cratered terrain and suggests this difference occurs over a relatively short distance of a few to several hundred kilometers

thrust faults along the dichotomy boundary in the eastern hemisphere of mars 1 mars orbiter laser altimetry mola data have been used to characterize lobate scarps along the dichotomy boundary in the eastern hemisphere of mars these structures are the surface expression of thrust faulting of the southern highlands displacement on the thrust faults is generally less than 1 km except in the case of the amenthes rupes thrust fault where the maximum displacement is on the order of 3 km the ratio of maximum displacement to fault length of the population of thrust faults is 6 2 10 3 n 26 consistent with previous estimates for martian thrust faults based on photoclinometrically derived topography and other planetary thrust fault populations thrust faulting is roughly coincident with late noachian to early hesperian fracturing and normal faulting along the dichotomy boundary this suggests that the formation of the present day dichotomy boundary involved compressional and extensional deformation the long wavelength topography suggests that the dichotomy boundary in the eastern hemisphere may have been formed by flexure of the southern highlands lithosphere lithospheric flexure alone cannot account for the thrust faulting of the highlands along the dichotomy boundary a combination of stresses due to bending erosion and global contraction may have formed the lobate scarp thrust faults

crater size frequency distributions and a revised martian relative chronology a revised martian relative chronology is determined which dates geologic units with respect to the end of the period of heavy bombardment this analysis differs from previous studies by using viking 1 2m photomosaics to map all 25 826 craters 8km diameter which postdate the lithologic units making up the martian surface the relative plotting technique is used because it shows changes in the shape and density of the crater size frequency distribution curves more clearly than cumulative plots the changes in shapes of the distribution curves are interpreted to be due to two different impacting populations one dominating during the heavy bombardment period and the other emplaced during the post heavy bombardment period the 8 km diameter limit was chosen to exclude the majority of secondary craters and to minimize obliteration effects acting over the age of the planet the results obtained are average ages valid for units with typical thicknesses between about 1 and 3 km and extending over an area of about 105 km2 the shapes and densities of the crater distribution curves indicate that approximately 60 of the martian lithologic units in which these craters occur formed during the period of heavy bombardment many of the small volcanoes those generally less than 200 km diameter including most in the tharsis and elysium regions date from this period the hellas and argyre impacts bracket the average age of the intercrater plains in the highlands the ridged plains distribution curves indicate formation during the period of heavy bombardment but their lower crater density indicates that they are the last geologic unit to retain the scars from this population of impactors they therefore likely formed near the end of heavy bombardment the remaining 40 of martian terrain units show distribution curves indicative of emplacement since the end of heavy bombardment the wide range in crater densities among different regions of plains indicate a range in formation ages although the general view of martian geologic evolution does not differ significantly from previous studies the present analysis explicitly states the relationship of numerous geologic units to the period of heavy bombardment and has revised the relative ages of localized regions across mars

channel slope reversal near the martian dichotomy boundary testing tectonic hypotheses abstract faults along the martian dichotomy boundary are evidence of tectonic activity which by analogy with terrestrial tectonism may cause changes in or even reversal of fluvial longitudinal profiles in the eastern hemisphere between 30 e and 150 e this tectonic activity has been hypothesized to result from lower crustal flow or from lithospheric flexure for which loading e g by material deposition of the northern lowlands is a possible cause watters 2003a nimmo 2005 the topographic slope changes resulting from these two different mechanisms are distinct and can provide a means for distinguishing between them although other processes may complicate interpretations of reversed longitudinal profiles two fan shaped networks of inverted channels are located near 150 e in the aeolis dorsa region just north of the dichotomy boundary their original flow directions inferred from planform morphology suggest flow to the northeast in contrast to their current longitudinal profiles sloping down to the southwest this contrast indicates slope reversal we investigate the lower crustal flow and flexure mechanisms for slope reversal by testing three different hypotheses 1 lower crustal flow 2 flexure caused by material erosion from the highlands and deposition in the lowlands and 3 flexure caused by highland erosion and deposition in the lowlands plus deposition of the medusae fossae formation in the lowlands we test these three hypotheses by comparing the inferred magnitudes of the slope reversals with predicted slope changes from geophysical models for these processes taking the possibility of non tectonic i e collapse processes into account our results suggest that among these three models the slope reversal is most consistent with the predicted tectonic response to erosion and deposition of highland material in conjunction with deposition of the medusae fossae formation contrary to previous findings our results do not support the mechanism of lateral crustal flow although they do not rule it out because lower crustal flow and erosion may both have been operating but on different time scales the result of this hypothesis testing provides insight into the evolution of the dichotomy boundary and martian crust in this location

large impact basins and the mega impact origin for the crustal dichotomy on mars the origin of the fundamental crustal dichotomy on mars remains the outstanding unsolved problem in martian geologic evolution the hypothesis that this dichotomy is due to a single giant mega impact early in martian history is tested by comparing the observed number of large impact basins on mars with the number expected from a d 2 distribution this test is based on the assumption that the mega impact borealis basin is the largest member of such a distribution the difference between observed and expected basins is the number of missing basins that might have been obscured by subsequent geologic events if the borealis basin were the largest member of a d 2 impact population there would be a very large number of missing basins on mars 1 larger than 5000 km 10 larger than 2000 km and 40 50 larger than about 1000 km diameter the area occupied by these missing basins allowing for overlap is too large to be hidden by the younger surface units a large population of presently unrecognized large impact basins would have to be located in the well preserved cratered highlands by contrast if chryse is the largest member of a d 2 impact population the more modest number of missing basins could in principle be confined to areas of mars that have been resurfaced or reworked by subsequent geologic processes a possible alternative to the mega impact hypothesis is that the crustal dichotomy is due to the cumulative effects of multiple overlapping large but not giant impacts

textures and c axis orientations of deformed quartz crystals from porphyric dikes of the alpine root zone western alps deformation textures and c axis preferred orientations of quartz phenocrysts from porphyric dikes of the alpine root zone have been formed under conditions of greenschist to low amphibolite facies it is shown that the deformation textures as shear planes and kink bands boudins grain boundary sutures subgrains coarse and fine recrystallized grains deformation lamellae and fractures are developed in a chronological sequence during first increasing and later decreasing temperatures

major late triassic strike slip displacement in the seven devils terrane oregon and idaho a result of left oblique plate convergence abstract the shortening direction in rocks deformed in collision or subduction zones is not directly related to the plate convergence vector rather it is perpendicular to the collision zone or subduction zone even in cases where plate convergence is oblique the component of convergence parallel to the subduction collision zone is expressed by strike slip displacement in the arc region behind the subduction zone such strike slip shear zones have been recognized in the seven devils terrane of northeastern oregon and adjacent idaho one of these the oxbow shear zone consisting of cataclasite mylonite and ultra mylonite trends northeasterly from oxbow oregon to cuprum idaho the original rock types of the shear zone were plagiogranite gabbro diabase bassalt and keratophyre the age of the mylonitization is constrained by 40ar 39ar dates as late triassic meso and microscopic structures textures and quartz c axes fabrics indicate that the shear zone was formed by left lateral strike slip motion a minimum left lateral displacement of 65 km has been estimated but the true displacement may have been much larger the oxbow shear zone is interpreted as an intra arc strike slip zone of the seven devils terrane related to left oblique plate convergence during the triassic

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

utopia and hellas basins mars twins separated at birth 1 using topography and gravity data as constraints we formulate spherical harmonic thin elastic shell models to determine the subsurface structure of the hellas and utopia basins for hellas we show that our model is consistent with the elastic thickness results of mcgovern et al 2002 2004 the thin elastic lithosphere at the time of formation implies that hellas is close to isostatic since utopia formed earlier we argue that an isostatic assumption is justified for the utopia basin before it was filled from this supposition we derive a system of equations that allows us to solve for the amount of fill the prefill topography and the amount of flexure due to the fill within the utopia basin an analysis of the parameter space shows that the fill density and the amount of fill is strongly dependent on the elastic thickness at the time of infilling a thinner elastic lithosphere favors a denser fill while a thicker lithosphere will allow for less dense material likewise larger crustal thickness values lead to smaller fill density values the presence of quasi circular depressions interpreted as impact craters within the utopia basin indicates that the majority of the material within utopia was deposited prior to 4 04 4 11 ga the early timing for the deposition combined with the heat imparted by the basin forming event argues for a thinner lithosphere which could in turn suggest fill densities that are more consistent with a volcanic load than with pure sediment or ice rich material these results are supported using an alternative method of determining the amount of fill and flexure within utopia this model assumes that hellas and utopia were initially identical and that the only difference in their subsequent evolution was the addition of material in the utopia basin the volume of material needed to fill utopia is immense on the order of 50 million km3 or more the high density obtained for the fill requires that it contain a large igneous component the source of which is problematic relaxing the isostatic assumption to a reasonable degree perturbs the density bound only slightly

stealth quasi circular depressions sqcds in the northern lowlands of mars 1 while qcds are quasi circular depressions revealed in the mars orbiter laser altimeter mola gridded data set stealth qcds sqcds are those qcds that have no obvious structural representation in visible images of mars it has been suggested that sqcds are surface representations of buried impact craters and on the basis of this assumption size frequency distributions of all qcds in the northern lowlands have been constructed to date the buried lowland floor expanding upon previous studies of stealth qcds around the utopia basin 446 sqcds with diameters ranging from 7 to 115 km have been identified in the northern lowlands of mars the direct relationship of sqcd surface relief to diameter indicates that stealth qcds form due to differential compaction of a cover material over buried impact craters a comparison of sqcd location to the geologic map of the northern lowlands reveals that stealth qcds are only located in geologic units predicted to be capable of differential compaction consistent with models of their formation the slope of the trend of the sqcds surface relief versus diameter has a coefficient of determination r2 approaching 1 when sqcds are evaluated by geologic unit and by regions of inferred comparable cover thickness trend slope analysis also indicates that the lowland cover material is thickest in the scandia region and the center of the borealis and utopia basins and thinnest in isidis planitia and north of arabia terra

quantification of waste morphology in martian fretted terrain extensive geomorphic mapping in the northern martian fretted terrain of deuteronilus and protonilus mensae shows details of the wastedominated landform association in this region wasting appears to have occurred along the cratered terrain boundary ctb at the expense of old cratered terrain north of the boundary remnants of old cratered terrain occur as island mesas surrounded by extensive debris aprons quantitative study using principal components analysis pca reveals distinct spatial variations in the morphology of waste forms in the egion and supports the qualitative interpretations made during geomorphic mapping these organized spatial patterns occur with respect to longitude and latitude and therefore with reference to distance from the ctb and provide clues to the degradational history of the region analyses of the individual landforms irrespective of their location to the ctb indicate strong structural control of their form mesas appear to evolve from equant forms to narrow ridges with large debris aprons after they become detached from the old cratered terrain the success of pca in explaining patterns in degradational landforms suggests that pca may be an important technique in quantifying the geomorphic evolution of the martian surface

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations

the kinematic insignificance of mineral lineations in a late jurassic thrust and fold belt in eastern oregon u s a abstract the kinematic significance of mineral lineations in tectonites has been discussed many times in the past while in some instances mineral lineations are parallel to the tectonic transport direction in other cases these lineations do not have such a relationship lower and middle jurassic flysch deposits in eastern oregon have been strongly deformed during the nevadan orogeny by south to southeast directed thrusting and by folding mega and mesoscopic folds are overturned to the south and southeast and they have a well developed axial planar slaty cleavage foldaxes and cleavage bedding intersections are generally subhorizontal but toward the oregon idaho border their orientation gradually changes from horizontal to steeply plunging the shape of quartz grains and of many other minerals can be represented by a triaxial ellipsoid with the shortest axis normal to the cleavage and the longest axis consistently parallel to the foldaxis and cleavage bedding intersection previously these mineral lineations were thought to have formed by arcuation or extension parallel to the foldaxis quartz c axes orientations measured in several samples consistently show the well known cross girdle pattern indicating that the major principal compressive strain is normal to the cleavage plane and the major principal extensile strain is parallel to the downdip direction of the cleavage plane the measured strain axes are thus in agreement with south to southeast directed thrusting and they are unrelated to the mineral lineations the mineral lineations may have formed by dissolution synchronous with the non coaxial deformation