imaging mapping and modelling continental lithosphere extension and breakup an introduction abstact this special publication is a direct outcome of a small but dedicated group of researchers who met in pontresina switzerland to review and define the fundamental observations characterizing extensional systems and their application in guiding and constraining modelling efforts and results the various summaries of the keynote addresses give an objective overview of the state of the art in modelling lithospheric extensional systems both from the regional scale using dynamic models to individual basins using kinematic models with an emphasis on capturing the extensional history of the iberia and newfoundland margins at the heart of all of these efforts is a simple question exactly what mechanisms allow the continental lithosphere to be thinned to the point of rupture related questions are 1 do crustal and mantle faults play a major role in this thinning process if so what is their geometry and does their importance and geometry change with time 2 are there other mechanisms of lithospheric and crustal thinning that cannot be imaged on seismic sections 3 how is deformation accommodated in space and time 4 what role do inherited mechanical thermal and or chemical heterogeneities play in controlling strain distribution and localization 5 when how and to what degree does magma production affect the distribution and localization of extension and 6 what is the stratigraphic record of continental extension and how does it document the extension of the crust and thinning of the lithospheric mantle the aim of this special publication is to address many of these fundamental questions concerning the extreme extension and thinning of continental lithosphere

crustal structure across the transition from rifting to spreading the woodlark rift system of papua new guinea summary the woodlark rift system is one of the few places where active ocean basin formation can be studied within this rift system continental extension rates are some of the fastest on the planet and extension progresses eastwards to full seafloor spreading we use results from a recent passive seismic experiment to address the role of magmatism prior to the onset of seafloor spreading we invert local earthquake p and s traveltimes for 3 d structure around and ahead of the active spreading tip from the local earthquake tomography we observe three main structures seismic velocities in the crust show a sharp contrast between regions that resemble continental crust ahead of the spreading tip and oceanic crust to the east in the continental portion p velocities increase from 5 6 km s 1 to 6 9 km s 1 between 10 and 25 km depth indicating a bulk felsic to intermediate composition similar to other continental regions beneath the seismic moho 23 28 km depth as defined by receiver functions a 10 15 kmthick gradient zone exist locally with velocities from 7 0 to 7 9 km s 1 which may reflect an underplated mafic granulite facies lowermost crust or perhaps magmatic intrusion into the lithospheric mantle in contrast farther east and closer to the active spreading tip velocities rapidly increase from 6 5 to 7 2 km s 1 between 8 and 18 km depth these fast crustal velocities appear in a narrow zone roughly 60 km wide and indicate a mafic crust similar to oceanic crust our velocity results suggest that the transition from diffuse continental rifting to localized seafloor spreading likely occurs across a narrow zone magmatism prior to the onset of seafloor spreading may not play a significant role in altering crust until the onset of seafloor spreading except through underplating at the base of the crust

the spatial distribution of grains and crystals in rocks characterisation and analysis of the spatial distribution pattern sdp of grains or crystals in rocks is potentially a powerful technique which can be used to constrain the processes which operate in the formation of rocks a method to quantify the sdp of grains in thin section is presented the distance betwen the centre of a grain and the centre of its nearest neighbour is calculated for all the grains in the sample area to produce a distribution of distances that characterises the spatial pattern of grains in the rock this distribution is then normalised to a random distribution of points with the same population density to give a descriptive value r values of r for rock samples are plotted against porosity modal abundance of other phases in igneous and metamorphic rocks to characterise the sdp the sdp of randomly packed distributions of equal size spheres varies systematically with porosity producing a line on a porosity versus r plot termed the random sphere distribution line rsdl rocks which plot above the rsdl have an ordered sdp and those that plot below a clustered sdp the effects of variation in grain packing order grain sorting compaction and random crystallisation overgrowth on determined r values were investigated using a combination of 3 d sphere models and 2 d texture models the maximum possible value of r is 2 148 corresponding to a perfect section through hexagonal cubic close packing of grains the minimum value of r is dependent on the proportion of grains in the sample volume and may be as low as 1 2 for a sample volume with 30 grains which are clustered variations in size sorting can cause r to vary by approximately 0 25 mechanical compaction of a loose framework of grains results in a higher packing order and an increase in r continued compaction creates a fabric in the texture and r decreases as cluster patterns are developed perpendicular to the principal stress the overgrowth of grains in a touching framework alters the location of the geometric grain centre as the crystal shape changes leading to an increase in r as porosity decreases results reveal that grain sorting compaction and grain overgrowth produce different r versus porosity relationships which can be identified in and therefore used to constrain the relative importance of these processes in rock samples

the timing of martian valley network activity constraints from buffered crater counting abstract valley networks concentrations of dendritic channels that often suggest widespread pluvial and fluvial activity have been cited as indicators that the climate of mars differed significantly in the past from the present hyperarid cold desert conditions some researchers suggest that the change in climate was abrupt while others favor a much more gradual transition thus the precise timing of valley network formation is critical to understanding the climate history on mars we examine thirty valley network incised regions on mars including both cratered upland valley networks and those outside the uplands and apply a buffered crater counting technique to directly constrain when valley network formation occurred the crater populations that we derive using this approach allow assessment of the timing of the last activity in a valley network independent of the mapping of specific geological units from these measurements we find that valley networks cluster into two subdivisions in terms characteristics and age 1 valley network activity in the cratered highlands has an average cessation age at the noachian hesperian boundary and all valleys that we crater counted are early hesperian or older no evidence is found for valley networks in the cratered uplands of late hesperian or amazonian age the timing of the cessation of cratered upland valley network activity at the noachian hesperian boundary also corresponds to a decline in the intensity of large crater formation and degradation and to the apparent end of phyllosilicate type weathering 2 a few valley network incised regions formed outside of the cratered uplands on volcanic edifices in association with younger impact craters and on the rim of valles marineris we applied our buffered crater counting technique to four such valleys on the volcanoes ceraunius tholus hecates tholus and alba patera and on the rim of echus chasma and find that each has distinctive and different late hesperian or early amazonian ages indicating that valley networks formed from time to time in the post noachian period unlike the cratered upland valley networks these isolated occurrences are very local and have been interpreted to represent local conditions e g snowpack melted during periods of intrusive volcanic activity in contrast to a gradual cessation in the formation of valley networks proposed by some workers our new buffered crater counting results indicate a relatively abrupt cessation in the formation of the widespread cratered upland valley networks at approximately the end of the noachian followed only by episodic and very localized valley network formation in later mars history very likely due to specific conditions e g local magmatic heating these valley network ages and correlations are thus consistent with a major change in the near surface aqueous environment on mars at approximately the noachian hesperian boundary the noachian environment supported surface running water and fluvial erosion across mars in the cratered uplands enhanced crater degradation and a weathering environment favoring the formation of phyllosilicates the hesperian amazonian environment was more similar to the hyperarid cold desert of today with valley networks forming only extremely rarely and confined to localized special conditions sources of water for these latter occurrences are likely to be related to periodic mobilization and equatorward migration of polar volatiles due to variations in spin axis orbital parameters and to periodic catastrophic emergence of groundwater

fate of outflow channel effluents in the northern lowlands of mars the vastitas borealis formation as a sublimation residue from frozen ponded bodies of water 1 we analyze the fate of the hesperian aged outflow channel effluents emplaced into the northern lowlands of mars we have modeled the evolution of these effluents under the assumption that they were emplaced under a range of atmospheric conditions comparable to those of today and thought to have prevailed in the hesperian under these conditions we find that the evolution of the sediment loaded water after it left the channels includes three phases phase 1 emplacement and intensive cooling violent emplacement of water followed by a short period of intensive evaporation from the surface and near surface layer and intensive convection during this phase the water maintained and redistributed its large suspended sediment load water vapor strongly influenced the climate at least for a geologically short time when the temperature of the water reached the temperature of the maximum density or the freezing point boiling and intensive convection ceased and the water deposited the sediments phase 2 freezing solid geologically rapid freezing of the water body accompanied by weak convective water movement occurred over a period of the order of 104 years phase 3 sublimation and loss this period involved sublimation of the ice and lasted longer than the freezing phase the rate and latitudinal dependence of the sublimation as well as the location of water vapor condensation crucially depend on the planetary obliquity climate and sedimentary veneering of the ice phase 3 would have been very short geologically 105 106 years if an insulating sedimentary layer did not build up rapidly if such an insulating layer did form rapidly sublimation could cease and residual ice hundreds of meters thick could remain below the surface today the northern lowlands of mars are largely covered by the late hesperian aged vastitas borealis formation vbf which has sharp boundaries clearly seen in the map of kilometer scale roughness and a distinctive kilometer scale roughness signature we examine detrended topography data and find evidence that the vbf is underlain at very shallow depths by an early hesperian ridged volcanic plains substrate rather than frozen water deposits remaining from the outflow channel events analysis of the vbf roughness characteristics suggests at least 100 m thickness of sediments on top of the underlying volcanic ridged plains the total inferred volume of the vbf material approximately corresponds to the volume of material removed from the outflow channels these results support a model in which the vastitas borealis formation predominantly represents the sedimentary residue remaining after the sediment laden water effluents of the outflow channels ponded in the northern lowlands and rapidly froze solid and sublimed the distinctive facies of the vbf are interpreted to be related to the freezing sublimation and residue of the outflow channel effluents ridged facies are dominantly marginal and are interpreted to be related to ice sheet lateral retreat knobby facies occur throughout and are interpreted to be due to a variety of causes including dewatering de icing and brine related processes and kame like ice residues grooved facies are interpreted to represent postsublimation uplift and fracturing due to load removal the mottled facies may represent bright ejecta whose albedo is related to buried sedimentary or evaporitic residues presently the most likely sites to find frozen remnants of the hesperian ocean are below the floors of stealth craters underlying the vbf on the basis of these findings and interpretations we make predictions for the fate of the outflow channel water vapor and the nature of the martian hydrologic cycle during the hesperian if noachian aged oceans existed conditions at that time would be similar to those described for a hesperian ocean as soon as a global cryosphere was formed if any residual deposits remain in the northern lowlands from a proposed noachian ocean they would lie below both the vbf and the underlying sequence of hesperian ridged plains

a crystal size distribution study of the kiglapait layered mafic intrusion labrador canada evidence for textural coarsening crystal size distributions csd of plagioclase olivine and clinopyroxene from the kiglapait intrusion labrador were measured for a series of 13 samples spanning most of the exposed intrusion all csds lack small crystals and are slightly curved to the right concave up all csds of a mineral sweep out a fan as expected where there is relatively little variation in phase proportion csd dispersion can be replicated using the communicating neighbours model of textural coarsening ostwald ripening most of the textural variation in the intrusion is the result of variable degrees of coarsening it is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling suppression of nucleation and coarsening particularly coarse grained areas reflect focusing of circulating magmatic fluids during coarsening the effects of crystal settling were not observed in the csds even within single well developed layers they were probably concealed by the pervasive textural coarsening csds do not suggest that mechanical compaction has played an important role in the formation of texture although it is very likely to have occurred however samples from the lower part of the intrusion show textural evidence of pressure solution compaction

comprehensive analysis of glaciated martian crater greg abstract the 66 km diameter martian crater greg east of hellas hosts various distinctive features including dendritic valleys filled with chevron textured masses south wall and lobate tongues a few kilometers long north wall we analyze these features by various quantitative techniques to illuminate martian geologic and climatic history crater retention model ages indicate that greg is at least 1 3 gy old but surface layers of mantles and glacial features are orders of magnitude younger properties of the dendritic valleys combined with climate models suggest that fluvial activity began under a thicker warmer atmosphere soon after the crater s formation the oldest exposed fluvial systems have surface crater retention ages of a few hundred my indicating runoff in recent geologic time much of greg is covered by ice rich mantle deposits for which we infer gradual accumulation and depths of order 30 85 m they mask pre existing landforms the lobate tongues are interpreted as glaciers with mean slope of 10 2 2 3 and average thickness of 33 19 m our calculations and data suggest that these glaciers were originally ice rich and that their surface layers have been depleted by volatile loss the glaciers probably formed when ice rich mantle deposits reached critical thickness and flowed downhill the top 5 10 m of the mantle and glaciers show crater survival times of order a few my to 15 my which remarkably is the time since the last 1 4 episodes of obliquity 45 global climate models affirm that greg lies in one of two non polar areas with extremes of ice deposition during high obliquity epochs this match with observations supports the use of such models in studies of planetary climate change

age and cooling history of the kiglapait intrusion from an study abstract 40 ar 39 ar age spectra of hornblende biotite and feldspars from the kiglapait layered intrusion labrador give closure ages that define the subsolidus cooling history of the intrusion and its surroundings biotites give very good plateaus but u shaped age spectra are typical for hornblende antiperthite and plagioclase and a spectrum of multi plateau segments is found in mesoperthites isochron studies reveal an age range from 1298 14 ma for hornblende to 1217 28 ma for plagioclase multiple linear arrays or clusters appear on the isochron diagrams of perthitic feldspars indicating the effect of excess argon for antiperthite this excess argon resides mainly in the plagioclase domains based on its inverse correlation with k ca with the highest closure temperature the hornblende age is very close to the published sm nd rockmineral isochron age of 1305 22 ma and gives a good approximation to the crystallization age calculated closure temperatures of different feldspars combined with published values for hornblende and biotite show that this intrusion cooled rapidly down to an ambient temperature of about 240 c in the first 20 ma after its crystallization then underwent extremely slow cooling for a long time combining these results with the published pre intrusion ambient temperature of about 350 c a regional slow cooling is revealed and this implies a late uplift of the region during and after the kiglapait magmatism

discordant bodies from olivine bearing zones iii and iv of the stillwater complex montana evidence for postcumulus fluid migration and reaction in layered intrusions mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35 olivine occur at three different levels in olivine bearing zones iii and iv of the middle banded series of the stillwater complex the discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses although the host olivine gabbros are foliated with tabular plagioclase the discordant bodies lack a discernible fabric and have blocky plagioclase average olivine in the host rocks is slightly more magnesian than that of the discordant bodies mg 75 8 0 7 versus mg 74 6 0 3 respectively but plagioclase compositions are indistinguishable an77 6 2 0 versus an76 6 4 3 average host and discordant bodies respectively whole rock major and trace element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance however bulk compositions of anorthositic cores from the discordant bodies are enriched in k na ba sr and p we conclude that the discordant bodies formed when cooler volatile fluids or fluid rich silicate liquids moved upward and encountered a hotter undersaturated solid plus liquid assemblage continued liquid fluid fluxing increased the permeability along the flow path and focused the flow allowing the original bulk compositions to be modified and leaving plagioclase rich troctolites and anorthosites the shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late stage liquid fluid flow chemical and mineralogical evidence from other parts of olivine bearing zones iii and iv suggests that the processes that formed the discordant bodies may have influenced other cumulates in fact it appears that the same processes that formed the discordant bodies operated within an anorthositic layer strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence

sedimentology and climatic environment of alluvial fans in the martian saheki crater and a comparison with terrestrial fans in the atacama desert abstract the deflated surfaces of the alluvial fans in saheki crater reveal the most detailed record of fan stratigraphy and evolution found to date on mars during deposition of at least the uppermost 100 m of fan deposits discharges from the source basin consisted of channelized flows transporting sediment which we infer to be primarily sand and gravel sized as bedload coupled with extensive overbank mud rich flows depositing planar beds of sand sized or finer sediment flow events are inferred to have been of modest magnitude probably less than 60 m3 s of short duration and probably occupied only a few distributaries during any individual flow event occasional channel avulsions resulted in the distribution of sediment across the entire fan a comparison with fine grained alluvial fans in chile s atacama desert provides insights into the processes responsible for constructing the saheki crater fans sediment is deposited by channelized flows transporting sand through boulder sized material and overbank mudflows sand size and finer and wind erosion leaves channels expressed in inverted topographic relief the most likely source of water was snowmelt released after annual or epochal accumulation of snow in the headwater source basin on the interior crater rim during the hesperian to amazonian periods we infer the saheki fans to have been constructed by many hundreds of separate flow events and accumulation of the necessary snow and release of meltwater may have required favorable orbital configurations or transient global warming

paleohydrology of eberswalde crater mars abstract eberswalde crater mars contains a well preserved fluvial distributary network in a likely deltaic setting the meandering inverted paleochannels and closed drainage basin of this deposit support relatively well constrained estimates of channel forming discharge over an individual event flood timescale runoff production event and annual timescales and longevity of deposition geologic timescale during the late hesperian to early amazonian epochs the width and meander dimensions of two inverted paleochannels reflect the channel forming discharge from event floods 200 to 400 m 3 s the deposit surface indicates the level 1400 to 1350 m and surface area 410 to 810 km 2 of the likely paleolake and the topography and mapped extent of tributaries constrain the watershed area 5000 to 17 000 km 2 based on these results and terrestrial empirical constraints on evaporation and sediment concentration we evaluated three hypothetical water sources meltwater liberated by the nearby holden crater impact continuous deposition over 10 1 10 2 years intermittent rainfall or snowmelt during finite periods controlled by orbital evolution deposition over 10 4 10 6 years and highly infrequent runoff or melting of accumulated snowpacks following distant impacts or secular changes in orbital parameters local impact generated runoff and highly infrequent rainfall or snowmelt require unreasonably high and low rates of evaporation respectively to maintain the paleolake level the local impact hypothesis alternatively depends on one flooding episode with very high concentrations of fluvial sediment that are inconsistent with morphologic considerations multiple primary impact craters in the area postdate holden ejecta but were later dissected indicating fluvial erosion long after the holden impact intermittent rainfall of 1 cm day and seasonal snowmelt are both consistent with our results over a deposition timescale totaling 10 4 10 6 years

degradation of mid latitude craters on mars abstract recent geomorphic remote sensing and atmospheric modeling studies have shown evidence for abundant ground ice deposits in the martian mid latitudes numerous potential water ice rich flow features have been identified in craters in these regions including arcuate ridges gullies and small flow lobes previous studies such as in newton basin have shown that arcuate ridges and gullies are mainly found in small craters 2 30 km in diameter these features are located on both pole facing and equator facing crater walls and their orientations have been found to be dependent on latitude we have conducted surveys of craters 20 km in diameter in two mid latitude regions one in the northern hemisphere in arabia terra and one in the southern hemisphere east of hellas basin in these regions prominent lobes potentially ice rich are commonly found on the walls of craters with diameters between 20 100 km additional water ice rich features such as channels valleys alcoves and debris aprons have also been found in association with crater walls in the eastern hellas study region channels were found to be located primarily on pole facing walls whereas valleys and alcoves were found primarily on equator facing walls in the arabia terra study region these preferences are less distinct in both study regions lobate flows gullies and arcuate ridges were found to have pole facing orientation preferences at latitudes below 45 and equator facing orientation preferences above 45 similar to preferences previously found for gullies and arcuate ridges in smaller craters interrelations between the features suggest they all formed from the mobilization of accumulated ice rich materials the dependencies of orientations on latitude suggest a relationship to differences in total solar insolation along the crater walls differences in slope of the crater wall differences in total solar insolation with respect to wall orientation and variations in topography along the crater rim can explain the variability in morphology of the features studied the formation and evolution of these landforms may best be explained by multiple cycles of deposition of ice rich material during periods of high obliquity and subsequent modification and transport of these materials down crater walls

errors in martian paleodischarges skew interpretations of hydrologic history case study of the aeolis dorsa mars with insights from the quinn river nv abstract changes in martian fluvial geomorphology with time stratigraphic age including decreases in paleochannel widths suggest waning paleodischarges through time where fluvial landforms do not preserve paleochannel widths e g meander deposits other landform dimensions i e radius of curvature may be used to estimate paleodischarges in the aeolis dorsa region topographically inverted and stacked fluvial deposits wide meander point bars overlain by thin channel fills preserve ostensible evidence of decreasing paleodischarges through time however a robust paleohydraulic analysis of these distinct deposits requires knowledge of the accuracy of a terrestrial based empirical relationship that estimates channel width from point bar radius of curvature we assess the accuracy of this radius width relationship by applying it to a well studied terrestrial analog the quinn river nevada we find that radii of curvature from the quinn river exceed the values predicted from the empirical relationship these anomalously high radii are associated with greater resistance in the channel cut banks indicating that bank strength is a confounding factor in the radius width relationship some deposits in the aeolis dorsa include irregular meander morphologies suggesting variably resistant channel banks and overestimates of both paleochannel widths and paleodischarges furthermore the morphometry of the overlying thin channel fills suggests their widths have been eroded such that their paleodischarges are underestimates these overestimates and underestimates when considered together suggest little change in paleodischarge during the stratigraphic transition from meander deposits to channel fills this work demonstrates the importance of terrestrial analog studies for revealing confounding factors in martian fluvial systems and cautions against simplistic interpretations of martian fluvial history the discovered inaccuracies of paleodischarge estimates expose sources of uncertainty in the extant paleodischarge data that bias inferences toward waning hydrologic activity through time

origin of megacrysts in granitoids by textural coarsening a crystal size distribution csd study of microcline in the cathedral peak granodiorite sierra nevada california microcline megacrysts in the cathedral peak granodiorite and other parts of the tuolumne intrusive suite were formed by textural coarsening ostwald ripening of earlier formed crystals the early formed crystals nucleated and grew in an environment of increasing undercooling probably during the ascent of the magma emplacement of the magma into warm host rocks promoted textural coarsening crystals smaller than a certain size the critical size dissolved in the interstitial melt whilst large crystals grew microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization positive feedback between textural coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts megacryst growth was halted when cooling resumed at the end of the intrusive cycle k feldspar nucleation was then renewed and k feldspar crystals grew to form part of the groundmass it was the particular thermal history of this pluton that promoted textural coarsening chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture

dominant symmetry of a conjugate southern australian and east antarctic magma poor rifted margin segment synthesis and modeling of published deep seismic and potential field data from the conjugate magma poor rifted margins of the great australian bight southern australia and central wilkes land east antarctica show that there is pronounced symmetry of structures in a 300 km wide zone straddling the axis of final breakup this symmetry is observed consistently for a distance of some hundreds of kilometers along strike from inboard to outboard both margins comprise a narrow zone of attenuation of the crystalline continental crust an approximately 4 km high basement ridge interpreted as unroofed peridotites at the location of maximum thinning of the continental crust and a 60 70 km wide continent ocean transition zone that contains a sedimentary basin that may be underlain by altered mantle and fragments of crystalline continental crust the marked breakup symmetry described here is in contrast to the asymmetry of the iberia newfoundland margin and is consistent with the operation of a symmetrical extensional detachment system deforming the whole crust in the center of the rift as envisaged by some numerical models for the continental rifting process

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

supraglacial and proglacial valleys on amazonian mars abundant evidence exists for glaciation being an important geomorphic process in the mid latitude regions of both hemispheres of mars as well as in specific environments at near equatorial latitudes such as along the western flanks of the major tharsis volcanoes detailed analyses of glacial landforms lobate debris aprons lineated valley fill concentric crater fill viscous flow features have suggested that this glaciation was predominantly cold based this is consistent with the view that the amazonian has been continuously cold and dry similar to conditions today we present new data based on a survey of images from the context camera ctx on the mars reconnaissance orbiter that some of these glaciers experienced limited surface melting leading to the formation of small glaciofluvial valleys some of these valleys show evidence for proglacial erosion eroding the region immediately in front of or adjacent to a glacier while others are supraglacial eroding a glacier s surface these valleys formed during the amazonian consistent with the inferred timing of glacial features based on both crater counts and stratigraphic constraints the small scale of the features interpreted to be of glaciofluvial origin hindered earlier recognition although their scale is similar to glaciofluvial counterparts on earth these valleys appear qualitatively different from valley networks formed in the noachian which can be much longer and often formed integrated networks and large lakes the valleys we describe here are also morphologically distinct from gullies which are very recent fluvial landforms formed during the last several million years and on much steeper slopes 20 30 for gullies versus 10 for the valleys we describe these small valleys represent a distinct class of fluvial features on the surface of mars glaciofluvial their presence shows that the hydrology of amazonian mars is more diverse than previously thought

late alluvial fan formation in southern margaritifer terra mars 1 crater statistics indicate alluvial fans crater floor and fill mantling deposits within impact craters 50 km in diameter in southern margaritifer terra were likely emplaced during multiple epochs fans formed during the amazonian or near the amazonian hesperian boundary crater floor deposits are likely hesperian in age and most fill mantling deposits are amazonian the regional distribution of fans points to a late period of widespread water driven degradation two of the final candidate landing sites for the 2011 mars science laboratory mission would land on or near some of these fans which appear younger than previously considered

timescales of alluvial fan development by precipitation on mars 1 dozens of large low gradient alluvial fans are present within impact crater basins on the cratered highlands of mars the timescales and climate conditions that were required to generate such fans are unknown but testable through our understanding of terrestrial hill slope erosion in the presence of precipitation previous estimates of fan formation time vary from years to millions of years here we use an idealised physical model of 2 d catchment fan evolution to present a framework within which the development of martian alluvial fans should be considered we simplify the erosional and depositional system so that there are only three variables erodibility due to gravity amount of water runoff due to precipitation and catchment fan boundary elevation within this framework to generate large low gradient 108 yr if the substrate is extremely erodible relative to terrestrial examples

light toned strata and inverted channels adjacent to juventae and ganges chasmata mars 1 light toned layered deposits on the plains adjacent to juventae and ganges chasmata have been identified and analyzed with several instruments onboard the mars reconnaissance orbiter including hirise ctx and crism beds exhibit variations in brightness color polygonal fracturing and erosional properties that are not seen in light toned layered deposits within the chasmata at both juventae and ganges the light toned layered deposits along the plains have associated features we interpret as fluvial including sinuous ridges inverted channels that exhibit light toned layering and an extensive valley system for the ganges deposit the strong correlation between the inverted channels and light toned layered deposits suggests an association with fluvial lacustrine processes although pyroclastic and eolian activity may also be involved in their emplacement the identification of potential fluvial lacustrine layered deposits and associated valley networks suggests surface water flow in the valles marineris region that occurred for sustained periods during the hesperian 3 7 3 0 gyr

methane bursts as a trigger for intermittent lake forming climates on post noachian mars lakes existed on mars later than 3 6 billion years ago according to sedimentary evidence for deltaic deposition the observed fluviolacustrine deposits suggest that individual lake forming climates persisted for at least several thousand years assuming dilute flow but the lake watersheds little weathered soils indicate a largely dry climate history with intermittent runoff events here we show that these observational constraints although inconsistent with many previously proposed triggers for lake forming climates are consistent with a methane burst scenario in this scenario chaotic transitions in mean obliquity drive latitudinal shifts in temperature and ice loading that destabilize methane clathrate using numerical simulations we find that outgassed methane can build up to atmospheric levels sufficient for lake forming climates if methane clathrate initially occupies more than 4 of the total volume in which it is thermodynamically stable such occupancy fractions are consistent with methane production by water rock reactions due to hydrothermal circulation on early mars we further estimate that photochemical destruction of atmospheric methane curtails the duration of individual lake forming climates to less than a million years consistent with observations we conclude that methane bursts represent a potential pathway for intermittent excursions to a warm wet climate state on early mars

cordierite bearing lavas from jaguarao southern brazil petrological evidence for crustal melts during early rifting of gondwana abstract the jaguarao stratoid dacites rio grande do sul brazil are limited in areal extent are comprised of about 3 2 km3 of preserved erupted material and outcrop only in areas of the region underlain by mylonitic and ultramylonitic rocks they are s type volcanic rocks containing cordierite orthopyroxene plagioclase and ilmenite as liquidus phases and partially melted granite gneiss and migmatite enclaves that are very similar to the precambrian basement rocks the jaguarao lavas have distinct geochemical signatures and sr nd isotopes with respect to other volcanic rocks of the region available geochronological data for jaguarao dacites range between 157 5 ma and 139 6 7 4 ma considering the errors the younger ages obtained for jaguarao lavas overlap the 138 128 ma age of rocks of the serra geral group and thus indicate that the dacites were erupted prior to the break up of gondwana in this region petrographic mineralogical and petrochemical data as well as the tectonic context of the jaguarao lavas suggest that magma genesis was linked at least in part to friction melts the dacitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartz feldspathic source terrane leaving a granulite facies residue in subsurface these melts were probably generated as a consequence of crustal thinning linked to simple shear extension just prior to gondwana break up and rifting of the southern atlantic ocean

a sedimentary origin for intercrater plains north of the hellas basin implications for climate conditions and erosion rates on early mars understanding the origin volcanic or sedimentary and timing of intercrater plains is crucial for deciphering the geological evolution of mars we have produced a detailed geological map of the intercrater plains north of the hellas basin based on images from the mars express high resolution stereo camera the mars reconnaissance high resolution imaging science experiment and context erosional windows and fresh impact craters provide a way of studying the lithology of intercrater plain units they are composed predominantly of light toned sedimentary rocks with subhorizontal bedding over a broad extent greater than tens of kilometers showing cross bedding stratifications locally the broad extent geometry and flat topography of these sediments favor a formation by aqueous processes alluvial and lacustrine rather than airfall eolian and volcaniclastic the late noachian 3 7 ga sedimentary plains are locally covered by dark toned rough textured lava flows of late hesperian age 3 3 ga fe mg phyllosilicates were detected within sedimentary rocks whereas volcanic rocks contain pyroxene and lack signatures of alteration in agreement with interpretations made from texture and morphology in erosional windows the superimposition of sedimentary rocks by younger volcanic flows enables the estimation of an erosion rate of 1000 nm yr 1 during the hesperian period 3 3 3 7 ga thus our study shows that an intense sedimentary cycle occurred on the northern rim of the hellas basin before and during the late noachian leading to the formation of widespread sedimentary plains which were then eroded in agreement with a gradual change in the climatic conditions in this period and later covered by volcanic flows

early cretaceous motion of flemish cap with respect to north america implications on the formation of orphan basin and se flemish cap galicia bank conjugate margins bouguer gravity anomalies together with deep seismic reflection and magnetic data on both sides of the north atlantic are used to locate the hinge zones of the flemish cap and galicia bank within the iberian and north american plates regions across which there were abrupt changes in lithospheric extension the characteristic shape and alignment of these hinge zones suggest that they were conjugate features generated during chrons m25 m0 late jurassic early aptian around a distally located euler pole of rotation rifting between iberia and north america involved these two larger plates and the two smaller microplates the flemish cap and galicia bank microplates the motion of the microplates which were adjacent to eurasia was much more complex than those of the larger plates the motion between the microplates from chron m25 or older to chron m0 was complicated by the fact that they remained attached to each other for most of the time when regions to the south were rifting apart as a result continental regions landward of these segments were subjected to extension that created the orphan and flemish pass basins on the north american side and the galicia interior basin on the iberian side by comparing the hinge zones delineated off galicia bank and flemish cap using the bouguer anomalies we were able to infer that flemish cap rotated approximately 438 relative to galicia bank and iberia and moved 200 300 km se with respect to north america such motions of flemish cap and galicia bank agree remarkably well with extensional episodes deduced from industry multichannel seismic reflection data acquired in the orphan basin normal fault orientations identified in the west orphan basin trend n0208 and are approximately perpendicular to the flow lines of our proposed flemish cap north american motion during the m25 m0 period which provides an independent constraint on our proposed kinematic model therefore extensional events affected not only the galicia bank flemish cap conjugate margins but also the galicia interior and orphan basins and need to be taken into account in any assessment of the geological development of the iberian and north american continental margins during the intermargins workshop meeting modelling the extensional deformation of the lithosphere imedl held in pontresina in 2004 a benchmark exercise was proposed to model the crustal and mantle rifting processes between the western galicia bank and the se flemish cap conjugate margins where high quality seismic reflection and refraction data exist screech line 1 funck et al 2003 hopper et al 2004 and ise line 17 perez gussinye et al 2003 henning et al 2004 during the meeting it became apparent that additional boundary conditions and constraints were needed to correctly characterize the known extension across the iberian newfoundland margin the aim of this paper is to provide some of these conditions and constraints namely a knowledge of the initial positions of plates and microplates before their separation by sea floor spreading processes the positions of flemish cap and galicia bank are well constrained at chron m0 fig 1a because of the identification of correlatable sea floor magnetic anomalies in both the bay of biscay sibuet et al 2004b and in the north atlantic srivastava et al 2000 the resulting euler pole position was constrained by the geometry of the triple junction that had remained at the mouth of the bay of from karner g d manatschal g pinheiro l m eds imaging mapping and modelling continental lithosphere extension and breakup geological society london special publications 282 63 76 doi 10 1144 sp282 4 0305 8719 07 15 00 the geological society of london 2007 fig 1 a reconstruction of the north atlantic between africa af iberia ib and north america na at chron m0 with respect to eurasia eu as given by srivastava et al 2000 ib eu 43 858n 5 838w 244 768 af eu 43 618n 6 938w 240 898 na eu 69 678n 154 268e 23 178 m0 picks west of ib crosses and east of j c sibuet et al 64

greater contrast in martian hydrological history from more accurate estimates of paleodischarge correlative width discharge relationships from the missouri river basin are commonly used to estimate fluvial paleodischarge on mars however hydraulic geometry provides alternative and causal width discharge relationships derived from broader samples of channels including those in reduced gravity submarine environments comparison of these relationships implies that causal relationships from hydraulic geometry should yield more accurate and more precise discharge estimates our remote analysis of a martian terrestrial analog channel combined with in situ discharge data substantiates this implication applied to martian features these results imply paleodischarges of interior channels of noachian hesperian 3 7 ga valley networks have been underestimated by a factor of several whereas paleodischarges for smaller fluvial deposits of the late hesperian early amazonian 3 0 ga have been overestimated thus these new paleodischarges significantly magnify the contrast between early and late martian hydrologic activity width discharge relationships from hydraulic geometry represent validated tools for quantifying fluvial input near candidate landing sites of upcoming missions

fluvial and lacustrine activity on layered deposits in melas chasma valles marineris mars 1 valley networks on mars are the most obvious features attesting that different geologic processes and possibly climatic conditions existed in the past themis images reveal valley networks within melas chasma in valles marineris a hesperian age canyon system the valley networks in melas chasma are dense and highly organized and the heads of the valleys are scattered at different elevations all these features suggest that the networks were fed by precipitation the morphological details reveal inner channels on some valley floors attesting that water flowed within these valleys on the dem the valleys flow into a completely enclosed depression the edge of this feature follows a mola contour line and the depression shows many sedimentary morphologies suggesting lacustrine environment these landforms are located on remnants of layered deposits possibly composed of sulfate layers suggesting that fluvial activity could have contributed to the erosion of the layered terrains in valles marineris collectively the features in melas chasma are a maximum of hesperian in age these results suggest that warm wet environmental conditions on mars persisted through the hesperian and were present during the formation of valles marineris the evidence for a paleolake in melas chasma attests to adequate environmental conditions for life development through the hesperian period

tropical to mid latitude snow and ice accumulation flow and glaciation on mars images from the mars express hrsc high resolution stereo camera of debris aprons at the base of massifs in eastern hellas reveal numerous concentrically ridged lobate and pitted features and related evidence of extremely ice rich glacier like viscous flow and sublimation together with new evidence for recent ice rich rock glaciers at the base of the olympus mons scarp superposed on larger late amazonian debris covered piedmont glaciers we interpret these deposits as evidence for geologically recent and recurring glacial activity in tropical and mid latitude regions of mars during periods of increased spin axis obliquity when polar ice was mobilized and redeposited in microenvironments at lower latitudes the data indicate that abundant residual ice probably remains in these deposits and that these records of geologically recent climate changes are accessible to future automated and human surface exploration

geomorphic study of fluvial landforms on the northern valles marineris plateau mars 1 fluvial landforms are observed on the plateau near echus chasma and other locations on the valles marineris plateau using high resolution 10 to 50 m pixel mars express images and topography branching valleys have a 20 to 100 m deep v shape profile typical of fluvial processes their incision occurred in a thin 150 m and weak dark unit that overlies the plateau basement the valleys are distributed close to watershed boundaries as expected for overland flows and different from pure glacial or hydrothermal processes the 2d geometry of valley networks show drainage densities reaching 1 3 km 1 that indicate a strong 2d extension stage that usually requires a minimum of thousands of years as established from terrestrial examples however the 3d valley geometry shows a limited incision and a lack of concavity suggesting a limited development in time millions of years of evolution are unlikely many outlets connect to the heads of canyons of echus chasma that might involve sapping processes these canyons might have formed coevally with shallow valleys their difference in geometry is a consequence of the difference in lithology which induced a difference in the erosion capacity and an enhanced infiltration conducing to sapping valleys are submitted to modification by aeolian processes sometimes leading to the formation of inverted channels as observed more broadly through the region these landforms formed late in the early mars history and might be considered as examples of episodic fluvial events due to short term climate changes and or regional fluvial activity after the noachian

geomorphology of ma adim vallis mars and associated paleolake basins 1 ma adim vallis one of the largest valleys in the martian highlands appears to have originated by catastrophic overflow of a large paleolake located south of the valley heads ma adim vallis debouched to gusev crater 900 km to the north the landing site for the spirit mars exploration rover support for the paleolake overflow hypothesis comes from the following characteristics 1 with a channel width of 3 km at its head ma adim vallis originates at two eastern and western gaps incised into the divide of the 1 1 m km2 enclosed eridania head basin which suggests a lake as the water source 2 the sinuous course of ma adim vallis is consistent with overland flow controlled by preexisting surface topography and structural control is not evident or required to explain the valley course 3 the nearly constant 5 km width of the inner channel through crater rim breaches the anastomosing course of the wide western tributary the migration of the inner channel to the outer margins of bends in the valley s lower reach a medial sedimentary bar 200 m in height and a step pool sequence are consistent with modeled flows of 1 5 106 m3 s peak discharges were likely higher but are poorly constrained by the relict channel geometry 4 small direct tributary valleys to ma adim vallis have convex up longitudinal profiles suggesting a hanging relationship to a valley that was incised quickly relative to the timescales of tributary development 5 the eridania basin had adequate volume between the initial divide and the incised gap elevations to carve ma adim vallis during a single flood 6 the eridania basin is composed of many overlapping highly degraded and deeply buried impact craters the floor materials of the six largest craters have an unusually high internal relief 1 km and slope 0 5 1 5 among degraded martian craters which are usually flat floored long term fluvial sediment transport appears to have been inhibited within these craters and the topography is inconsistent with basaltic infilling 7 fluvial valleys do not dissect the slopes of these deeper crater floor depressions unlike similar slopes that are dissected at higher levels in the watershed these characteristics 6 7 suggest that water mantled at least the lower parts of the eridania basin floor throughout the period of relatively intense erosion early in martian history the lake level increased and an overflow occurred near the close of the noachian age determined using 5 km crater counts initially the eridania basin debouched northward at two locations into the intermediate basin a highly degraded impact crater 500 km in diameter as this intermediate basin was temporarily filled with water erosion took place first along the lower northern reach of ma adim vallis debouching to gusev crater the western overflow point was later abandoned and erosion of the intermediate basin interior was concentrated along the eastern pathway subsequent air fall deposition impact gardening tectonism and limited fluvial erosion modified the eridania basin region so evidence for a paleolake is restricted to larger landforms that could survive post noachian degradation processes

amazonian aged fluvial system and associated ice related features in terra cimmeria mars abstract the martian climate throughout the amazonian is widely believed to have been cold and hyper arid very similar to the current conditions however ubiquitous evidence of aqueous and glacial activity has been recently reported including channels that can be tens to hundreds of kilometres long alluvial and fluvial deposits ice rich mantles and glacial and periglacial landforms here we study a 340 km long fluvial system located in the terra cimmeria region in the southern mid latitudes of mars the fluvial system is composed of an upstream catchment system with narrow glaciofluvial valleys and remnants of ice rich deposits we observe depositional features including fan shaped deposits and erosional features such as scour marks and streamlined islands at the downstream section of this fluvial system is an outflow channel named k r n valles which displays a unique braided alluvial fan and terminates on the floor of the ariadnes colles basin our observations point to surface runoff of ice snow melt as the water source for this fluvial activity according to our crater size frequency distribution analysis the entire fluvial system formed during early to middle amazonian between 1 8 0 2 0 2 ga to 510 40 40 ma hydraulic modelling indicates that the k r n valles and consequently the alluvial fan formation took place in geologically short term event s we conclude that liquid water was present in terra cimmeria during the early to middle amazonian and that mars during that time may have undergone several episodic glacial related events

late hesperian to early amazonian midlatitude martian valleys evidence from newton and gorgonum basins 1 numerous shallowly incised valleys extend from the upper interior rims of newton and gorgonum basins across smooth interior basin deposits these valleys are a few meters to 300 m wide and may have experienced discharges over all or most of their width implying that they are in fact incised channels in newton crater these valleys extend up to 75 km to near the center of the basin floor in gorgonum basin the valleys terminate at what we interpret to be a former ice covered lake these valleys appear to be examples of scattered shallowly incised valleys found throughout the midlatitudes of mars superimposed upon the widespread mantling deposits within the region on the basis of crater count age dating the interior valleys in newton and gorgonum basins were formed at about the hesperian to amazonian transition the runoff through the valleys may have occurred due to episodic melting of snow and ice that had accumulated on the crater rims temperatures warm enough to cause extensive melting may have occurred during optimal orbital and obliquity configurations because of intensive volcanism releasing greenhouse gasses or as a result of a brief episode of warming from a large impact somewhere on mars the valleys were formed at about the same time as major outflow channels were active along the highlands lowlands boundary water delivered to the northern lowlands by the outflow channels may have been recycled as snow and ice deposits within the martian midlatitudes the episodic melting of such deposits may have formed the midlatitude valleys

amazonian aged fluvial valley systems in a climatic microenvironment on mars melting of ice deposits on the interior of lyot crater valley networks regional drainage patterns suggesting liquid water stability at the surface are confined to early in the history of mars the noachian hesperian boundary and before prior to a major climate transition to the hyperarid cold conditions of the amazonian several later fluvial valley systems have been documented in specific hesperian and early amazonian environments and are thought to have formed due to local conditions here we describe fluvial valley systems within lyot crater that have the youngest well constrained age reported to date middle or late amazonian for systems of this size tens of km these valleys are linked to melting of near surface ice rich units extend up to 50 km in length follow topographic gradients and deposit fans the interior of lyot crater is an optimal micro environment since its low elevation leads to high surface pressure and temperature conditions at its location in the northern mid latitudes are sufficient for melting during periods of high obliquity this micro environment in lyot apparently allowed melting of surface ice and the formation of the youngest fluvial valley systems of this scale yet observed on mars

the circum chryse region as a possible example of a hydrologic cycle on mars geologic observations and theoretical evaluation the transection and superposition relationships among channels chaos surface materials units and other features in the circum chryse region of mars were used to evaluate relative age relationships and evolution of flood events channels and chaos in contact with one another were treated as single discrete flood carved systems some outflow channel systems form networks and are inferred to have been created by multiple flood events within some outflow channel networks several separate individual channel systems can be traced to a specific chaos which acted as flood source area to that specific flood channel individual flood carved systems were related to widespread materials units or other surface features that served as stratigraphic horizons chryse outflow channels are inferred to have formed over most of the perceivable history of mars outflow channels are inferred to become younger with increasing proximity to the chryse basin in addition outflow channels closer to the basin show a greater diversity in age the relationship of subsequent outflow channel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile near surface water supply instead we propose the circum chryse region as a candidate site of past hydrological recycling the discharge rates necessary to carve the circum chryse outflow channels would have inevitably formed temporary standing bodies of h2o on the martian surface where the flood waters stagnated and pooled the chryse basin is topographically enclosed these observations and inferences have led us to formulate and evaluate two hypotheses 1 large amounts of the sublimated h2o off the chryse basin flood lakes precipitated snowed onto the flood source highlands and this h2o was incorporated into the near surface recharging the h2o sources making possible subsequent deluges and 2 ponded flood water in chryse basin drained back down an anti basinward dipping subsurface layer accessed long the southern edge of the lake recharging the flood source aquifers h2o not redeposited in the flood source region was largely lost to the hydrologic cycle this loss progressively lowered the vitality of the cycle probably by now killing it our numerical evaluations indicate that of the two hypotheses we formulated the groundwater seep cycle seems by far the more viable optimally approximately 3 4 of the original mass of an ice covered cylindrical lake albedo 0 5 1 km deep 100 km radius draining along its rim for one quarter of its circumference into substrata with a permeability of 3000 darcies can be modeled to have moved underground on timescales of the order of 10 3 years before the competing mechanisms of sublimation and freeze down choked off further water removal once underground this water can travel distances equal to the separation between chryse basin and flood source sites in geologically short approximately 10 6 year scale times conversely we calculate that optimally only approximately 40 of the h2o carried from chryse can condense at the highlands and most of the precipitate would either collect at the base of the highlands lowlands scarp or sublimate at rates greater than it would accumulate over the flood source sites further observations from forthcoming missions may permit the determination of which mechanisms may have operated to recycle the chryse flood waters

origin of strongly reversed rims on plagioclase in cumulates abstract narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the kiglapait intrusion and may be a common feature of all such cumulates the rims occur at plag plag plag ol and less strongly at plag aug grain boundaries they are optically obvious at an and can reach an 32 mol or more in parallel k na drops sharply although ubiquitous from sample to sample the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact they are prominent in linear networks suggesting the last trace of intercumulus liquid a subsolidus origin is ruled out by the absence of reactants at plag plag and plag ol boundaries and by the local rather than pervasive development of rims the rims are required to grow from intercumulus liquid in which the partition of an component between crystals and liquid increases with the trapped augite component of the liquid calculations from published experimental data show that an 30 can easily be achieved by such a process it is also probable that the trapped liquid is part of an an rich boundary layer generated by solute rejection during adcumulus growth the ability of the rims to sustain steep k na gradients despite a long subsolidus cooling history proves that the k na exchange rate is vanishingly small over a geologic time scale in an rich feldspar suggesting that at low k content the potassium is site bound to the tetrahedral al si distribution reversed rims therefore provide important information on diffusion limits as well as on the late stage solidification history of plagioclase rich cumulates moreover they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored for example by augite content

seismic stratigraphy and subsidence analysis of the southern brazilian margin campos santos and pelotas basins abstract the campos santos and pelotas basins have been investigated in terms of 2d seismo stratigraphy and subsidence the processes controlling accommodation space e g eustacy subsidence sediment input and the evolution of the three basins are discussed depositional seismic sequences in the syn rift barremian to the drift holocene basin fill have been identified in addition the subsidence uplift history has been numerically modeled including i sediment flux ii sedimentary basin framework iii relation to plate tectonic reconfigurations and iv mechanism of crustal extension although the initial rift development of the three basins is very similar basin architecture sedimentary infill and distribution differ considerably during the syn rift sag to the drift basin stages after widespread late aptian early albian salt and carbonate deposition shelf retrogradation dominated in the campos basin whereas shelf progradation occurred in the santos basin in the tertiary these basin fill styles were reversed since the paleogene shelf progradation in the campos basin contrasts with overall retrogradation in the santos basin in contrast long term cretaceous paleogene shelf retrogradation and intense neogene progradation characterize the pelotas basin its specific basin fill and architecture mainly resulted from the absence of salt deposition and deformation these temporally and spatially varying successions were controlled by specific long term subsidence uplift trends onshore and offshore tectonism in the campos and santos basins affected the sediment flux history distribution of the main depocenters and occurrence of hydrocarbon stratigraphic structural traps this is highlighted by the exhumation and erosion of the serra do mar serra da mantiqueira and ponta grossa arch in the hinterland as well as salt tectonics in the offshore domain the pelotas basin was less affected by changes in structural regimes until the eocene when the andean orogeny caused uplift of the source areas flexural loading largely controlled its development and potential hydrocarbon traps are mainly stratigraphic

fresh shallow valleys in the martian midlatitudes as features formed by meltwater flow beneath ice significant numbers of valleys have been identified in the martian midlatitudes 30 60 n s spatially associated with extant or recent ice accumulations many of these valleys date to the amazonian but their formation during these cold dry epochs is problematic in this study we look in detail at the form distribution and quantitative geomorphology of two suites of these valleys and their associated landforms in order to better constrain the processes of their formation since the valleys themselves are so young and thus well preserved uniquely we can constrain valley widths and courses and link these to the topography from the mars orbiter laser altimeter and high resolution stereo camera data we show that the valleys are both qualitatively and quantitatively very similar despite their being 5000 km apart in different hemispheres and around 7 km apart in elevation buffered crater counting indicates that the ages of these networks are statistically identical probably forming during the late amazonian 100 ma in both localities at least tens of valleys cross local drainage divides apparently flowing uphill we interpret these uphill reaches to be characteristic of flow occurring beneath a now absent relatively thin order 101 102 m regionally extensive ice cover ridges and mounds occasionally found at the foot of these valley systems are analogous to eskers and aufeis like refreezing features on the basis of their interaction with these aufeis like mounds we suggest that this suite of landforms may have formed in a single short episode perhaps order of days probably forced by global climate change

the origin and evolution of the peace vallis fan system that drains to the curiosity landing area gale crater mars the landing site for the curiosity rover is located at the distal end of the peace vallis fan in gale crater peace vallis fan covers 80 km2 and is fed by a 730 km2 catchment which drains an upland plains area through a 15 km wide gap in the crater rim valley incision into accumulated debris delivered sediment through a relatively low density valley network to a main stem channel to the fan an estimated total fan volume of 0 9 km3 matches the calculated volume of removal due to valley incision 0 8 km3 and indicates a mean thickness of 9 m the fan profile is weakly concave up with a mean slope of 1 5 for the lower portion numerous inverted channels outcrop on the western surface of the fan but on the eastern portion such channels are rare suggesting a change in process from distributary channel domination on the west to sheet flow on the eastern portion of the fan runoff discharge watershed area to produce the fan is estimated to be more than 600 m perhaps as much as 6000 m indicating a hydrologic cycle that likely lasted at least thousands of years atmospheric precipitation possibly snow not seepage produced the runoff based on topographic data peace vallis fan likely onlapped bradbury rise and spilled into a topographic low to the east of the rise this argues that the light toned fractured terrain within this topographic low corresponds to the distal deposits of peace vallis fan and in such a setting lacustrine deposits are expected

origin of the valley networks on mars a hydrological perspective abstract the geomorphology of the martian valley networks is examined from a hydrological perspective for the compatibility with an origin by rainfall globally higher heat flow and localized hydrothermal systems comparison of morphology and spatial distribution of valleys on geologic surfaces with terrestrial fluvial valleys suggests that most martian valleys are probably not indicative of a rainfall origin nor are they indicative of formation by an early global uniformly higher heat flow in general valleys are not uniformly distributed within geologic surface materials as are terrestrial fluvial valleys valleys tend to form either as isolated systems or in clusters on a geologic surface unit leaving large expanses of the unit virtually untouched by erosion with the exception of fluvial valleys on some volcanoes most martian valleys exhibit a sapping morphology and do not appear to have formed along with those that exhibit runoff morphology in contrast terrestrial sapping valleys form from and along with runoff valleys the isolated or clustered distribution of valleys suggests localized water sources were important in drainage development persistent groundwater outflow driven by localized but vigorous hydrothermal circulation associated with magmatism volcanism impacts or tectonism is however consistent with valley morphology and distribution snowfall from sublimating ice covered lakes or seas may have provided an atmospheric source of water for the formation of some valleys in regions where the surface is easily eroded and where localized geothermal hydrothermal activity is sufficient to melt accumulated snowpacks

updated global map of martian valley networks and implications for climate and hydrologic processes 1 martian valley networks have long been viewed as some of the best evidence of prolonged surface water on mars analysis from viking data showed that valleys were primarily contained on the ancient crust drainage networks were immature and large undissected regions occurred in between individual systems these observations led many to propose that they formed primarily by groundwater processes and limited or no climate change from the present state was required using more recent and higher resolution data sets including visible infrared and topographic data we have manually remapped valleys on a global scale more than eight times as many valleys have been identified on the surface and calculated drainage densities are on average higher by a factor of 2 further regions previously thought to be undissected have significant incision by fluvial processes most of these systems seem to have formed around the noachian hesperian boundary 3 8 3 6 ga minor valley formation continued through the hesperian and into the early amazonian epoch approximately 2 8 ga ago the new data reveal characteristics of sustained precipitation and surface runoff including inner braided channels terraces multiple periods of formation complex network morphology and correlation with other fluviosedimentary features and chloride salts on mars groundwater processes or a transient steam atmosphere generated by impacts played at most a minor role these new results imply that mars had a long lived period or periods of clement conditions toward the end of the noachian epoch that supported a hydrologic cycle and potentially a biosphere

origin of anorthosite by textural coarsening quantitative measurements of a natural sequence of textural development the textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite such textures have been quantified in a troctolite anorthosite from the proterozoic lac st jean anorthosite complex canada crystal size distributions csds indicate that initially plagioclase nucleated and grew in an introduction environment of linearly increasing undercooling producing a straight the textures of plutonic rocks are one of their most line csd during this phase latent heat of crystallization was striking aspects but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush by about 25 solidification the crystallinity was such that isotopic methods where used discussion has tended it reduced but did not eliminate the circulation of magma resulting to be qualitative in contrast to quantitative chemical in the retention of more latent heat within the crystal pile the methods hence reducing the weight of this approach temperature rose until it was buvered by the solution of plagioclase indeed it is not possible to verify physical models if close to the liquidus temperature of plagioclase nucleation of quantitative data are not available quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur ostwald and these methods are applied here to the development ripening in this process small crystals were resorbed whereas of anorthosite larger crystals grew from both material recycled by the resorption of quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with jackson s studies of the stillwater complex by the circulating magmatic fluid the results of this process resemble in 1961 jackson 1961 he measured crystal size disthose of high temperature metasomatism but there is no necessity for tributions but did not record the area that he measured a magmatic fluid with a diverent composition the shapes of the hence his data cannot be completely compared with plagioclase csds fit better the communicating neighbours equation of modern data his pioneering work was not really followed textural coarsening rather than the classic lifshitz slyozov wagner up and since then most studies have looked only at equation as do other examples drawn from the literature if olivine average or typical crystal sizes these data are much started to nucleate at a higher temperature than plagioclase then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions partly because they only give one parameter undercooled than plagioclase and would have had a higher maximum for each sample the few existing studies of crystal size growth rate in these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it hence the order of crystallization plagioclase determined from the textures would be the reverse of the order of one problem especially pertinent to quantitative texfirst nucleation of the two phases from equilibrium phase diagrams tural studies is the way that many igneous petrologists maintenance of the temperature near the plagioclase liquidus may look at rocks they commonly try to reconstruct how a rock has formed by examining the final fully crystallized also inhibit the nucleation and growth of other phases

discontinuous drainage systems formed by highland precipitation and ground water outflow in the navua valles and southwest hadriacus mons regions mars abstract the navua valles are systems of paleodrainages located north of dao vallis which empty into hellas planitia the largest impact basin on mars in this study we mapped and characterized the navua valles region s individual drainage systems including drainages along the southwestern flank of hadriacus mons and one valley network from the same source as navua valles but flowing in the opposite direction the major drainage systems share morphological characteristics common to both outflow channels and valley networks the slopes in this region are dissected by two major navua drainage systems here navua a and b and several shorter sub parallel valleys formed on the highest gradient approximately 20 m km 1 15 slopes at the lowest part of hellas basin s rim the two major drainage systems originate in the highlands and empty into the basin our mapping suggests that water in navua valles reached the basin floor in a complicated descent and included several episodes of surface ponding surface runoff infiltration subsurface flow and subsequent outflow the most prominent channel system navua a forms a repetitive sequence of deep incision into bedrock followed by a transition into broad channels in erodible materials and then into unconfined deposits this successive erosion transport deposition sequence continues to repeat along the valley s entire length forming a discontinuous pattern that is consistent with classical fluvial process models the channels cut into volcanic plains likely emplaced from the formation of tyrrhenus and hadriacus montes the dendritic source valleys of navua a originate from the rim of a highland crater while the rest of this subsystem consists of a single discontinuous channel which is consistent with a single water source zone that likely supplied water for all channels downslope these drainages may have formed as discontinuous channels revealing the potential existence of subsurface drainage pathways located within an approximately 1 5 10 5 km 2 area water transported underground emerged in segments that are either isolated or aligned downslope from each other we have identified several causes of discontinuities located the zones of potential ground water discharge and recharge and a site of localized precipitation at the headwaters of navua a

sub kilometer fans in mojave crater mars in the xanthe terra region of mars two forms of flow fields are observed on the walls of mojave crater a fresh impact site with a maximum age of late hesperian flow fields with steep lobate margins are consistent with emplacement of a highly viscous medium the focus of this report is on fan shaped landforms that share many morphologic attributes in common with terrestrial alluvial fans including a semi conical form branching tributary networks distributary channels and incised channels collectively these sub kilometer scale landforms have attributes consistent with overland flow of fluids and formation of fans by water and gravity driven alluvial sedimentation superposition and cross cutting relationships indicate that fan formation occurred in multiple phases that may have been a single event or multiple temporally distinct episodes many aspects of the fan formation are ill constrained including the amount and source of fluid as well as the duration of fan formation and modification fans are concentrated on the crater walls and the ejecta blanket shows minimal evidence of fluvial erosion similar fan shaped landforms to those in mojave crater are extremely rare on mars the localization of fans to mojave crater implies that the impact event played a role in the formation of these sub kilometer fans this is the first geologic evidence on mars that tentatively supports a link between impact crater events and the liberation of water for surface runoff

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

a large paleolake basin at the head of ma adim vallis mars at 8 to 15 kilometers wide ma9adim vallis is one of the largest valleys in the martian highlands although a groundwater source was previously suggested the channel originates at a spillway in the divide of a 3 000 000 square kilometer closed drainage basin the interior morphology of this source basin including likely shoreline features following topographic contours suggests that ma9adim vallis was created through catastrophic overflow of a 1 100 000 square kilometer highland lake the size constant levels and interior morphology of three regional paleolake basins require a warmer paleoclimate and a long term recharged stable highland water table more than 3 5 billion years ago

origin and evolution of valleys on martian volcanoes morphological analyses of six martian volcanoes ceraunius tholus hecates tholus alba patera hadriaca patera apollinaris patera and tyrrhena patera indicate that fluvial processes were the dominant influence in the initiation and subsequent development of many dissecting valleys lava processes and possibly volcanic density flows were also important as valley forming processes fluvial valleys are especially well developed on alba patera ceraunius tholus and hecates tholus these valleys are inset into the surrounding landscape they formed in regions of subdued lava flow morphology contain tributaries and tend to widen slightly in the downstream direction lava channels on alba patera are located on the crest of lava flows and have a discontinuous irregular surface morphology and distributary patterns these channels sometimes narrow toward their termini possible volcanic density flow channels are located on the northern flank of ceraunius tholus valleys dissecting apollinaris patera hadriaca patera and tyrrhena patera appear to have a complex evolution probably a mixed fluvial and lava origin they are inset into a subdued possibly mantled surface lack tributaries and either have fairly constant widths or widen slightly downvalley valleys surrounding the caldera of apollinaris appear to have formed by fluvial and possibly by volcanic density flow processes while those on the apollinaris fan structure may have a mixed lava and fluvial origin valleys on tyrrhena have broad flat floors and theater heads which have been extensively enlarged probably by sapping hadriaca valleys have a trough shaped form like those on the fan structure on apollinaris but are more enlarged and have theater heads fluvial valleys on the martian volcanoes generally followed a two stage evolutionary sequence similar to that of the hawaiian volcano valleys including 1 initial formation by surface runoff and 2 subsequent enlargement by groundwater sapping because basalt is highly permeable valleys on hawaii island formed only in regions where lava flows were mantled by less permeable volcanic ash similarly fluvial valleys on the martian volcanoes particularly those on alba patera formed in regions where lava flow morphology is subdued probably by ash mantling thus the presence or absence of this mantling may explain why fluvial valleys are present on some martian volcanoes but not on others of the same age the formation of fluvial or fluvial modified valleys on the older volcanoes ceraunius hecates tyrrhena apollinaris and hadriaca implies that surface and near surface water activity was widespread during late noachian to early hesperian period of heavy bombardment however the presence of anomalously young fluvial valleys on alba patera indicates that fluvial valley activity did not entirely cease near the end of late heavy bombardment but rather persisted in isolated regions until mid to late amazonian

scarp bounded benches in gorgonum chaos mars formed beneath an ice covered lake 1 level bench like platforms edged with scarps facing the interior of the martian gorgonum chaos basin may have formed in conjunction with an ancient ice covered lake these benches however lack the typical features of shorelines associated with wave and current transport and erosion such as crescentic embayments spits barrier islands and wave cut cliffs rather the basin facing platform edges are commonly rounded and cumulate in planform often evenly encircling buttes protruding above the level of the benches the benches are postulated to have been formed by outward growth in a quiescent environment possibly by deformative lateral flow of sediment below the ice water interface in a perennially frozen lake due to the weight of the ice overburden

martian cratering 8 isochron refinement and the chronology of mars abstract this paper reviews and refines the technique of dating martian surfaces by using impact crater isochrons defined as size distributions of impact craters on undisturbed martian surfaces of specified ages in the 1970s this system identified not only abundant ancient martian volcanic surfaces but also extensive lava plains with ages of a few 108 y old this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites the present update utilizes updated estimates of the mars moon cratering ratio the most important calibration factor improves treatment of gravity and impact velocity scaling effects combines aspects of the crater size distribution data from earlier work by both neukum and hartmann and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from popova et al 2003 meteorit planet sci 38 905 925 the updated isochrons are not radically different from the previous 2002 iteration but fit observed data better and give somewhat older model ages for features dated from small craters diameter d 100 m crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in d confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y interpreted as lava flow ages more complex older units are also discussed uncertainties are greatest if only small craters d 100 m are used suggestions by other workers of gross uncertainties due to local secondary craters and deposition exhumation are discussed they do not refute our conclusions of significant volcanic fluvial and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation indeed crater count data suggest certain very recent episodes of deposition exhumation and ice flow possibly associated with obliquity cycles of 10 7 y timescale evidence from ancient surfaces suggests higher rates of volcanism fluvial activity glaciation and other processes in noachian hesperian time than in amazonian time

eridania basin an ancient paleolake floor as the next landing site for the mars 2020 rover abstract the search for traces of past martian life is directly connected to ancient paleolakes where ponding water or low energy water fluxes were present for long time intervals the eridania paleolakes system located along the 180 meridian is one of the largest lacustrine environments that were once present on mars morphological features suggest that it was constituted by connected depressions filled by water to maximum depths of 2400 m and a volume of at least 562 000 km3 we focused our attention on the northern side of the eridania basin where high albedo uneven patches of material characterized by the absence of dust are present based on omega and crism orbital imaging spectroscopy data a large clay bearing unit has been identified there in particular a set of aqueous minerals in present in the stratigraphy being visible through erosional windows in the first several tens of meters of the sedimentary sequence below this capping unit a thin al rich clay stratum attributable to al smectite and or kaolins is present this overlies a fe rich clay stratum attributable to the nontronite smectite at the base of the mineralogic sequence a stratum that could be either a zeolite or more likely a hydrated sulfate is present in addition small deposits of alunite a rare phase on mars and jarosite are here found at several locations such stratigraphy is interpreted as originating from a surface weathering process similar to terrestrial abiotic pedogenesis nonetheless possible exobiologic processes can be also invoked to explain it nasa s spirit rover landed on gusev crater in 2004 near the mouth of the ma adim vallis which connects this crater with the considered paleolakes system the eridania site provides the unique opportunity to complete the measurements obtained in gusev crater while investigating the exposed mineralogical sequence in its depositionary setting in addition the extremely favorable landing parameters such as elevation slope roughness rock distribution thermal inertia and dust coverage support this location as a possible landing site for the nasa mars 2020 rover

pervasive aqueous paleoflow features in the aeolis zephyria plana region mars abstract a survey of themis visible wavelength images in the aeolis zephyria plana region over the two western lobes of the equatorial medusae fossae formation mff shows 150 sinuous ridges having a variety of morphologies and contexts to systematize investigation we use a classification scheme including both individual ridge and ridge network types as well as associations with impact craters and fan shaped features the morphology of the ridges their location downslope from higher topography e g crater rims and scarps and their association with fan shaped forms indicate that most sinuous ridges formed through overland aqueous flow analysis of observations by individual ridge type leads to interpretation of most of these sinuous ridges as inverted fluvial channels or floodplains and a few as possible eskers with the origin of the remaining ridges under continuing investigation about 15 of the sinuous ridges are associated with impact craters but data analysis does not support a genetic relationship between the craters and the sinuous ridges instead analysis of one sinuous ridge network associated with a crater indicates that the water source for the network was atmospheric in origin namely precipitation runoff the broad areal distribution of these 150 ridges and the network morphologies in particular the branched and subparallel types suggest that an atmospheric water source is generally applicable to the population of sinuous ridges as a whole this concentration of sinuous ridges is the largest single population of such landforms on mars and among the youngest these ridges are situated at a paleoscarp between cerberus palus and the aeolis highlands suggesting that the precipitation that formed them was orographic in origin the ages of the equatorial mff units in which this population of sinuous ridges is found imply that this orographic rain and or snow fell during some period from the late hesperian through the middle amazonian

constraints on the history of open basin lakes on mars from the composition and timing of volcanic resurfacing 1 abundant evidence exists for valley network related fluvial activity near the noachian hesperian transition on mars and areally significant quantities of volcanic ridged plains were emplaced during this period as well thus it is worthwhile to explore the hypothesis that lava water interaction occurred on the surface of mars at this time we analyzed the morphology physical properties composition and surface ages of thirty open basin lakes topographic lows with both an inlet valley and an outlet valley that were also resurfaced by volcanic flows hyperspectral imaging data from the compact reconnaissance imaging spectrometer for mars crism and observatoire pour la mineralogie l eau les glaces et l activite omega instruments indicate that of the 30 basins 12 exhibit the spectral properties of basaltic lithologies with diagnostic absorptions of olivine high calcium pyroxene and low calcium pyroxene an olivine high calcium pyroxene mixture is the most commonly identified mineral assemblage consistent with other hesperian aged volcanic units therefore our mineralogical results for over a third of the open basin lake floors analyzed support prior interpretations that the basins were resurfaced by volcanic flows supporting evidence for resurfacing by volcanic flows is also given by the observed morphology and physical properties e g surface roughness thermal inertia of the resurfaced open basin lake floors in these 30 examples however no evidence was found for lava water interaction the ages of emplacement derived through counts of superposed craters of all 30 of the open basin lake volcanic resurfacing units show that resurfacing began in the late noachian near the noachian hesperian boundary was concentrated in the hesperian and continued into the early amazonian the lack of geologic features indicative of lava water interaction suggests that the basins were likely to have been mostly devoid of water at the time of the latest phase of volcanic resurfacing we conclude that there is no geologic evidence that suggests the fluvial activity associated with the studied paleolakes was coeval with the emplacement of the observed volcanic resurfacing units several of which date to the late noachian early hesperian

mantle compensation of active metamorphic core complexes at woodlark rift in papua new guinea in many highly extended rifts on the earth tectonic removal of the upper crust exhumes mid crustal rocks producing metamorphic core complexes these structures allow the upper continental crust to accommodate tens of kilometres of extension but it is not clear how the lower crust and underlying mantle respond also despite removal of the upper crust such core complexes remain both topographically high and in isostatic equilibrium because many core complexes in the western united states are underlain by a flat moho discontinuity it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating these processes should decouple upper crust extension from that in the mantle in contrast here we present seismic observations of metamorphic core complexes of the western woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension these core complexes are actively being exhumed at a rate of 5 10 km myr 1 and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density as indicated by low seismic velocities we conclude that at least in this case the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon but must involve mantle extension

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

the cumulate paradigm reconsidered the cumulate terminology outlined by wager and coworkers provided a logical system for describing the textures of the rocks of layered intrusions but recent work shows that this model fails to accommodate all the processes responsible for the petrographic features of coarse grained rocks textures of adcumulates and orthocumulates in the skaergaard intrusion cannot be correlated with concentrations of incompatible elements representing trapped liquid compaction and open system percolative processes have altered their textures modes and even bulk compositions although a rock may be a cumulate in the absence of other evidence it can be misleading to refer to it in terms of the cumulate nomenclature

knob fields in the terra cimmeria terra sirenum region of mars stratigraphy mineralogy and morphology abstract we investigate the stratigraphy morphology and mineralogy of five major knob fields in the region between terra cimmeria and terra sirenum on mars based on hrsc ctx moc and hirise imagery together with hyperspectral data from crism the knob fields comprise ariadnes colles atlantis chaos and gorgonum chaos and further unnamed fields of mounds they have been mapped in previous studies as hesperian or amazonian units and are located within the shoreline of the proposed eridania lake the putative source of ma adim vallis the mounds contain mg fe bearing phyllosilicates and locally al rich phyllosilicates our geological mapping shows that the knob fields have a late noachian age which indicates later phyllosilicate formation than typically observed on mars the knob fields formed by alteration of the electris deposit an airfall deposit possibly rich in basaltic glass grant j a schultz p h 1990 icarus 84 166 195 in local depressions possibly in the eridania lake the spectroscopic detection of phyllosilicates here may indicate that liquid water persisted longer in this region than elsewhere on mars the knob fields are embayed by the hesperian ridged plains numerous valleys carve into the ridged plains and document that the aqueous history of this region continued into the hesperian and amazonian the study area is traversed by the sirenum fossae these graben appear to post date the aqueous activity in the study area except in the gorgonum basin where a lake developed after their formation

the origin and timing of fluvial activity at eberswalde crater mars the fan deposit in eberswalde crater has been interpreted as strong evidence for sustained liquid water on early mars with a paleolake formed during the noachian period 3 7 gy this location became a key region for understanding the mars paleo environment eberswalde crater is located 50 km north of the rim of the 150 km diameter crater holden stratigraphic relationships and chronology obtained using recent mars express high resolution stereo camera and mars reconnaissance orbiter context camera images show that eberswalde fluvial activity crosscuts holden ejecta and thus postdates holden crater whose formation age is estimated from crater counts as late hesperian 3 5 gy depending on models fluvial modeling shows that short term activity over several years to hundreds of years involving dense flows with sediment water ratio between 0 01 and 0 3 may be as good an explanation of the fluvial landforms as dilute flow over longer durations modeling of the thermal effect of the holden impact in the eberswalde watershed is used to evaluate its potential role in aqueous activity the relative timing of the holden impact and eberswalde s fan is a constraint for future studies about the origin of these landforms holden ejecta form a weak and porous substrate which may be easy to erode by fluvial incision in a cold climate scenario impact heating could have produced runoff by melting snow or ground ice any attempt to model fluvial activity at eberswalde should take into account that it may have formed as late as in the late hesperian after the great majority of valley network formation and aqueous mineralization on mars this suggests that hypotheses for fan formation at eberswalde by transient and or localized processes i e impact volcanism unusual orbital forcing should be considered on a par with globally warmer climate

hydrology of early mars valley network incision 1 widespread occurrences of valley networks on mars provide geomorphic evidence for an active hydrologic cycle to constrain the climatic conditions capable of forming the valley networks a hydrological model was used to analyze the valley incision depth and volume of eroded valleys because the absolute magnitudes of precipitation runoff and evaporation are uncertain we have used the ratio of these quantities the x ratio to express climatic conditions the spatial distribution and strength of the correlations between 1 the estimated depth and volume of eroded material and 2 estimated flood magnitude and valley gradients were investigated as a function of the assumed x ratio we also conducted an analysis of conditions required to have appreciable discharge in selected valley networks which provided the most definitive constraint on martian paleoclimate the other methods show a relatively weak dependency of incision depths and volumes upon the assumed x ratio the multiple regression analyses indicate that incision depth is strongly influenced by gradient and weakly related to modeled flood discharge the factors determining relative depth of incision depend partly on the type of channel bed however postflow modification of the valley networks by mass wasting cratering aeolian infilling and ice related processes precludes direct determination of bed morphology our hydrological analyses suggest that climatic conditions on early mars were at least as moist as those that occurred in the great basin region of the u s when large lakes were present during the pleistocene in terms of the balance of runoff and lake evaporation

inverted fluvial features in the aeolis zephyria plana region mars formation mechanism and initial paleodischarge estimates 1 a subset of the sinuous ridges srs in the aeolis zephyria plana azp region of mars has been previously hypothesized to be inverted fluvial features although the precise induration and erosion mechanisms were not specified morphological observations and thermal inertia data presented here support this hypothesis a variety of mechanisms can cause inversion and identification of the specific events that lead to fluvial sr formation can provide insights into the sedimentological geochemical and climatic processes of the region reconnaissance of two terrestrial lava capped ridges suggests some criteria that may be used to identify inverted fluvial features formed by lava infill on mars but these criteria are not satisfied by the majority of the azp fluvial srs armoring also appears inconsistent with terrestrial analogs layering and surface textures of fluvial srs indicate that the most likely induration mechanism was geochemical cementation of fluvial sediments and that the primary erosional mechanism that exposed the fluvial srs was aeolian abrasion this analysis of formation mechanism provides a foundation for estimating paleodischarge using an empirical form discharge approach to which we have applied scaling for martian gravity for those fluvial srs meeting a set of criteria for accurate paleodischarge estimates paleodischarge values generally range between 101 and 103 m3 s 1 the largest of these initial estimates are comparable to paleodischarge estimates for some late stage noachian fluvial channels on mars and provide a constraint on the atmospheric conditions at this equatorial location during the late hesperian to early amazonian time frame

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished

marginal rocks resembling the estimated bulk composition of the kiglapait intrusion abstract the southern margin of the kiglapait intrusion labrador contains fine grained rocks 5 6 m above a contact with anorthosite of the nain complex and a laminated unit 3 m from the contact three fine grained samples and one of the laminated unit have a mean composition closely resembling that previously calculated by summation of the exposed layered group analyzed values agree closely with the earlier estimates for alkalies sr and y thus they support the mineral liquid partition coefficients previously calculated for these elements and for the rare earth elements based on new calculations the value for ferric oxide is revised to 1 04 from its original value of 1 91 the new samples are depleted in the incompatible elements titanium and phosphorus suggesting that they have lost some residual liquid by sidewall fractional crystallization and downward motion of rejected solute the marginal rocks provide no evidence for repeated injection of magma during lower zone time

sinton crater mars evidence for impact into a plateau icefield and melting to produce valley networks at the hesperian amazonian boundary abstract the majority of martian valley networks are found on noachian aged terrain and are attributed to be the result of a warm and wet climate that prevailed early in mars history younger valleys have been identified though these are largely interpreted to be the result of localized conditions associated with the melting of ice from endogenic heat sources sinton crater a 60 km diameter impact basin in the deuteronilus mensae region of the dichotomy boundary is characterized by small anastomosing valley networks that are located radial to the crater rim large scale deposits interpreted to be the remains of debris covered glaciers have been identified in the area surrounding sinton and our observations have revealed the occurrence of an ice rich fill deposit within the crater itself we have conducted a detailed investigated into the sinton valley networks with all the available remote data sets and have dated their formation to the amazonian hesperian boundary the spatial and temporal association between sinton crater and the valley networks suggest that the impact was responsible for their formation we find that the energy provided by an asteroid impact into surficial deposits of snow ice is sufficient to generate the required volumes of melt water needed for the valley formation we therefore interpret these valleys to represent a distinct class of martian valley networks this example demonstrates the potential for impacts to cause the onset of fluvial erosion on mars our results also suggest that periods of glacial activity occurred throughout the amazonian and into the hesperian in association with variations in spin orbital parameters

evidence for a short period of hydrologic activity in newton crater mars near the hesperian amazonian transition 1 hesperian amazonian aged valleys and alluvial fans distributed in regional clusters throughout the southern middle to low latitudes were formed during a period of fluvial runoff and erosion which acted over a smaller spatial and temporal scale than the older classical martian valley networks dated to the noachian hesperian boundary in order to explore the potential sources of water which formed these younger valleys we calculated the expected sediment transport and water discharge rates for a valley and alluvial fan located in newton crater 40 s 159 e over a wide range of water filled channel depths and sediment grain sizes in order to constrain the formation timescale and required water volume depending on the depth of the water filled channel within the valley the alluvial fan was likely emplaced over 0 1 to 10 years of fluvial activity involving between 1 8 and 5 7 km 3 of water these results imply water runoff rates of between 1 and 10 cm d over a typical 300 km2 drainage area possible processes for delivering water to these drainages include high obliquity snowpack melting via volcanism or impacts resulting in either scattered local to regional melting events or a brief global warming event an extended perhaps episodic period of fluvial activity lasting hundreds of years driven by insolation induced melting of high obliquity snowpacks is another possibility

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow