the global martian volcanic evolutionary history abstract viking mission image data revealed the total spatial extent of preserved volcanic surface on mars one of the dominating surface expressions is olympus mons and the surrounding volcanic province tharsis earlier studies of the global volcanic sequence of events based on stratigraphic relationships and crater count statistics were limited to the image resolution of the viking orbiter camera here a global investigation based on high resolution image data gathered by the high resolution stereo camera hrsc during the first years of mars express orbiting around mars is presented additionally mars orbiter camera moc and thermal emission imaging system themis images were used for more detailed and complementary information the results reveal global volcanism during the noachian period 3 7 ga followed by more focused vent volcanism in three tharsis elysium and circum hellas and later two tharsis and elysium volcanic provinces finally the volcanic activity became localized to the tharsis region about 1 6 ga ago where volcanism was active until very recently 200 100 ma these age results were expected from radiometric dating of martian meteorites but now verified for extended geological units mainly found in the tharsis montes surroundings showing prolonged volcanism for more than 3 5 billions years the volcanic activity on mars appears episodic but decaying in intensity and localizing in space the spatial and temporal extent of martian volcanism based on crater count statistics now provides a much better database for modelling the thermodynamic evolution of mars

evolution of mica fish in mylonitic rocks abstract mineral fish are lozenge shaped porphyroclasts single crystals in a finer grained matrix which occur in ductile shear zones and which are commonly used as shear sense indicators mineral fish of biotite tourmaline k feldspar garnet hypersthene and quartz occur in mylonites but most common are white mica fish these mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations the principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation concomitant selective grain size reduction occurs by recrystallisation cataclastic separation pressure solution and diffusional mass transfer microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds all the presented fish shaped porphyroclasts or mineral fish have a specific shape preferred orientation with their long axis at a small antithetic angle with respect to the foliation they represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands

composition morphology and stratigraphy of noachian crust around the isidis basin definitive exposures of pristine ancient crust on mars are rare and the finding that much of the ancient noachian terrain on mars exhibits evidence of phyllosilicate alteration adds further complexity we have analyzed high resolution data from the mars reconnaissance orbiter in the well exposed noachian crust surrounding the isidis basin we focus on data from the compact reconnaissance imaging spectrometer for mars as well as imaging data sets from high resolution imagine science experiment and context imager these data show the lowermost unit of noachian crust in this region is a complex brecciated unit of diverse compositions breccia blocks consisting of unaltered mafic rocks together with rocks showing signatures of fe mg phyllosilicates are commonly observed in regions of good exposure layered or banded phyllosilicate bearing breccia rocks are observed suggestive of pre isidis sedimentary deposits in places the phyllosilicate bearing material appears as a matrix surrounding mafic blocks and the mafic rocks show evidence of complex folded relationships possibly formed in the turbulent flow during emplacement of basin scale ejecta these materials likely include both pre isidis basement rocks as well as the brecciated products of the isidis basin forming event at 3 9 ga a banded olivine unit capped by a mafic unit covers a large topographic and geographic range from northwest of nili fossae to the southern edge of the isidis basin this olivine mafic cap combination superimposes the phyllosilicate bearing basement rocks and distinctly conforms to the underlying basement topography this may be due to draping of the topography by a fluid or tectonic deformation of a previously flatter lying morphology we interpret the draping superposed olivine mafic cap combination to be impact melt from the isidis basin forming event while some distinct post isidis alteration is evident carbonate kaolinite and serpentine the persistence of olivine from the time of isidis basin suggests that large scale aqueous alteration processes had ceased by the time this unit was emplaced

watershed modeling in the tyrrhena terra region of mars 1 watershed analyses from high resolution image viking mars orbiter camera and thermal emission imaging system and topographic mars orbiter laser altimeter mola data are used to qualitatively and quantitatively characterize highland fluvial systems and analyze the role of water in the evolution of tyrrhena terra 13 s 30 s 265 w 280 w mars in this study geographical information system software is used in conjunction with mola digital elevation models to delineate drainage basin divides extract valley networks and derive basin and network morphometric parameters e g drainage density stream order bifurcation ratio and relief morphometry useful in characterizing the geologic and climatic conditions of watershed formation as well as for evaluating basin maturity and processes of watershed development model predicted valley networks and watershed boundaries which are dependent on the degree to which pixel sinks are filled in the topographic data set and a channelization threshold are evaluated against image and topographic data slope maps and detailed maps of valley segments from photogeologic analyses valley morphologies crater valley relationships and impact crater distributions show that valleys in tyrrhena terra are ancient based on geologic properties of the incised materials valley and network morphologies morphometric parameters and the presence of many gullies heading at or near crater rim crests surface runoff derived from rainfall or snowmelt was the dominant erosional process sapping may have only played a secondary role in valley formation in tyrrhena terra furthermore spatial and temporal relationships of dissected highland materials and impact craters suggests widespread but relatively short lived erosion by runoff with most activity in the noachian period

morphologic and topographic analyses of debris aprons in the eastern hellas region mars this paper presents new detailed analyses of small scale morphologic and topographic characteristics of martian debris aprons that support viking based interpretations of debris aprons as ice rich flow features derived from local uplands fifty four debris apron complexes in the eastern hellas region of mars were examined using mars global surveyor data sets including mars orbiter camera images and mars orbiter laser altimeter topographic profiles consistent patterns in a suite of small scale surface textures and geomorphic features observed throughout the population reflect a history of viscous flow and surface degradation through wind ablation and loss of contained ice a wide variety of shapes seen in topographic profile reveal variations in distribution of contained ice and different stages of apron development and degradation the results of this study provide new evidence consistent with multiple models of apron formation including rock glacier debris covered glacier and ice rich landslide models typical eastern hellas debris aprons formed from a series of large scale events emplacing debris that was enriched initially or later by ground ice complemented by small scale mass wasting of multiple styles and postemplacement flow of apron masses

evolution of the navua valles region implications for mars paleoclimatic history abstract the navua valles are comprised of a system of channels and valleys on the inner northeastern rim of hellas basin which is a 1500 km long sloping terrain drainage systems and regional geology in this unique setting were not previously mapped in detail we mapped this region using ctx 6 m px as the base map and assessed surface unit ages resulting from our crater counting we found that the timing of the deposit forming episodes in this region during the hesperian and early to middle amazonian largely correlated to active phases of the hadriacus mons volcanic center we found evidence for several episodes of fluvial activity hesperian to the amazonian with declining intensity and transitioning to ice dominated processes the channels in the navua valles region erode into deposits dating from the noachian to early amazonian including the noachian highlands noachian to early amazonian crater ejecta and likely volcanic plains formed from the hesperian to the hesperian amazonian transition channels directly originating from hadriacus mons are younger while precipitation fed channels at larger distance from the volcanic center are older indicating different triggers for fluvial activity crater counting results indicate that almost all channel floors were at least partially resurfaced during the amazonian and that several channel deposits formed during the last 0 5 gyr water pathways likely included surface channels lakes and subsurface flow the navua valles channel system is discontinuous and the number of terminal deposits sink locations is almost as high as the number of channel sources which is unusual for valley networks elsewhere on mars interior channels formed only in the major navua channels they are even more fragmented than their parent channels but occur along their entire length channels and valley systems within the navua valles are potential targets for in situ astrobiological studies as they could have provided potential habitats at least periodically from the late hesperian to the late amazonian

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

evidence for multiple ice deposits on the northeastern rim of hellas basin mars abstract four prominent fluvial systems dao niger and harmakhis valles and a system of smaller channels between niger and harmakhis valles characterize a wide topographic trough that connects hesperia planum and hellas planitia hesperia hellas trough hht the source depressions of the canyons of dao and harmakhis valles are at the lowermost level about 5 4 km and lower the system of niger vallis is at medium elevation about 3 to 4 km and the small channels occur at the highest topographic levels about 2 5 to 3 2 km all systems are among the youngest features in the region they formed due to the release of subsurface water and apparently have their sources at different topographic levels below or within the layered suite of volcanic deposits in hht two scenarios can explain the subsurface nature of their sources 1 flow of liquid ground water from extensive watershed toward the source areas and accumulation of water there and 2 deposition of ice bearing materials in and around the sources the first scenario requires the existence of extensive confining layers at different topographic levels corresponding to the levels of the floors of the canyons channels because the presence of such confining layers on mars is problematic we consider the first scenario to be less likely we believe that the deposition of ice bearing materials at different levels during the growth of the lava plateau in hht can better explain the key features of the fluvial systems the subsurface nature of the sources and the relatively young age in this scenario the higher topographic position of the ice bearing deposits corresponds to the younger age of deposition the possible accumulations of ice were buried by successive volcanic deposits including lava flows in hht and later were heated from below by magmatic intrusions that caused melting of the icy deposits release of water and formation of the fluvial structures

catastrophic erosion of hellas basin rim on mars induced by magmatic intrusion into volatile rich rocks 1 malea and hesperia plana form large sectors of the rim of hellas basin that display partly eroded volcanic shields and plains these regions have topographic profiles that appear to be several hundred meters lower than those of adjacent rim sectors and lack prominent massifs of remnant basement that would be expected to stand above the lava plains we interpret that before the volcanic edifices were constructed these regions were denuded by an early stage of voluminous sill intrusion into friable volatile rich impact breccia magma volatile interactions may have resulted in catastrophic generation of debris flows deposited into the adjacent basin particularly if co2 were involved later lavas covered the eroded terrain in turn the lavas were eroded locally by volatile interactions across mars huge channel systems erosional features in volcanic terranes and vast layered deposits may be due to magma volatile interactions

reading history of folding from porphyroblasts abstract the observation that inclusion trails in porphyroblasts commonly exhibit a more or less constant orientation over a large area even though the enclosing rocks have been folded has been considered puzzling by geologists and has generated considerable debate an analysis of vorticity during multilayer folding in response to layer parallel shortening shows that the observation is in fact theoretically expected folding of multilayers in response to layer parallel shortening bulk folding can be considered kinematically as a time variable combination of flexural flow and pure shear components the rotation of a spherical porphyroblast during folding is related to the vorticity history in the vicinity of the porphyroblast by a set of differential equations for likely folding histories in nature the solutions to these equations are in agreement with naturally observed patterns therefore it is unnecessary to invoke other mechanisms such as assuming porphyroblasts to be in islands irrotational with respect to the fold axial plane more importantly the theory allows constraints to be placed from current data sets of porphyroblast inclusion trail geometry on the history of folding and the competence properties of rocks at the time of folding

ancient volcanism and its implication for thermal evolution of mars abstract volcanism plays an important role in the formation and thermal evolution of the crusts of all terrestrial planets martian volcanoes have been extensively studied and it has been suggested that the volcanism on mars that created the visible volcanic features was initiated in the noachian 3 8 ga and continued to the late amazonian 4 0 ga volcanoes are preserved in the heavily cratered southern highlands most of these are central volcanoes with diameters ranging from 50 to 100 km and heights of 2 3 km most of them are spatially adjacent to and temporally continuous with the tharsis and circum hellas volcanic provinces suggesting that these two volcanic provinces have experienced more extensive and longer duration volcanism than previously thought these edifices are heavily cut by radial channels suggesting that an early phase of aqueous erosion occurred and ended prior to the emplacement of the encircling hesperian lava fields

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

terrane accretion and upward extrusion of high pressure granulites in the neoproterozoic nappes of southeast brazil petrologic and structural constraints the high grade crystalline nappes exposed southeast of the sao francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of neoproterozoic deep continental crust the socorro guaxupe nappe above derived from an arc terrane and the tres pontas varginha nappe below metamorphism in the tres pontas varginha nappe is characterized by the exceptional preservation of kyanite granulites 700 750 c 15 kbar and followed by limited retrogression maximum temperatures around 900 950 c were reached toward the base of the overlying socorro guaxupe nappe during the intrusion of charnockitic mangeritic magmas lower pressure metamorphism accompanied by anatexis prevailed at shallower crustal levels our petrological results document an inverted thermal structure with isobaric heating of the top of the high pressure granulite nappe both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units both metamorphosed to high pressure but lower temperature conditions significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes the proposed geodynamic scenario considers that continental subduction took place westward underneath neoproterozoic oceanic lithosphere the two granulite units crystallised at 45 km depths under distinct paleogeotherms within this subduction zone around 630 ma the kyanite granulites were rapidely exhumed through the mechanism of low angle forced extrusion whereas syncollisional collapse affected the soft anatectic middle crust of the overlying arc terrane the final emplacement of the thinned nappe pile onto the cold sao francisco craton and its platform cover with at most anchizonal to greenschist facies metamorphism occurred around 600 ma

topographic and morphologic characteristics of reull vallis mars implications for the history of the reull vallis fluvial system 1 the eastern rim region of hellas basin is characterized by the four prominent and quite extensively researched cf crown et al 2005 and references therein outflow channels in this work we focus on the reull vallis on the basis of observations from available data sets we present a hypothesis for the evolution of reull vallis and its complimentary fluvial system we suggest that this system consists of parts that were formed during several phases rather than being a single continuous channel our results show that the fluvial system of reull vallis consists of two main parts and likely had independent formation phases and different sources of water our results also show that the upper portion of the reull vallis was formed by outflow from beneath hesperia planum as proposed already in earlier works but the suggested segments 1 and 2 mest and crown 2001 of the vallis are not directly linked there seems to have been an on surface source for the formation of the segment 2 in the form of a topographic depression that was filled before the subsequent draining and formation of segment 2 our interpretation of the evolution and formation implies a complex history for the reull vallis system

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

photogeologic mapping and the geologic history of the hellas basin floor mars abstract the hellas basin on mars is the second largest topographically well defined impact structure in the solar system and has repeatedly been interpreted as a major sink of volcanic glacio fluvial and eolian materials based on established guidelines for planetary mapping we compiled a comprehensive photogeological map of hellas planitia i e the hellas basin floor 1 2 000 000 1 8 106 km2 see supplementary online material using the thermal emission imaging system themis ir day time mosaic as basemap supplemented by several other datasets we identified 33 units which were categorized into a rim assemblage containing dissected units a layered rim sequence and other basin rim units as well as a floor assemblage containing the honeycomb formation an interior formation and a plains sequence relative dating of units was performed wherever contacts revealed stratigraphic relationships and was complemented by absolute model ages amas of all units that lend themselves to reliable crater size frequency distribution csfds measurements on the basis of our results as well as amas of circum hellas volcanic provinces by previous authors we compiled a chronostratigraphic model of the hellas basin floor the northern basin rim shows evidence vast layered hydrous mineral bearing deposits containing meandering channel like valleys that the early history of the basin until 3 8 ga ago experienced extended periods of low energy fluvial and possibly lacustrine activity superposing the layered rim sequence the majority of the hellas basin infill 1 5 1 7 106 km3 consists of volcanic material a lower and an upper wrinkle ridged plains unit which was shortened by a compressive stress field relatively soon after its emplacement based on their ages and stratigraphic considerations we identified malea patera and possibly also tyrrhena patera as a suitable source for the older lower plains 3 8 ga and hadriaca and or amphitrites paterae for the younger upper plains 3 7 ga shortly after these volcanic episodes the entire basin floor was covered by large amounts of deposits 106 km3 containing aqueously altered mafic materials we interpret the deposits to have originated from hesperia planum and the hesperia hellas trough where they might have been removed by glacio fluvial processes which also sculpted the hellas rim sections along northern promethei terra and malea planum around the same time such an apparent correlation between nearby volcanic activity and increased erosion deposition is in agreement with previous models which suggest outgassing by a maximum in global volcanism at that time caused warmer episodes enabling repeated subaerial water run off after the emplacement of these deposits intense erosion likely deflation along a rim parallel annulus began to exhume the outer parts of the wrinkle ridged plains again forming an interior formation of more elevated erosional remnants including the alpheus colles since 3 7 ga ago fluvial activity of dao niger and harmakhis valles dissected the eastern plains and emplaced pancake shaped deposits in the basin center later deflation and abrasion by katabatic winds moving through the hellas basin in a clock wise direction parallel to the basin floor outline carved out the northwestern hellas planitia trough called peneus palus possibly within a time span of few hundreds of ma or less within the amazonian being consistent with erosional patterns we observed within the basin circulation models predict such winds to be ongoing today and might thus have recently exhumed the so called honeycomb formation on parts of peneus palus this enigmatic unique terrain was possibly formed by ductile deformation of pre 3 8 ga material and appears to be partially covered by another unique landform the banded terrain although we cannot rule out that it is the surface expression of a thicker unit the banded terrain might represent flows of volatile rich airfall deposits emplaced along the edges of the hellas wind alley by turbulent marginal wind currents which currently recently prevail on both sides of the hellas planitia trough in summary our investigations enabled us to draw an updated comprehensive and self consistent picture of the geologic evolution of the hellas basin floor including volcanic peri glacial fluvial and eolian processes their possible interactions and the implications on the climatic and geologic development of the circum hellas region and the entire planet

mantle and gully associations along the walls of dao and harmakhis valles mars 1 evaluation of high resolution images allows a systematic characterization of the walls of dao and harmakhis valles mars variations in wall morphology are classified into three archetypes exposed mantled and incised gullies are exposed by removal of semi competent mantling deposits suggesting progressive development of feature types in an idealized sequence from i mantled walls to ii incised mantle and eventually iii walls that display exposed gullies analysis of latitude elevation and wall orientation of 420 representative sample sites reveals spatial patterns that display preferential clustering of the two end member morphologies exposed gullies are favored at lower latitude higher elevation and on east facing walls mantled walls are more prevalent at higher latitude lower elevation and on south and southwest facing walls these morphologies and their patterns are consistent with the hypothesis that gullies emerge from beneath an ice rich mantle that is degrading in response to local variations in total solar insolation

strain insensitive preferred orientation of porphyroclasts in mont mary mylonites abstract the shape preferred orientation spo of porphyroclasts was determined in high temperature mylonites the porphyroclasts approach rhomboidal sillimanite or elliptical garnet plagioclase sillimanite shapes and exhibit aspect ratios r as high as 11 particles with r 3 are dominantly rhomboidal the long axis of the best fit ellipse defines a very strong spo inclined at 5 10 to the mylonitic foliation in an antithetic sense with respect to the shear direction this angle is independent of r the inclination of the long sides of rhomboidal sillimanite increases from 10 to 20 with decreasing r in contrast the short sides have a constant orientation of 15 to 17 irrespective of r and are parallel to extensional crenulation cleavage low aspect ratio mainly elliptical objects show low intensity spo close to the shear plane the two spos appear strain insensitive in the case of r r 3 a stable position is acquired this is not explained by any of the current theoretical and experimental models of spo

hadriaca patera insights into its volcanic history from mars express high resolution stereo camera 1 high resolution stereo camera hrsc images of hadriaca patera mars in combination with mars orbiter camera moc mars orbiter laser altimeter mola and thermal infrared imaging system themis data sets reveal morphologic details about this volcano and enable determination of a chronology of the major geologic events through new cratering age assessments new topographic measurements of the hadriaca edifice were also made from a hrsc based high resolution 125 m digital terrain model dtm and compared to the mola dtm we find evidence for a complex formation and erosional history at hadriaca patera in which volcanic fluvial and aeolian processes were all involved crater counts and associated model ages suggest that hadriaca patera formed from early shield building volcanic likely explosive pyroclastic eruptions at 3 7 3 9 ga with caldera formation no later than 3 5 ga a variety of geologic activity occurred in the caldera and on the northern flank and plains at 3 3 3 5 ga likely including pyroclastic flows that partially filled a large crater nw of the caldera and plains to the ne and differential erosion deposition by aeolian and or fluvial activity there were some resurfacing event s in the caldera and on the eastern flank at 2 4 2 6 ga in which the eastern flank s morphology is indicative of fluvial erosion the most recent dateable geologic activity on hadriaca patera includes caldera resurfacing by some process most likely differential aeolian erosion deposition in the amazonian period as recent as 1 5 ga this is coincident with the resurfacing of the heavily channeled south flank by fluvial erosion unlike the tharsis shields major geologic activity ended at hadriaca patera over a billion years ago

reference frame angular momentum and porphyroblast rotation abstract rotation of small rigid objects in a deforming ductile matrix can produce two different types of microstructure a shape fabric due to alignment of the principal axes of a population of elongate objects and the inclusion trail microstructure preserved in syntectonic porphyroblasts we use numerical modeling to show that inclusion trails of elongate porphyroblasts are expected to be extremely complex in contrast snowball garnets are readily interpretable but misuse of reference frame and kinematic misconceptions have obfuscated the discussion on the formation of porphyroblast inclusion trails in general and snowball garnet inclusion trails in particular we clarify this point models for snowball garnet formation that are based on the notion of garnets being irrotational with respect to the earth can be rejected on a geometrical and kinematic basis further the notion that rigid objects embedded in a deforming ductile matrix generally do not rotate is unsound it violates the fundamental physical law of balance of angular momentum

the influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones abstract mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution they are oriented with their long axes tilted with respect to the mylonitic foliation and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio this orientation seems to be a stable orientation for the mica and tourmaline fish analogue experiments with two rheologically different matrix materials were performed to explain the data one material was pdms a linear viscous polymer the other was tapioca pearls a granular material with low cohesion and mohr coulomb type behaviour in contrast to a fairly homogeneous strain distribution in pdms distinct small scale shear bands developed in tapioca pearls during deformation experiments modelled different vorticity numbers and parallelogram shaped rigid objects with different aspect ratios were used rotation rates of objects in a viscous matrix are very similar to analytical solutions for ellipses in viscous flow but stable orientations differ from data of natural examples in all experiments with a mohr coulomb matrix elongated objects had a stable orientation due to small scale strain localisation we therefore suggest that small scale strain localisation mm that might be hidden by ongoing deformation and recrystallisation processes is an important characteristic of the rheology of mylonites

a new global database of mars impact craters 1 km 1 database creation properties and parameters 1 impact craters have been used as a standard metric for a plethora of planetary applications for many decades including age dating geologic mapping and stratigraphic relationships as tracers for surface processes and as locations for sampling lower crust and upper mantle material utilizing craters for these and other investigations is significantly aided by a uniform catalog of craters across the surface of interest consequently catalogs of craters have been developed for decades for the moon and other planets we present a new global catalog of martian craters statistically complete to diameters d 1 km it contains 384 343 craters and for each crater it lists detailed positional interior morphologic ejecta morphologic and morphometric data and modification state information if it could be determined in this paper we detail how the database was created the different fields assigned and statistical uncertainties and checks in our companion paper robbins and hynek 2012 we discuss the first broad science applications and results of this work

stable positions of rigid objects in non coaxial flow a study in vorticity analysis abstract distortions of a homogeneous fabric which occur around rigid objects in rocks deformed by non coaxial ductile flow are useful to determine the sense of shear but also contain information on other aspects of the flow rigid objects in a non coaxial flow fall into two categories those which are permanently rotating and those which can reach stable positions at high finite strain the orientation of such immobilized objects with respect to eigenvectors of instantaneous flow is a function of the vorticity number of the flow the ratio of instantaneous stretches and the axial ratio of the object in naturally deformed rocks immobilized rigid objects may be recognized from the geometry of the surrounding fabric tails of recrystallized material around porphyroclasts in a mylonite rotate towards parallellism with the extensional eigenvector of the flow and the shape of tails reflects the rotational behaviour of the porphyroclast the axial ratio of immobilized porphyroclasts and their orientation with respect to the tails can theoretically be used to determine the vorticity number of the flow and deviations from isochoric plane strain in practice such an analysis is as yet difficult but a tentative example is given of the way in which the vorticity number can be calculated from a population of feldspar porphyroclasts in a quartzite mylonite from the french pyrenees

2 d shape preferred orientations of rigid particles in transtensional viscous flow ghosh and ramberg tectonophysics 34 1976 1 70 studied the two dimensional 2 d rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression sr 0 at low medium shear strains we now extend this theoretical study to transtension sr 0 by deriving a new analytical solution to non interacting populations of rigid particles and to high shear strains we used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock our results show that 1 shape preferred orientations spos developed in viscous simple shear flow are transient and cyclical except for particles with aspect ratio or material lines which tend to the shear direction 2 stable spos can develop in transpression they dip towards the shear sense but when defining an s c structure with the shear foliation this composite fabric is always antithetic and thus indicates the wrong sense of shear 3 stable spos can also develop in transtension they dip opposite to the shear sense but when defining an s c structure with the shear foliation this composite fabric is always synthetic and thus indicates the correct sense of shear 4 spos produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations which must be demonstrated a priori

rotated garnets a mechanism to explain the high frequency of inclusion trail curvature angles around 90 and 180 abstract this paper presents numerical data from garnets with inclusion trail curvature angles of up to 260 three hundred and twenty five garnets were studied from an outcrop of greenschist facies phyllite in southern minas gerais state brazil apart from the inclusion trail curvature angle also the aspect ratio r and the angle between the long axis of the garnets and the foliation were measured the results show a remarkable concentration of at 180 and a minor one at 90 r varies between 1 and 2 showing that the garnets deviate from sphericity and shows that all garnets have their long axis in the forward rotated quadrant supporting the rotational interpretation a model is proposed to explain the concentrations of based on preferential growth of the garnets into the mica rich strain caps orthogonal to the foliation causing elongated crystals that because of their shape and position would experience accelerated rotation until relatively stable positions with their long axes parallel to the foliation would be attained renewed growth again into the mica rich strain caps orthogonal to the foliation would first restore the spherical shape and then produce an elongated shape again perpendicular to the foliation forcing a repetition of the process it is concluded that this model is capable of explaining the concentration of in multiples of 90 in a rotational model where this concentration was considered earlier as an argument in favor of the non rotational model

extensive middle amazonian mantling of debris aprons and plains in deuteronilus mensae mars implications for the record of mid latitude glaciation abstract the mid latitudes of mars are host to a record of recent episodes of accumulations of ice rich materials the record includes debris aprons interpreted to be debris covered glaciers that may represent the preserved remnants of a much more extensive ice sheet we assessed the possibility of former glacial extents by examining debris aprons and the surrounding plains in deuteronilus mensae geomorphic units and stratigraphic relationships were mapped and documented from mars reconnaissance orbiter mro context ctx and high resolution imaging science experiment hirise camera images and crater retention ages were estimated from crater size frequency distributions three major units are observed within the study area debris aprons lower plains and upper plains debris aprons exhibit characteristics typical for these features documented elsewhere and in previous studies including integrated flow lineations and patterns convex upward profiles and knobby and brain terrain surface textures a lower bound on the age for debris aprons is estimated to be 0 9 ga debris aprons are superposed on a lower plains unit having a lower bound age of 3 3 3 5 ga a 50 100 m thick upper plains unit superposes both debris apron landforms and lower plains units and has a best fit minimum age of 0 6 ga the upper plains unit exhibits characteristics of atmospherically emplaced mantle material including fine grained nature sublimation textures cyclic layering draping character and widespread spatial distribution fracturing and subsequent sublimation erosion of upper plains on debris aprons has contributed to many of the surface textures on debris aprons the upper plains unit has also been eroded from the lower plains and plateaus evidenced by isolated blocks of upper plains in the interiors of craters and on the walls and tops of plateaus while no conclusive evidence diagnostic of former cold based ice sheets are observed in the plains within the study region such landforms and units may have been poorly developed or absent as is often the case on earth and would have been covered and reworked by later mantling episodes these observations suggest that emplacement of thick ice rich mantle deposits extended at least to near the early middle amazonian boundary and overlapped with the waning stages of glaciation in deuteronilus mensae

styles and timing of volatile driven activity in the eastern hellas region of mars 1 recent analyses of mars global surveyor and mars odyssey data sets provide new insights into the geologic evolution of the eastern hellas region of mars in particular the role of volatiles here we present results of our recent work and integrate these with previous studies by various investigators to provide a synthesis of the history of volatile driven activity of the region we utilize high resolution images from the mars orbiter camera and thermal emission imaging system combined with mars orbiter laser altimeter digital elevation models and profiles to examine fluvial systems that dissect the circum hellas highlands to characterize stages in the development of the dao niger harmakhis and reull valles canyon systems and to evaluate evidence for ancient lakes in hellas planitia the occurrence of valley networks dissected highland crater rims and crater interior deposits such as layered plateaus suggests widespread ancient degradation of the circum hellas highlands canyon development which represents subsequent more localized activity may have included an early fluvial phase followed by the collapse and sapping dominated stages that along with recent wall erosion and floor resurfacing produced the currently observed morphologies the prominent role of collapse and sapping along the east rim of hellas along with the presence of numerous channels extending toward the basin and sequences of finely layered deposits along the basin rim suggests a volatile rich substrate across a broad depositional shelf the east rim of the basin was an accumulation zone for atmospheric volatiles and or the edge of volatile rich deposits associated with the basin floor this evidence combined with topographic data and cratered terrain preservation around the basin is consistent with a lacustrine period or periods in early martian history the style magnitude and spatial extent of volatile driven activity in eastern hellas have varied considerably with time and these variations may represent a transition from a water to an ice dominated surface environment

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

evidence for middle amazonian catastrophic flooding and glaciation on mars abstract early geologic investigations of mars revealed some of the largest channels in the solar system outflow channels which appear to have mostly developed 3 byr ago these channels have been the subject of much scientific inquiry since the 1970s and proposed formative processes included surface erosion by catastrophic floods glaciers debris flows and lava flows based on the analysis of newly acquired mars reconnaissance orbiter mro context ctx 5 15 5 91 m pixel and high resolution imaging science experiment hirise 25 50 cm pixel image data we have identified a few locations contained within relatively narrow canyons of the southern circum chryse outflow channels that retain well preserved decameter hectometer scale landform assemblages these terrains include landforms consistent in shape dimension and overall assemblage to those produced by catastrophic floods and at one location to glacial morphologies impact crater statistics for four of these surfaces located within upstream midstream and downstream outflow channel surfaces yield an age estimate of 600 myr this suggests that the southern circum chryse outflow channels were locally resurfaced by some of the most recent catastrophic floods on the planet and that these floods coexisted within regional glacier environments as recently as during the middle amazonian

geology of the reull vallis region mars abstract the geology and stratigraphy of the reull vallis region 27 5 47 5 s 245 270 w of the southern highlands of mars are documented through geomorphic analyses and geologic mapping at 1 2 000 000 scale crater size frequency distributions are used to constrain relative ages of geologic units and determine the timing and duration of the various geologic events the origin and evolution of the reull vallis outflow system and the extent to which reull vallis and other fluvial features modified the surrounding highlands are also evaluated rugged highland terrains were formed by repeated impact events and were extensively degraded and embayed by a series of plains highland terrains incised with well developed valley networks record some of the earliest fluvial activity in the region reull vallis originates in hesperia planum and dissects units of various types and ages truncated and bisected wrinkle ridges in hesperia planum indicate that the formation of reull vallis postdates ridge formation the morphology of reull vallis suggests it had a long and complex history including subsurface and surface movement of fluids followed by extensive modification by mass wasting volcanic and sedimentary plains units in the region record evidence of erosion by surface flow possibly related to development of reull vallis features produced by mass wasting such as debris aprons and crater fill material and aeolian processes appear to form the youngest deposits exposed in the region

unravelling the spirals a serial thin section study and three dimensional computer aided reconstruction of spiral shaped inclusion trails in garnet porphyroblasts seventy seven spatially orientated serial thin sections cut from a single rock reveal changes in the geometry of spiral shaped inclusion trails sslts in garnet porphyroblasts the observed sits are doubly curved non cylindrical surfaces with total inclusion trail curvature decreasing systematically from the cores to the rims of porphyroblasts the three dimensional geometry of the ssits reconstructed with the aid of computer graphics shows that the orientations of spiral axes defined by the ssits are not related in any expected nor predictable way to the main foliation in the matrix this suggests continued deformation after or during the latest stages of porphyroblast growth which has important implications for the use of ssits as shear sense indicators whether the formation of ssits involves significant porphyroblast rotation with respect to a geographically fixed reference frame cannot be determined from the available data key wocds computer graphics garnet porphyroblasts serial thin sections spiral shaped inclusion trails three dimensional reconstruction

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

porphyroblast microstructures a review of current and future trends many recent papers show how porphyroblast microstructures play an important role in a wide range of structural and metamorphic studies this paper reviews ten current applications of these microstructures 1 porphyroblast growth timing criteria 2 tracking progressive foliation development relative to changing metamorphic conditions 3 timing of pluton emplacement relative to deformation and metamorphism 4 finite longitudinal strain determinations 5 kinematics and porphyroblast rotation 6 use of linear fabrics preserved in porphyroblasts 7 porphyroblasts and folding mechanisms 8 inclusion trail orientations and orogenic processes 9 inferring shear strain rates from porphyroblast growth rates and 10 in situ age determinations although there is still no concensus on the interpretation of some porphyroblast microstructures a bright future lies ahead as traditional and newly developed techniques of microstructural analysis are combined with modern chemical and microprobe techniques to provide an increased understanding of the relationships between deformation and metamorphism in a wide range of metamorphic settings

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

near orthogonal foliation development in orogens meaningless complexity or reflection of fundamental dynamic processes abstract orogenic belts are geometrically complex owing to repeated deformation within this complexity there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations seven possible explanations are presented for the sequential development of these foliations which can probably be reduced to four of general importance 1 the passage of thrust sheets over flats and ramps 2 switching of the maximum and minimum compressive stress orientations during orogenesis 3 reversal of structural development owing to strong rheological anisotropy and 4 back rotation of crenulation hinges during crenulation cleavage development it is suggested that all four of these reflect fundamental dynamic processes at work to build an orogen and therefore that sequentially overprinted steeply and gently dipping foliations also reflect such fundamental processes this illustrates that detailed geometrical analysis at the meso and microscales can provide valuable input to dynamic models of orogenesis

major episodes in the geologic history of western promethei terra mars 1 geologic evolution of the western portion of promethei terra 40 50 s 90 105 e consists of two major episodes the first was the formation of a late noachian early hesperian volcanic province during this episode a kilometer thick suite 0 3 106 km3 of lavas accumulated to the south of reull and harmakhis valles within a regional depression that probably was formed earlier in a massive erosion a topographic barrier of the cratered terrains separates the volcanic western promethei plateau from hesperia planum thus the lavas could not originate from hesperia planum the topographic position of western promethei terra excludes a possible lava feeding from hadriaca patera dikes from tyrrhena patera potentially could be the lava source the massive erosion before formation of the lava plateau in western promethei terra is poorly consistent however with the hypothesis that the distal dikes served as the source and favors emplacement of sills or a local source for the lavas the second episode after the lava plateau emplacement was surface modification by glacial fluvial and aeolian processes the time of the episode is poorly known and it could continue from the late hesperian and through the entire amazonian epoch mesas channels and esker like ridges suggest that at the earlier stages of this episode late hesperian early amazonian a glacier or ice dust deposits covered the western promethei terra area studied the 50 60 103 km3 formation could contain 15 18 to 40 50 103 km3 of ice this is comparable with the ice involved into of lobate apron formation 5 13 103 km3 at the later amazonian stages of western promethei terra because of the uncertainties in the ice volume estimates the total amount of water in the earlier glacier and later aprons was approximately the same

geometric relationships of pristine martian complex impact craters and their implications to mars geologic history 1 based on the depths of pristine complex craters on mars we have estimated the final post modification phase depth dr to diameter d function of martian complex craters we suggest that the dr d function determined here holds globally for complex craters 12 km diameter the global nature of this function may be due to the stability of slopes in the highly fractured rock that makes up the steep interior walls of post modification phase craters the stability of such slopes should be controlled mainly by such factors as the nature of the fractures and not target strength this function is an essential benchmark for studies that utilize crater geometry as their basis for example the function can be used in gradation history of mars to predict the magnitude of degradation for instance an example 20 km diameter middle early noachian age crater that initially formed in the mid latitudes of mars with a depth of 1800 m instead of 1404 m as predicted by the fresh craters function has lost 1450 m of its depth over its history 350 m in the noachian 400 m during an intense episode of degradation at the end of the noachian and 700 m thereafter no evidence of the period of intense degradation is found in the high latitudes although those regions appear to have been affected by a process that reduced crater depth soon after they form

carbon isotope fractionation between dissolved and gaseous carbon dioxide abstract fractionation of the stable carbon isotopes between dissolved and gaseous carbon dioxide has been measured at temperature 25 c by two methods in the first method the open system conditions and different methods of co 2 sampling were arranged in the second method the closed system conditions and co 2 gas extraction were used the results obtained by these methods are very consistent gaseous co 2 is enriched in heavy isotope 13 c by 1 03 0 02 permil in comparison to dissolved carbon dioxide

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

neoproterozoic high pressure metamorphism and tectonic constraint from the nappe system south of the sao francisco craton southeast brazil abstract the neoproterozoic giant nappe system exposed south of the sao francisco craton underwent a minimum of 150 km of near horizontal northeastward displacement this nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high pressure high temperature intermediate to mafic granulites at its base socorro guaxupe nappe underlain in sequence by a high pressure kyanite granulite nappe an amphibolite facies metapelite nappe and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton the kyanite granulites crystallized ca 13 kbar 750 c and have only at their top sillimanite related to t increase overprinting t 890 c a consequence of downward heat advection from the overlying socorro guaxupe granulites equilibrated at 900 c pressures of 13 14 kbar are documented in the underlying metapelite nappe with temperatures of 640 670 c in which lenses of eclogite indicate p max of ca 17 5 kbar phyllites of the parautochthonous unit are directly overlain by hp granulites in the north and record p 7 kbar and t 500 c the distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction related metamorphism and kyanite granulites from early extrusion of subducted continental crust the metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile the overall picture suggests that high pressure units recrystallized in a subduction zone located to the wsw the frontal thrusts of the nappe system show a late north northeastward transported thin skinned pattern

testing models for the development of spiral shaped inclusion trails in garnet porphyroblasts to rotate or not to rotate that is the question the formation of spiral shaped inclusion trails ssits is problematical and the two viable models for their formation involve opposite shear senses along the foliation in which the porphyroblasts are growing one model argues for porphyroblast rotation with respect to a geographically fixed reference frame whereas the other argues for no such porphyroblast rotation but instead rotation of the matrix foliation around the porphyroblast thus porphyroblasts with ssits cannot be used as shear sense indicators until it is conclusively determined which model best explains them any successful model must explain features associated with ssits including 1 foliation truncation zones 2 smoothly curving ssits 3 millipede microstructure 4 total inclusion trail curvature in median sections 5 porphyroblasts with ssits that have grown together 6 evidence for relative porphyroblast displacements 7 shear sense indicators inside and outside porphyroblasts 8 crenulations associated with porphyroblasts and 9 geometries in sections subparallel to spiral axes axes of rotation a detailed study of these features suggests that most if not all can be explained by both the rotational and non rotational models in spite of these models involving diametrically opposed movement senses therefore geometrical analysis of individual porphyroblast microstructures may not determine which model best explains ssits until the kinematics required to form these microstructures are better understood in particular the sense of shear along a developing crenulation cleavage specific tests for determining the shear sense along crenulation cleavages are proposed and results of such tests may conclusively resolve the debate over how ssits form

development of snowball structure numerical simulation of inclusion trails during synkinematic porphyroblast growth in metamorphic rocks abstract this paper presents a hydrodynamic analysis that simulates the development of snowball structures in porphyroblasts the porphyroblast is represented by a rigid sphere that grows at a constant volumetric rate while the matrix is represented by a newtonian fluid that deforms by simple shear well away from the sphere the marker particles used to track the motion of matrix and porphyroblast are arranged in a straight line before deformation those in the matrix move along flow lines that deflect around the sphere while those in the sphere passively rotate at a constant angular velocity those in the matrix are progressively captured and incorporated within the growing sphere the resultant trails of the marker particles in the sphere simulate the inclusion trails of synkinematic porphyroblasts three main types of trails are distinguished by the appearance of curvature in the sphere single double and triple rotation types single rotation types show very simple spiral trails formed when the initial angle between the marker line and the shear plane is between 0 and 45 double rotation types show a complicated pattern with the marker line showing a reversal of curvature from the porphyroblast center to its margin this type is formed when the angle between the marker line and the shear plane was initially between 45 and 135 when this angle exceeds 135 then triple rotation types appear which are characterized by two reversals of curvature of the marker line from center to the margin the calculated results are found to be very similar to some natural examples implying that such natural spiral patterns in the porphyroblast are produced in the similar way to what happens in the computer simulation

tyrrhena patera geologic history derived from mars express high resolution stereo camera 1 we used mars express high resolution stereo camera images of the tyrrhena patera volcano to assign cratering model ages to material units defined in the vikingorbiter based geologic mapping cratering model ages are generally consistent with their stratigraphy we can identify three key intervals of major activity at tyrrhena patera 1 formation of the volcanic edifice in the noachian period 3 7 4 0 ga shortly following the hellas impact 4 ga and coincident with the formation of hadriaca patera 3 9 ga 2 modification of the edifice and formation of the caldera rille and channels in the hesperian period possibly extending into the amazonian period and 3 a final stage of modification in the late amazonian epoch 0 8 1 4 ga early to mid hesperian activity on tyrrhena patera is consistent with similar activity on hadriaca patera at 3 3 3 7 ga the most recent dateable event on tyrrhena patera is modification on the upper shield caldera rille and channel floors at 800 ma this coincidence of resurfacing in three units suggests a widespread process es which we speculate involved preferential aeolian erosion of small craters on these flatter surfaces relative to the other units on the volcano alternatively some combination of pyroclastic flow emplacement on the upper shield and fluvial activity in the caldera rille and channels followed by differential aeolian erosion and deposition could have produced the present surface regardless major geologic resurfacing ended at tyrrhena patera nearly a billion years ago

planetary surface dating from crater size frequency distribution measurements multiple resurfacing episodes and differential isochron fitting abstract the analysis of crater size frequency distributions and absolute densities forms the basis of current approaches for estimating the absolute and relative ages of planetary surfaces users of the neukum system of crater dating have conventionally used a cumulative presentation of the data but because of the recent proliferation of interest in identifying resurfacing ages it is worth emphasising the utility of the differential presentation of crater data in identifying resurfacing events and particularly in distinguishing the signature of short lived events such as volcanic flows from long acting processes such as aeolian erosion the work describes some additional considerations for making isochron fits to differentially plotted crater populations with respect to the removal of a binning bias for incrementally differentially plotted data the hartmann approach has not typically employed the fitting of isochrons but differential fitting would be a natural choice for this system and is implemented in the craterstats software a revised calculation of the martian epoch boundary times in both chronology systems is provided

deposition and degradation of a volatile rich layer in utopia planitia and implications for climate history on mars 1 we investigate the surface morphology of a study area in western utopia planitia mars which is characterized by a variety of landforms that resemble those of terrestrial periglacial landscapes polygonally fractured ground and thermokarst like depressions are found to be located in a young mantling deposit with a thickness of several tens of meters we observe a latitudinal dependence of the degradation of this mantling deposit larger depressions whose floors reveal the underlying basement rocks cover a higher fraction of the total terrain in the southern part of the study area than in the northern part indicating a more intense degradation of the mantle in the south all depressions have an asymmetric cross section in north south direction interpreted to be the result of the different solar radiation on differently oriented slopes on the basis of our morphological observations we develop a conceptual model for landscape evolution in western utopia planitia it involves subaerial deposition of a layered ice rich mantle and its subsequent degradation by polygon formation and sublimation a terrestrial analog to the polygonally fractured mantling deposit and its thermokarst like depressions is the siberian ice complex or yedoma which consists of subaerial ice rich deposits and is connected to nonglaciated landscapes with highly continental cold climatic environmental conditions our comparison suggests that no unusual or exotic processes need to be invoked to explain the current morphology of western utopia however the young age of the deposition and degradation implies climatic excursions in the very recent past on mars

titanite petrochronology of the southern brasilia orogen basement effects of retrograde net transfer reactions on titanite trace element compositions abstract combining u pb geochronology and trace element characteristics of accessory minerals more reactive during metamorphism and deformation than zircon such as titanite catisio5 is a powerful tool for elucidating pressure temperature time p t t evolution of orogenic basement complexes this study reports in situ titanite u pb and trace element data from upper amphibolite facies metatexitic orthogneisses of the pouso alegre complex in the basement of the neoproterozoic southern brasilia orogen se brazil the titanite data is combined with thermodynamic modeling major phase compositions and u pb zircon dating and used to constrain the p t t path of the complex during the neoproterozoic orogenesis the investigated garnet amphibole gneisses record peak metamorphic conditions of 750 775 c and 10 5 12 5 kbar associated with partial melting and dated at ca 630 620 ma based on u pb analyses of hree y depleted titanite cores inferred to have crystallized in the presence of garnet near the peak p t conditions extensive melt crystallization at ca 616 ma is constrained by u pb dating of leucosome zircons the exhumation of the complex and re equilibration to ca 550 625 c at 6 9 kbar led to widespread generation of biotite epidote clinozoisite partially replacing amphibole garnet along the main foliation trace element signatures of these retrograde net transfer reactions are recorded in the titanite rims which are hree y enriched strontium depleted and were dated at 605 7 6 2 ma the average zirconium contents of the titanite rims are higher than cores suggesting disequilibrium partitioning of zirconium into titanite during the retrograde path resulting in overestimated zr in titanite temperatures for the rims the exhumation of the complex was concomitant with the intrusion of folded and recrystallized leucogranite dykes dated at 607 0 5 7 ma attesting the persistence of deformation after their intrusion assuming that the pouso alegre complex is part of the sao francisco paleocontinent reworked during the brasilia orogeny the data constrain the minimum age for the collision between the sao francisco paleocontinent and the paranapanema block at ca 620 630 ma and agree with the idea of long lived metamorphism and deformation in the southern brasilia orogen