loess and life out of earth abstract loess is a homogeneous typically nonstratified porous friable slightly coherent often calcareous fine grained silty pale yellow wind blown aeolian and often by water reworked sediment that is distributed on continents world wide recent studies have highlighted the importance of biological loess crusts in trapping and transforming aeolian material into loess and also in prevention from wind and water erosion of deposited material some planets and satellites in our solar system are characterized by conditions comparable to those related to loess formation in terrestrial environments given the requirements for loess deposition on earth this study aims to ask the following questions 1 if loess like sediment formation is possible outside earth what are the mechanisms of sediment deposition and preservation 2 could the possible loess like sediments be stabilized through the activity of extraterrestrial equivalents of biological loess crusts found on earth as loess formation on earth is related to biological activity we propose a new direction in the search for extraterrestrial life evidence about extraterrestrial loess might indicate extraterrestrial life connected to loess formation on the basis of comparative analysis of some astrophysical conditions of seven celestial bodies we propose mars and titan and possibly venus and io to be assessed for evidence of loess deposition and the possible presence of related life forms and their biomarkers following recent advances in our understanding of terrestrial loess deposition we propose new indicators of extraterrestrial life research biomarkers in loess cyanobacterial and other microbial

the early environment and its evolution on mars implication for life there is considerable evidence that the early climate of mars was very different from the inhospitable conditions there today this early climate was characterized by liquid water on the surface and a dense atmosphere composed predominantly of co2 the duration of these warm initial conditions on the surface of mars is uncertain but theoretical models suggest that they could have persisted for hundreds of millions up to a billion years from studies of the earth s earliest biosphere we know that by 3 5 gyr ago life had originated on earth and reached a fair degree of biological sophistication surface activity and erosion on earth make it difficult to trace the history of life before the 3 5 gyr time frame if mars did maintain a clement environment for longer than it took for life to originate on earth then the question of the origin of life on mars follows naturally the fossil evidence of early life on earth provides clues as to what form fossils on mars might take of particular interest are stromatolites macroscopic layered structures that result from the anchoring of sediments by microorganisms living in the photic zone since over two thirds of the martian surface is more than 3 5 gyr old the possibility exists that mars may hold the best record of the events that led to the origin of life even though there may be no life there today

nonplanar faults mechanics of slip and off fault damage stress interactions and sliding characteristics of faults with random fractal waviness in a purely elastic medium differ both qualitatively and quantitatively from those of faults with planar surfaces with nonplanar fault models solutions for slip diverge as resolution of the fractal features increases and the scaling of fault slip with fault rupture dimension becomes nonlinear we show that the nonlinear scaling of slip and divergence of solutions arise because stresses from geometric interactions at irregularities along nonplanar faults grow with increasing slip and produce backstresses that progressively impede slip however in real materials with finite strength yielding will halt the growth of the interaction stresses which will profoundly affect slip of nonplanar faults we infer that in the brittle seismogenic portion of the earth s crust off fault yielding occurs on pervasive secondary faults predicted rates of stress relaxation with distance from major faults with random fractal roughness follow a power law relationship that is consistent with reported clustering of background seismicity up to 15 kilometers from faults

ubiquitous weakening of faults due to thermal pressurization faults weaken during earthquakes analysis of the amount of energy released during earthquakes globally suggests that heat induced pressurization of pore fluids can weaken faults during earthquakes of all sizes laboratory simulations of earthquakes show that at high slip rates faults can weaken significantly aiding rupture1 2 3 various mechanisms such as thermal pressurization and flash heating have been proposed to cause this weakening during laboratory experiments1 4 5 6 yet the processes that aid fault slip in nature remain unknown measurements of seismic radiation during an earthquake can be used to estimate the frictional work associated with fault weakening known as an event s fracture energy7 8 9 here we compile new and existing8 9 measurements of fracture energy for earthquakes globally that vary in size from borehole microseismicity to great earthquakes we observe a distinct transition in how fracture energy scales with event size which implies that faults weaken differently during small and large earthquakes and earthquakes are not self similar we use an elastodynamic numerical model of earthquake rupture to explore possible mechanisms we find that thermal pressurization of pore fluid by the rapid shear heating of fault gouge can account for the observed scaling of fracture energy in small and large earthquakes over seven orders of fault slip magnitude we conclude that thermal pressurization is a widespread and prominent process for fault weakening

effect of the greenhouse gases co2 h2o so2 on martian paleoclimate there is general agreement that certain surface features on mars are indicative of the presence of liquid water at various times in the geologic past in particular the valley networks are difficult to explain by a mechanism other than the flow of liquid water it has been suggested in several studies that a thick co2 atmosphere on mars early in its history could have provided a greenhouse warming that would have allowed the flow of water either on the surface or just below the surface however we have examined this effect with a detailed radiation model and have found that if reduced solar luminosity early in the history of the solar system is taken into account even three bars of co2 will not provide sufficient greenhouse warming the addition of water vapor and sulfur dioxide both plausible gases that may have been emitted by martian volcanoes to the atmosphere also fail to warm the surface above 273k for reduced solar luminosity conditions the increase in temperature may be large enough however for the formation of these features by brines

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

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

under the hood of the earthquake machine toward predictive modeling of the seismic cycle advances in observational laboratory and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power to explore that possibility we developed an integrative and fully dynamic model of the parkfield segment of the san andreas fault the model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude m w 6 0 main shocks including events similar to the ones in 1966 and 2004 and provides an excellent match for the detailed interseismic coseismic and postseismic observations collected along this fault during the most recent earthquake cycle such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins

global and regional ridge patterns on mars abstract over 16 000 wrinkle ridges on mars have been classified mapped and digitized to provide a large computer accessible data base for analyzing regions subjected to possible compressive stresses the survey has revealed major compressive structures that occur well beyond the tharsis dominated hemisphere the large variety of affected geologic terrains indicate stresses not simply localized in the intercrater plains one major area of inferred compression occurs in the southern cratered highlands near longitude 180 w where major ridges and scarps extend over 3000 km the occurrence and orientation of many ridges are locally controlled by ancient impact basins the chryse basin in particular has an important effect on ridges in northern lunae planum the removal of all basin concentric ridges reveals however a complex global pattern although such patterns may yet be controlled by heavily degraded impact basins major regional trends also emerge that appear to require broader scale global stresses most ridges in the western hemisphere are shown to be orthogonal to three centers corresponding to tharsis 1 n 122 w syria 12 s 104 w and chryse 19 n 47 w ridges not included in these three sets are generally more random and highly localized most but by no means all ridges in the eastern hemisphere are controlled by the hellas and isidis basins a simple global grid is not yet identifiable and may not be recognizable owing to the large number of regional patterns

nearly magnitude invariant stress drops in simulated crack like earthquake sequences on rate and state faults with thermal pressurization of pore fluids stress drops inferred to be magnitude invariant are a key characteristic used to describe natural earthquakes theoretical studies and laboratory experiments indicate that enhanced dynamic weakening such as thermal pressurization of pore fluids may be present on natural faults at first glance magnitude invariance of stress drops and enhanced dynamic weakening seem incompatible since larger events may experience greater weakening and should thus have lower final stresses and higher stress drops we hypothesize that enhanced dynamic weakening can be reconciled with magnitude invariant stress drops due to larger events having lower average prestress when compared to smaller events we conduct numerical simulations of long term earthquake sequences in fault models with rate and state friction and thermal pressurization and in the parameter regime that results mostly in crack like ruptures we find that such models can explain both the observationally inferred stress drop invariance and increasing breakdown energy with event magnitude smaller events indeed have larger average initial stresses than medium sized events and we find nearly constant stress drops for events spanning up to two orders of magnitude in average slip comparable to approximately six orders of magnitude in seismic moment segment spanning events have more complex behavior which depends on the properties of the arresting velocity strengthening region at the edges of the faults

volcanism on mars more than 15 years of planetary exploration of mars have given insight into the geologic processes that have shaped its surface the newly acquired viking data have shown that volcanism is one of the most important geologic processes operating on mars throughout its history in situ chemical analyses of martian soil by the viking lander spacecraft indicate mafic to ultramafic source rocks this is consistent both with available remote sensing data which indicate the presence of mafic minerals such as pyroxene and olivine and with petrologic modeling based on available geophysical data which suggest that martian lavas are probably iron rich and ultramafic these data strongly suggest that basaltic volcanism is widespread on mars and much of the photogeological data may be studied in this context photogeological analysis of the martian surface has shown two main types of volcanic morphologies the first type is central volcanoes which are volcanic landforms developed by continued and prolonged eruption from a point source vent this category includes 1 shields the classic low profile volcanic mountains of which olympus mons is the most spectacular example 2 domes steep sided constructs such as tharsis tholus that may represent lower rates of eruption than the shields or possibly more silicic lava compositions 3 highland patera radially textured low profile volcanoes that occur in the cratered terrain and are interpreted as ash shields 4 alba patera an apparently unique volcanic landform consisting of a vast volcanic center over 1500 km across with flank slopes of less than a tenth of a degree and 5 various small features such as cinder cones the second major category is volcanic plains which are units recognized by several criteria of which the presence of mare ridges and flow lobes are the most useful volcanic plains are subdivided into four main groups 1 simple flows broad smooth to rolling plains that contain numerous mare type ridges but no flow lobes interpreted as being composed of thick single cooling units 2 complex flows displaying multiple overlapping flow lobes interpreted to be indicative of thin multiple cooling units 3 undifierentiated flows plains that typically lack any morphologic identifying feature but are considered to be volcanic partly on the basis of their association with large volcanic centers and 4 questionable plains volcanic units heavily modified by other processes erosion tectonism etc so that their origins are uncertain when these categories of volcanic morphologies are combined with relative age data provided by crater statistics a volcanic history for mars can be derived as follows early heavy bombardment of mars was accompanied and followed by small scale fluvial channeling extensive flood volcanism the plateau plains and ash shield volcanism in the cratered terrain shortly after this time less extensive flood volcanism continued to resurface the planet during formation of the northern southern hemisphere dichotomy central volcanism became more prominent with the development of the alba patera center as well as the older shields and domes of the northern hemisphere early tharsis and elysium regions the development of the tharsis and elysium uplifts may have triggered the release of large scale catastrophic floods producing large channels continued uplift and lithospheric thinning concentrated volcanic activity in the tharsis region producing large shield volcanoes and extensive lava plains both central vent and plains volcanism have been active throughout martian history but the volumes of extrusion have gradually decreased with time this is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time gradually turning off the volcanism although many questions remain regarding martian volcanism the viking data have provided a remarkable detailed overview of the probable nature of the volcanic history of mars

the influence of ambient fault temperature on flash heating phenomena 1 recent high velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface i e flash heating here we explore the effects of elevated ambient fault temperature up to that at the base of the earth s seismogenic zone on the flash heating behavior of quartzite westerly granite and india gabbro the experiments demonstrate that elevated ambient fault temperature causes 1 an increase in the characteristic weakening velocity for flash heating and 2 an increase in the friction coefficient at a given velocity above this characteristic weakening velocity these unexpected results are reconciled with flash heating theory when temperature specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone

the shallow structure of the martian lithosphere in the vicinity of the ridged plains wrinkle ridges in the smooth plains of the coprates and lunae planum regions of mars form linear or concentric patterns with a regular spacing of 25 50 km the authors test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression that occurred prior to probable ridge related faulting of a strength stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement results show that a range of models with both rigid and deformable megaregolith basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials in models that incorporate viscous and plastic rheologies uniform and exponentially varying vertical strength distributions and the presence or absence of interfacial slip at the base of the surface plains unit the ridge spacing is primarily controlled by the power law exponent of the lithosphere the megaregolith plains unit thickness ratio and the plains unit megaregolith strength contrast deformable basement models with a viscous rheology 1 le n approx le 3 can explain the ridge spacing if the megaregolith was thicker than and approximately 1 3 orders of magnitude weaker than themore plains unit similar models with a perfectly plastic n yields infinity rheology require plains unit thicknesses greater than 5 km which exceeds most observational estimates ridge spacing in the combined viscous and plastic models can be consistent with either a dry water rich or ice rich megaregolith at the time of ridge formation less

thermal decomposition of carbonates in fault zones slip weakening and temperature limiting effects 1 during an earthquake the heat generated by fault friction may be large enough to activate the devolatilization of minerals forming the fault rocks in this paper we model the mechanical effects of calcite thermal decomposition on the slip behavior of a fault zone during an earthquake to do so we introduce the coupled effects of calcite volume loss heat consumption and co2 production in the theoretical analysis of shear heating and thermal pressurization of pore fluids we consider a rapidly deforming shear band consisting of a fluid saturated carbonate rock the equations that govern the evolution of pore pressure and temperature inside the band and the mass of emitted co2 are deduced from the mass and energy balance of the multiphase saturated medium and from the kinetics of the chemical decomposition of calcite numerical simulation of seismic slip at depths of 5 to 8 km show that decarbonation has two critical consequences on fault slip first the endothermic reaction of calcite decomposition limits the coseismic temperature increase to less than 800 c corresponding to the initiation of the chemical reaction inside the shear band second the rapid emission of co2 by decarbonation significantly increases the slip weakening effect of thermal pressurization the pore pressure reaches a maximum and then decreases due to the reduction of solid volume causing a restrengthening of shear stress our theoretical study shows on the example of decarbonation that the thermal decomposition of minerals is an important slip weakening process and that a large part of the frictional heat of earthquakes may go into endothermic devolatilization reactions

3d structure of the gusev crater region abstract gusev crater lies within the aeolis quadrangle of mars at the boundary between the northern lowlands and southern highlands the ancient valley ma adim vallis dissects the highlands south of gusev crater and is thought to have fed the crater with sediments high resolution stereo camera data and digital elevation models were used to construct a geologic geomorphic map 173 5 178 5 e 10 18 s and cross sections complemented by data from mars orbiter camera mars orbiter laser altimeter and thermal emission imaging system three geologic domains are recognised the highlands in the south gusev crater and lowlands in the north twelve units are mapped with thicknesses ranging from hundred meters to several kilometres thicknesses of units and their bedding attitude are estimated combining the geologic map and topographic information relative ages are determined from crater counts ranging from early noachian for highland units to middle amazonian for units in gusev crater and in lowlands episodes of intense geologic activity deposition volcanism deformation occur at around 4 0 ga 3 7 ga and 3 5 ga comparing the geometry of the gusev crater with similar sized filled and un filled martian craters suggests that the columbia hills are relics of the original central peak of gusev crater

sequence rheological properties and effusion rates of volcanic flows at alba patera mars volcanic flows with very large areas characterize the martian volcano alba patera photogeologic analysis indicates a change of eruptive style with time from early flood lavas to relatively short narrow leveed summit flows during the dying phase of activity estimates of mass effusion rate and rheological properties derived from dimensional parameters confirm the view that the magmas involved were mafic and indicate extremely high effusion rates 5 1610 103m3 sec 1 for the earlier flows the data substantiates the view that during the active life of alba volcano mass eruption rates in the tharsis region of mars were significantly higher than in most analogous terrestrial provinces but probably not as high as those for lunar mare basalts

rupture dynamics with energy loss outside the slip zone 1 energy loss in a fault damage zone outside the slip zone contributes to the fracture energy that determines rupture velocity of an earthquake a nonelastic two dimensional dynamic calculation is done in which the slip zone is modeled as a fault plane and material off the fault is subject to a coulomb yield condition in a mode 2 crack like solution in which an abrupt uniform drop of shear traction on the fault spreads from a point coulomb yielding occurs on the extensional side of the fault plastic strain is distributed with uniform magnitude along the fault and it has a thickness normal to the fault proportional to propagation distance energy loss off the fault is also proportional to propagation distance and it can become much larger than energy loss on the fault specified by the fault constitutive relation the slip velocity function could be produced in an equivalent elastic problem by a slip weakening friction law with breakdown slip dc increasing with distance fracture energy g and equivalent dc will be different in ruptures with different initiation points and stress drops so they are not constitutive properties they are determined by the dynamic solution that arrives at a particular point peak slip velocity is however a property of a fault location nonelastic response can be mimicked by imposing a limit on slip velocity on a fault in an elastic medium

major episodes of the hydrologic history in the region of hesperia planum mars 1 the high resolution stereo color camera hrsc data over hesperia planum and its surroundings reveal important details of geologic episodes and water related processes in this region 1 the noachian fluvial events of hesperia planum depression included accumulation of water and formation of a water ice reservoir there later the reservoir was depleted in several phases reflecting diminishing amounts of water climate changes and or volcanism were important in these volatile releases 2 the massive 0 45 1 5 106 km3 erosion from the hesperia depression before the main lava eruption possibly resulted in thick 0 5 1 5 km deposits in hellas planitia 3 measurements of the flooded craters within hesperia planum provide the estimates of the thickness of lavas there about 250 500 m the final volume of lavas within hesperia planum 0 4 0 7 106 km3 is comparable with the range of some terrestrial igneous provinces such as columbia river basalts 4 extended magmatism possibly triggered formation of the outflow channel in a few locations after the lava emplacement during this episode about 0 04 106 km3 of material about 4 5 8 9 of the volume eroded in the episode of massive erosion were removed the thickness of the composite lava layer exposed on the walls of the outflow channels a few hundreds of meters corresponds well to the thickness estimates made by the measurements of the flooded craters 5 dispersed viscous flows debris aprons flow like deposits reflect the final fluvial events viscous flows from the subsurface sources in the southwestern trough associate with dao niger and harmakhis valles these flows represent the final volatile discharge from the hesperia reservoir that mostly was depleted by the earlier events of massive erosion and formation of the outflow channels viscous surface flows are mostly associated with reull vallis and probably reflect redistribution of volatiles related to the late episodes of evolution of this outflow channel

plateaus and sinuous ridges as the fingerprints of lava flow inflation in the eastern tharsis plains of mars abstract the tharsis montes rift aprons are composed of outpourings of lava from chaotic terrains to the northeast and southwest flank of each volcano sinuous and branching channel networks that are present on the rift aprons suggest the possibility of fluvial processes in their development or erosion by rapidly emplaced lavas but the style of lava flow emplacement throughout rift apron development is not clearly understood to better characterize the style of lava emplacement and role of fluvial processes in rift apron development we conducted morphological mapping of the pavonis mons southwest rift apron and the eastern tharsis plains using images from the high resolution imaging science experiment hirise mars orbiter camera moc context camera ctx thermal emission imaging system themis and high resolution stereo camera hrsc along with the mars orbiter laser altimeter mola precision experiment data records pedrs and gridded data our approach was to 1 search for depositional fans at the slope break between the rift apron and adjacent low slope plains 2 determine if there is evidence that previously formed deposits might have been buried by plains units 3 characterize the tharsis plains morphologies east of pavonis mons and 4 assess their relationship to the rift apron units we have not identified topographically significant depositional fans nor did we observe evidence to suggest that plains units have buried older rift apron units flow features associated with the rift apron are observed to continue across the slope break onto the plains in this area the plains are composed of a variety of small fissures and low shield vents around which broad channel fed and tube fed flows have been identified we also find broad flat topped plateaus and sinuous ridges mixed among the channels tubes and vents flat topped plateaus and sinuous ridges are morphologies that are analogous to those observed on the coastal plain of hawai i where lava flows have advanced from the volcano s several degree flank onto the nearly zero degree coastal plain when local volumetric flow rates are low flow fronts tend to spread laterally and often thicken via endogenous growth or inflation of the sheet like flow units if flow advance is restricted by existing topography into narrow pathways inflation can be focused into sinuous elongate ridges the presence of plateaus and ridges emplaced from the rift zones across the plains to the east of pavonis mons and a lack of fan like features or evidence for their burial are consistent with rift apron lavas crossing a slope break with low local volumetric flow rates that led to inflation of sheet like and tube fed lava flows

three dimensional earthquake sequence simulations with evolving temperature and pore pressure due to shear heating effect of heterogeneous hydraulic diffusivity 1 a new methodology for three dimensional 3 d simulations of earthquake sequences is presented that accounts not only for inertial effects during seismic events but also for shear induced temperature variations on the fault and the associated evolution of pore fluid pressure in particular the methodology allows to capture thermal pressurization tp due to frictional heating in a shear zone one dimensional 1 d diffusion of heat and pore fluids in the fault normal direction is incorporated using a spectral method which is unconditionally stable accurate with affordable computational resources and highly suitable to earthquake sequence calculations that use variable time steps the approach is used to investigate the effect of heterogeneous hydraulic properties by considering a fault model with two regions of different hydraulic diffusivities and hence different potential for tp we find that the region of more efficient tp produces larger slip in model spanning events the slip deficit in the other region is filled with more frequent smaller events creating spatiotemporal complexity of large events on the fault interestingly the area of maximum slip in model spanning events is not associated with the maximum temperature increase because of stronger dynamic weakening in that area the region of more efficient tp has lower interseismic shear stress which discourages rupture nucleation there contrary to what was concluded in prior studies seismic events nucleate in the region of less efficient tp where interseismic shear stress is higher in our model hypocenters of large events do not occur in areas of large slip or large stress drop

recent and episodic volcanic and glacial activity on mars revealed by the high resolution stereo camera the large area coverage at a resolution of 10 20 metres per pixel in colour and three dimensions with the high resolution stereo camera experiment on the european space agency mars express mission has made it possible to study the time stratigraphic relationships of volcanic and glacial structures in unprecedented detail and give insight into the geological evolution of mars here we show that calderas on five major volcanoes on mars have undergone repeated activation and resurfacing during the last 20 per cent of martian history with phases of activity as young as two million years suggesting that the volcanoes are potentially still active today glacial deposits at the base of the olympus mons escarpment show evidence for repeated phases of activity as recently as about four million years ago morphological evidence is found that snow and ice deposition on the olympus construct at elevations of more than 7 000 metres led to episodes of glacial activity at this height even now water ice protected by an insulating layer of dust may be present at high altitudes on olympus mons

earthquake nucleation on rate and state faults aging and slip laws we compare 2 d quasi static earthquake nucleation on rate and state faults under both aging and slip versions of the state evolution law for both versions mature nucleation zones exhibit 2 primary regimes of growth well above and slightly above steady state corresponding respectively to larger and smaller fault weakening rates well above steady state aging law nucleation takes the form of accelerating slip on a patch of fixed length this length is proportional to b 1 and independent of a where a and b are the constitutive parameters relating changes in slip speed and state to frictional strength under the slip law the nucleation zone is smaller and continually shrinks as slip accelerates the nucleation zone is guaranteed to remain well above steady state only for values of a b that are low by laboratory standards near steady state for both laws the nucleation zone expands the propagating front remains well above steady state giving rise to a simple expression for its effective fracture energy g c this fracture energy controls the propagation style for the aging law g c increases approximately as the square of the logarithm of the velocity jump this causes the nucleation zone to undergo quasi static crack like expansion to a size asymptotically proportional to b b a 2 for the slip law g c increases only as the logarithm of the velocity jump and crack like expansion is not an option instead the nucleation zone grows as an accelerating unidirectional slip pulse under both laws the nucleation front propagates at a velocity larger than the slip speed by roughly b divided by the logarithm of the velocity jump where is the effective elastic shear modulus for this prediction to be consistent with observed propagation speeds of slow slip events in subduction zones appears to require effective normal stresses as low as 1 mpa

the geological history of nili patera mars nili patera is a 50 km diameter caldera at the center of the syrtis major planum volcanic province the caldera is unique among martian volcanic terrains in hosting i evidence of both effusive and explosive volcanism ii hydrothermal silica and iii compositional diversity from olivine rich basalts to silica enriched units we have produced a new geological map using three mosaicked 18 m pixel context camera digital elevation models supplemented by compact remote imaging spectrometer for mars hyperspectral data the map contextualizes these discoveries formulating a stratigraphy in which nili patera formed by trapdoor collapse into a volcanotectonic depression the distinctive bright floor of nili patera formed either as part of a felsic pluton exposed during caldera formation or as remnants of welded ignimbrite s associated with caldera formation both scenarios deriving from melting in the noachian highland basement after caldera collapse there were five magmatic episodes 1 a basaltic unit in the caldera s north 2 a silica enriched unit and the associated nili tholus cone 3 an intrusive event forming a 300 m high elliptical dome 4 an extrusive basaltic unit emplaced from small cones in the east and 5 an extreme olivine bearing unit formed on the western caldera ring fault the mapping together with evidence for hydrated materials implies magmatic interaction with subsurface volatiles this in an area of elevated geothermal gradient presents a possible habitable environment sampled by the hydrothermal deposits additionally similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices

variability of seismic source spectra estimated stress drop and radiated energy derived from cohesive zone models of symmetrical and asymmetrical circular and elliptical ruptures large variability of earthquake stress drops and scaled energy has been commonly reported in the literature but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties here we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry rupture directivity and rupture speeds we numerically simulate earthquake source scenarios using a cohesive zone model with the small scale yielding limit where the solution approaches a singular crack model with spontaneous healing of slip compared to symmetrical circular source models asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere the general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime although shear mach waves generated by the propagation of supershear rupture lead to much higher corner frequency and scaled energy estimates locally our results suggest that at least a factor of 2 difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops translating into a factor of 8 difference in estimated stress drops furthermore radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry

physical interpretation of the breakdown process using a rate and state dependent friction law abstract we study the dynamic traction and the slip velocity evolution within the cohesive zone during the propagation of a dynamic rupture using rate and state dependent constitutive laws we solve the elastodynamic equation for a 2 d in plane crack using a finite difference algorithm we show that rate and state constitutive laws allow a quantitative description of the dynamic rupture growth we confirm the findings of previous studies that slip weakening sw is a characteristic behavior of rate and state friction our simulations show that the state variable evolution controls the slip acceleration and the slip weakening behavior these modeling results help in understanding the physical interpretation of the breakdown process and the weakening mechanisms we compare the time histories of slip velocity state variable and total dynamic traction to investigate the temporal evolution of slip acceleration and stress drop during the breakdown time because the adopted analytical expression for the state variable evolution controls the slip velocity time histories we test different evolution laws to investigate slip duration and the healing mechanisms we show that the classic slowness or slip laws do not yield fast restrengthening or self healing although they appropriately describe rupture initiation propagation and the long term restrengthening during the interseismic period self healing rupture mode yielding to short slip durations has been obtained for homogeneous faults by modifying the evolution law introducing a fast restrengthening of dynamic traction immediately after the weakening phase in this study we discuss how the direct effect of friction and the friction behavior at high slip rates affect the weakening and healing mechanisms

elastohydrodynamic lubrication of faults the heat flow paradox provides evidence that a dynamic weakening mechanism may be important in understanding fault friction and rupture we present here a specific model for dynamic velocity weakening that uses the mechanics of well studied industrial bearings to explain fault zone processes an elevated fluid pressure is generated in a thin film of viscous fluid that is sheared between nearly parallel surface this lubrication pressure supports part of the load therefore reducing the normal stress and associated friction across the gap the pressure also elastically deforms the wall rock the model is parameterized using the sommerfeld number which is a measure of the lubrication pressure normalized by the lithostatic load for typical values of the material properties slip distance and velocity the sommerfeld number suggests that lubrication is an important process if the lubrication length scales as the slip distance in an earthquake the frictional stress during dynamically lubricated large earthquakes is 30 less than the friction with only hydrostatic pore pressure elastohydrodynamic lubrication also predicts a decrease in high frequency 1 hz radiation above a critical slip distance of a few meters this prediction is well matched by the strong motion data from the 1999 taiwan earthquake the observed 2 orders of magnitude variation in scaled radiated energy between small m w 6 is also predicted by the lubrication model

segregation of olivine grains in volcanic sands in iceland and implications for mars abstract basaltic sands cover several plains in volcanic regions on earth and dominate the aeolian sediments on mars but basaltic sands are not as well characterized as felsic sands the lambahraun sandy lava plain in iceland was chosen as a martian analog to study the physical sorting of basaltic sands the strong winds affecting the plain and its young age

tectonic implications of mars crustal magnetism mars currently has no global magnetic field of internal origin but must have had one in the past when the crust acquired intense magnetization presumably by cooling in the presence of an earth like magnetic field thermoremanent magnetization a new map of the magnetic field of mars compiled by using measurements acquired at an 400 km mapping altitude by the mars global surveyor spacecraft is presented here the increased spatial resolution and sensitivity of this map provide new insight into the origin and evolution of the mars crust variations in the crustal magnetic field appear in association with major faults some previously identified in imagery and topography cerberus rupes and valles marineris two parallel great faults are identified in terra meridiani by offset magnetic field contours they appear similar to transform faults that occur in oceanic crust on earth and support the notion that the mars crust formed during an early era of plate tectonics

variability in earthquake stress drop and apparent stress 1 we apply empirical green s function coda based analysis to four earthquake sequences in japan that span a magnitude range of 1 8 to 6 9 to measure radiated energy corner frequency and stress drop we find no systematic dependence of apparent stress or stress drop on seismic moment for these sequences and find they both are log normally distributed however we identify several anomalous events both energetic and enervated that show sharply different spectral signatures from the rest of the population these events indicate that much of the variation in apparent stress and stress drop is statistically significant which may have important implications for seismic hazard analysis

hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on mars hydrovolcanism is a common natural phenomenon on earth and should be common on mars too since its surface shows widespread evidence for volcanism and near surface water we investigate fields of pitted cones in the nephentes amenthes region at the southernmargin of the ancient impact basin utopia which were previously interpreted as mud volcanoes the cone fields contain pitted and breached cones with associated outgoing flow like landforms based on stratigraphic relations we determined ahesperian or youngermodel age we test the hypothesis of a hydro volcanic origin based on a detailed morphological and morphometrical analysis and an analysis of the regional context an igneous volcanic origin of these cones as hydrovolcanic edifices produced by phreatomagmatic eruptions is plausible several lines of evidence suggest the existence of subsurface water ice the pitted cones display well developed wide central craters with floor elevations below the preeruptive surface their morphometry and the overall appearance are analogous to terrestrial tuff cones and tuff rings mounds that are also observed in the same region resemble terrestrial lava domes the hydrovolcanic interaction between ascending magma and subsurface water and or water ice may explain the formation of the pitted cones although other scenarios such as mud volcanism cannot be ruled out together with the mounds the cones might represent effusive and explosive edifices of amonogenetic volcanic field composed of lava domes tuff rings tuff cones and possibly maars

a systematic mapping procedure based on the modified gaussian model to characterize magmatic units from olivine pyroxenes mixtures application to the syrtis major volcanic shield on mars clenet et al 2011 have developed an adapted version of the modified gaussian model mgm the improvements allow the characterization of spectra of olivine pyroxene s mixtures addressing both modal composition and individual minerals chemical composition this version of mgm is fully automated and operational with large amounts of hyperspectral imaging data two natural cases are considered the first one is the sumail massif in the oman ophiolite earth based on our approach applied to hymap data two contrasted lithologic units are mapped the mantle which is harzburgite dominated and the crust made of gabbros and clinopyroxene rich cumulates with spectral variations interpreted in terms of pyroxenes chemical compositions once this new mgm mapping approach has been validated on a controlled natural situation we map the distribution of mafic assemblages across the syrtis major volcano on mars using a visible and near infrared vnir short wave infrared swir observatoire pour la mineralogie l eau les glaces et l activite omega mars express mex mosaic our results are in agreement with previous work but olivine appears to be more abundant than previously estimated in the central part of the volcanic edifice especially in ternary mafic assemblages augite olivine enstatite based on these results we propose a possible scenario for the igneous and thus thermal evolution of the syrtis major region surrounding terrains have formed first and local heterogeneity can be observed between northern and southern areas of the noachian crust we also observe variations in the mineral assemblages within syrtis major lavas which can be interpreted in terms of differentiation from a common parent melt and or of a progressive evolution of the mantle source composition and temperature key points mgm systematic mapping on syrtis major to characterize mafic minerals mixtures olivine and pyroxenes mixture is detected in the central lavas of the volcano pyroxenes have chemical compositions spanning the enstatite augite range

layered megablocks in the central uplifts of impact craters abstract a database of bedrock exposed in crater central uplifts has been compiled from multiple orbital datasets in this study we focus on uplifts which show decameter scale layers within the exposed megablocks derived from the bedrock of the preexisting target this distinctive morphology found in 41 craters globally occurs mostly in regions mapped as hesperian plains material generally interpreted as regions of extensive flood lavas the association with volcanic units coupled with morphology and mineralogy that is consistent with flood lava emplacement suggests that the layers are stacks of dense lava interbedded with weaker materials estimates for the uplift of stratigraphic sections coupled with morphologic and morphometric analyses lead to estimated thicknesses of the layered unit these estimates indicate hesperian flood lavas are 23 thicker than a previous minimum estimate with a volume of 5 5 10 7 km 3 this increases the known extrusive volcanic production for the history of mars and volatile release to past martian atmospheres

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

absence of silicic volcanism on mars implications for crustal composition and volatile abundance on earth eruptions of silicic magmas with volumes of 1 100 km3 are generally associated with stratovolcanoes or composite cones whereas larger 100 1000 km3 ignimbrite deposits are erupted from resurgent calderas and related structures small volumes of silicic rocks are generated by differentiation within basic magma chambers associated with stratovolcanoes but large silicic deposits are largely confined to areas where the granitic crust is thicker than 25 km as mantle derived magmas rise through the thick granitic crust fractional crystallization and assimilation of hydrous crustal rocks concentrate volatiles and sio2 in the magma yielding explosive silicic eruptions on mars under similar chemical conditions ignimbrite eruptions could take place over a broader range of physical conditions than on earth but no large volume ignimbrite deposits can be identified positively explosive basic to ultrabasic eruptions may be responsible for small cinder conelike features on mars and perhaps for the ancient flank scoured paterae such eruptions may have been driven by near surface water ice magma interactions or volatile rich magmas the paterae represent a unique style of volcanism that stopped 2 b y ago we suggest that the volatiles associated with these features were derived from mantle sources and that the cessation of patera formation may coincide with the termination of the period of maximum planetary degassing volcanism since 2 b y ago has been dominantly effusive except for minor explosive ash events such as those which blanketed part of the summit of hecates tholus we propose that such eruptions were driven by volatiles generated by differentiation of a magma chamber similar to icelandic explosive eruptions the apparent lack of large scale silicic volcanism on mars is circumstantial evidence that mars is deficient in volatiles and lacks a granitic crust

geodetic constraints on the composition of mars the mean density and mean moment of inertia are remotely accessible geodetic parameters which provide two integral constraints on the radial density profile of mars the mean density is presently well known but until the axial precession rate is better determined the amount of inertia will remain uncertain values anywhere in the interval from 0 345 mr sup 2 to 0 365 mr sup 2 can be obtained from the observed gravitational field depending on how that field is partitioned into hydrostatic and nonhydrostatic components even if the moment were accurately known a considerable degree of nonuniqueness would still remain concerning the density and composition this nonuniqueness is illustrated by construction of a family of orthogonal polynomials in the normalized radius which make no contribution to either the mean density or the mean inertial moment if the mean moment is near to the lower end of its plausible range the nonuniqueness in density and composition is considerably diminished in order to avoid unrealistically high densities in the deep interior the martian mantle must have a density appreciably lower than the terrestrial mantle and must consequently be significantly depleted in iron on the other hand if the mean moment is near the upper end ofmore its plausible range a much broader range of density profiles can be accommodated including but not restricted to iron enriched mantles less

scaling of small repeating earthquakes explained by interaction of seismic and aseismic slip in a rate and state fault model because of short recurrence times and known locations small repeating earthquakes present a rare predictable opportunity for detailed field observations they are used to study fault creeping velocities earthquake nucleation stress drops and other aspects of tectonophysics earthquake mechanics and seismology an intriguing observation about repeating earthquakes is their scaling of recurrence time with seismic moment which is significantly different from the scaling based on a simple conceptual model of circular ruptures with stress drop independent of seismic moment and no aseismic slip here we show that a model of repeating earthquakes based on laboratory derived rate and state friction laws reproduces the observed scaling in the model a small fault patch governed by steady state velocity weakening friction is surrounded by a much larger velocity strengthening region long term slip behavior of the fault is simulated using a methodology that fully accounts for both aseismic slip and inertial effects of occasional seismic events the model results in repeating earthquakes with typical stress drops and sizes comparable with observations for a fixed set of friction parameters the observed scaling is reproduced by varying the size of the velocity weakening patch in simulations a significant part of slip on the velocity weakening patches is accumulated aseismically even though the patches also produce seismic events the proposed model supplies a laboratory based framework for interpreting the wealth of observations about repeating earthquakes provides indirect evidence that rate and state friction acts on natural faults and has important implications for possible scenarios of slip partition into seismic and aseismic parts

evolution of the olympus mons caldera mars synoptic images of the martian volcano olympus mons are of a quality and quantity that are unique for mars and somewhat surprisingly are appreciably better than image data that exist for many volcanoes on earth useful information about the evolution of shield volcanoes on earth can thus be derived from the investigation of this extraterrestrial example we have used shadow length measurements and photoclinometrically derived profiles to supplement and refine the topographic map of the olympus mons caldera as much as 2 5 km of collapse took place within the 80 65 km diameter caldera and the elevation of the caldera rim varies by almost 2 0 km low around the oldest collapse events high around the youngest an eight stage evolutionary sequence for the caldera of olympus mons is identified which shows that caldera subsidence was a longterm process rather than the near instantaneous event that has been interpreted from comparable terrestrial examples tectonic features on the caldera floor indicate a transition from an extensional environment graben formation around the perimeter of the caldera to compression ridge formation towards the caldera center this transition from a compressional to extensional environment is surprisingly sudden occurs at a radial distance of 17 km from the caldera center and is import because it can be used to infer that the magma chamber was relatively shallow thought to be at a depth of 16 km beneath the caldera floor zuber and mouginis mark 1990 ample evidence is also found within the olympus mons caldera for solidified lava lakes more than 30 km in width and for the localzed overturning and or withdrawal of lava within these lakes

layering stratigraphy of eastern coprates and northern capri chasmata mars abstract distinct competent layers are observed in the slopes of eastern coprates chasma part of the valles marineris system on mars our observations indicate that the stratigraphy of coprates chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers the strong competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes the topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of coprates chasma less than 100 m below the southern ophir planum surface the volume of the topmost emplaced layer is at least 70 km 3 and may be greater than 2100 km 3 if the unit underlies most of ophir planum the broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within coprates chasma where the layer is also observed rim outcrop morphology and elevation differences between ophir and aurorae plana may be indicative of the easternmost extent of the topmost competent layer these observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the valles marineris is carved and they present a picture of some of the last volcanic activity in this area furthermore the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material

the stratigraphy of the amenthes region mars time limits for the formation of fluvial volcanic and tectonic landforms abstract we present results of our morphologic and stratigraphic investigations in the amenthes region for which our observations suggest a complex spatial and temporal interrelation between volcanic and possibly water related processes we have produced a series of self consistent geological maps and a stratigraphic correlation chart that show the spatial and temporal distribution of volcanic fluvial and tectonic processes the amenthes region consists of a broad trough like topographic depression that has served as a path for the supply of materials from hesperia planum to isidis planitia it is most likely that hesperia planum and in particular the area north of hesperia planum including tinto vallis palos crater and the surrounding dissected highlands have acted as a source region for materials that were transported into the amenthes trough and farther into the isidis basin the amenthes trough as well as the graben of amenthes fossae were formed after the isidis impact in the noachian and represent likely the oldest features in the amenthes region dendritic valley networks that bear evidence for surface runoff have dissected the highlands adjacent to amenthes planum and within the tinto vallis and palos crater region before 3 7 ga the ridged volcanic plains located near the palos crater and tinto vallis region within amenthes planum as well as within the isidis transitional plains were formed between 3 5 and 3 2 ga and represent the volcanic activity which resulted in the flooding of the amenthes trough the sinuous channel of tinto vallis was formed in the hesperian 3 5 ga and shows characteristics which are consistent with both ground water sapping and igneous processes the palos crater outflow channel was formed nearly at the same time as tinto vallis between 3 5 ga and 3 2 ga and postdates the volcanic flooding of the amenthes trough in the hesperian small valleys 3 4 2 8 ga incised into the ridged plains of amenthes planum appear also within the transitional plains located between the amenthes plains and the isidis interior plains our model ages show that tinto vallis the palos crater outflow channel as well as the small valleys are unlikely formed at the same time and by the same processes as the dendritic valley networks and represent an episode that clearly postdates the volcanic activity

state of stress faulting and eruption characteristics of large volcanoes on mars the formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting in turn lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes we develop models of time dependent stress and deformation for the tharsis volcanoes on mars by means of a finite element code we calculate stresses and displacements due to a volcano shaped load emplaced on an elastic plate overlying a viscoelastic mantle models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere the models illustrate the manner in which time dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes as a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting according to the mohr coulomb criterion at the surface of the plate just outward of the volcano near the base of the elastic lithosphere beneath the center of the volcano and on the upper flanks of the volcano early in its growth history normal faulting is the dominant mode of failure predicted for the first region consistent with circumferential graben observed around the tharsis montes and with the scarp at the base of olympus mons interpreted as a large offset listric normal fault normal faulting mostly radially oriented is predicted for the second region failure in the third region is predicted to consist of thrust faulting circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks in models simulating a growing volcano this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail this volume of early failure remains the most highly stressed area in the edifice concentric terraces interpreted by some workers as thrust faults on the upper flanks of olympus mons may correspond to the predicted circumferential thrust features if the most recent increments of volcano growth were relatively large or in the presence of local material property or stress field variations for volcanoes detached from the plate predicted failure in the edifice takes the form of radial normal faulting near the volcano base the addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the tharsis montes including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben this stress state is also consistent with an interpretation of the aureole deposits of olympus mons as the result of gravity sliding along a basal detachment our models also suggest an explanation for the lack of strike slip features predicted by previously published flexural models around the tharsis volcanoes for a given load increment the first mode of near surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation as the volcano grows and the flexural response to the increasing load proceeds the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike slip faulting because normal faulting has been predicted to have taken place earlier however it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike slip faults

late noachian to hesperian climate change on mars evidence of episodic warming from transient crater lakes near ares vallis 1 the ares vallis region is surrounded by highland terrain containing both degraded and pristine large impact craters that suggest a change in climate during the late noachian early hesperian from warmer wetter conditions to colder dryer conditions however the regional occurrence of hesperian age crater outlet channels indicates that this period on mars was characterized by episodic climate fluctuations that caused transient warming facilitating the stability of liquid water at the surface an extensive survey of the morphology and topography of 75 impact basins in the region indicates that of the largest degraded craters 4 were identified with single outlet channels that suggest the former presence of water infill these basins lack inlets indicating that water influx was likely derived from sapping of groundwater a comparison of measured crater rim heights to modeled rim heights suggests that the bulk of the depth diameter reduction on these craters was the result of infilling possibly by sediments crater statistics indicate that crater degradation and infill occurred during a short 200 ma interval in the late noachian from 3 8 ga to 3 6 ga craters that formed after 3 6 ga exhibit a near pristine morphology our results support the hypothesis of rapid climate change at the end of the noachian period however geologic relationships between the crater outlet channels and ares vallis indicate that drainage occurred only after the period of intense crater modification during the hesperian 3 5 2 9 ga this suggests a delay between the time of infill of the craters and the time of drainage

what we know about mars from its impact craters martian impact craters provide insights into formation ages of terrain units nearsurface properties and the planet s degradational history crater size frequency distribution analyses allow determination of terrain unit relative ages and extrapolation of the lunar crater chronology can provide estimates of absolute ages for these units such analyses indicate that mars has been geologically active throughout its history but it also retains extensive regions dating back to ca 4 ga higher cratering rates during the late heavy bombardment lhb helped to create the martian regolith and formation of large impact basins helped to erode the initially denser martian atmosphere and may have created the hemispheric dichotomy low global degradation rates since the end of lhb ca 3 8 ga resulted in well preserved impact crater morphologies the presence of layered ejecta blankets and central pits associated with fresh to moderately eroded craters suggests a volatile rich crust impact crater depth diameter relationships allow estimation of the vertical extent of crustal volatiles to depths greater than those probed by current spacecraft instrumentation temporal changes in subsurface ice reservoir depths are revealed by differences in layered ejecta onset diameter as a function of crater age an understanding of the morphology and morphometry of pristine impact craters facilitates efforts to determine the amount and types of degradation and the periods over which they operated impact craters therefore play a major role in unraveling the climatic and geologic history of mars

geology and landscape evolution of the hellas region of mars hellas basin on mars has been the site of volcanism tectonism and modification by fluvial mass wasting and eolian processes over its more than 4 b y existence our detailed geologic mapping and related studies have resulted in the following new interpretations the asymmetric distribution of highland massifs and other structures that define the uplifted basin rim suggest a formation of the basin by the impact of a low angle bolide having a trajectory heading s60 e during the late noachian the basin was infilled perhaps by lava flows that were sufficiently thick 1 km to produce wrinkle ridges on the fill material and extensional faulting along the west rim of the basin at about the same time deposits buried northern malea planum which are interpreted to be pyroclastic flows from amphitrites and peneus paterae on the basis of their degraded morphology topography and the application of a previous model for pyroclastic volcanism on mars peneus forms a distinctive caldera structure that indicates eruption of massive volumes of magma whereas amphitrites is a less distinct circular feature surrounded by a broad low dissected shield that suggests generally smaller volume eruptions during the early hesperian a 1 to 2 km thick sequence of primarily fined grained eolian material was deposited on the floor of hellas basin subsequently the deposit was deeply eroded except where armored by crater ejecta and it retreated as much as 200 300 km along its western margin leaving behind pedestal craters and knobby outliers of the deposit local debris flows within the deposit attest to concentrations of groundwater perhaps in part brought in by outflow floods along the east rim of the basin these floods may have deposited 100 200 m of sediment subduing wrinkle ridges in the eastern part of the basin floor during the late hesperian and amazonian eolian mantles were emplaced on the basin rim and floor and surrounding highlands their subsequent erosion resulted in pitted and etched plains and crater fill irregular mesas and pedestal craters local evidence occurs for the possible former presence of ground ice or ice sheets 100 km across however we disagree with a hypothesis that suggests that the entire south rim and much of the floor of hellas have been glaciated orientations of dune fields and yardangs in lower parts of hellas basin follow directions of the strongest winds predicted by a recently published general circulation model gcm transient frost and dust splotches in the region are by contrast related to the gcm prediction for the season in which the images they appear in were taken

alba patera mars topography structure and evolution of a unique late hesperian early amazonian shield volcano 1 new spacecraft data especially mola topography reveal important characteristics of alba patera that were either unknown or poorly resolved in previous data sets the major and consistent breaks of slope and the specific pattern of lava flows within a broad region around alba patera permit division of this area into two major parts that appear to be related to different episodes of volcanism at alba patera a low lying apron of lava flows that broadly surrounds the edifice a high standing main construct consisting of a central edifice western and eastern flanking lobes and a summit area characterized by a summit plateau and a summit dome topped by a caldera complex volcanic activity apparently began as massive effusive eruptions that formed the broad apron of lava flows followed by more focused effusive eruptions that built the central construct the construct spread gravitationally in a northerly direction to favor e w dike propagation leading to the formation of tharsis montes like flanking lobes the final stages of volcanic activity at alba patera were concentrated in the summit area forming a summit dome 400 km in diameter a caldera complex and two small shields near and inside the caldera which underwent late stage tilting and deformation the load of the newly documented summit dome may have led to the development of alba fossae reactivation of graben within tantalus fossae and tilting of the summit dome to the east a clear sequence in eruptive style is observed early massive sheet flows later tube fed flows and late shorter length and lower volume sheet flows dendritic valley networks on the northern flanks of alba patera have been interpreted to be evidence of easily erodable material of pyroclastic origin but new data suggest that these friable deposits and the valley networks may have formed instead during amazonian climate excursion periods the extensive apron of lava flows combined with the flanking lava lobes and the tharsis montes scale summit dome make alba patera much more similar to the tharsis montes than to the hesperian aged paterae the alba patera lava apron was emplaced on early hesperian ridged plains largely during the middle to late hesperian stratigraphic relationships suggest that effusive eruptions from alba patera interacted with ice rich deposits in the northern lowlands alba patera flanking lobes and the summit dome were emplaced in the earliest amazonian accompanied by late stage loading graben formation and edifice tilting

earthquake slip weakening and asperities explained by thermal pressurization for the earth s crust to release accumulated stress in the form of an earthquake a fault must weaken and give way otherwise the movement would quickly grind to a halt due to the surrounding pressure this weakening is not really understood laboratory experiments simply don t match seismologists estimates of the distance a fault must slip before it becomes very weak a new study concludes that the missing ingredient is water geological studies and measurements show the slip zones to be narrow impermeable structures water already in the fault zone can quickly become pressurized by frictional heating as the rocks start to move catalysing large earthquakes from minor movements an earthquake occurs when a fault weakens during the early portion of its slip at a faster rate than the release of tectonic stress driving the fault motion1 2 this slip weakening occurs over a critical distance dc understanding the controls on dc in nature is severely limited however because the physical mechanism of weakening is unconstrained conventional friction experiments typically conducted at slow slip rates and small displacements have obtained dc values that are orders of magnitude lower than values estimated from modelling seismological data for natural earthquakes3 4 5 6 here we present data on fluid transport properties of slip zone rocks and on the slip zone width in the centre of the median tectonic line fault zone japan we show that the discrepancy between laboratory and seismological results can be resolved if thermal pressurization of the pore fluid7 8 9 10 is the slip weakening mechanism our analysis indicates that a planar fault segment with an impermeable and narrow slip zone will become very unstable during slip and is likely to be the site of a seismic asperity

tharsis tholus an unusual martian volcano tharsis tholus is unusual martian shield volcano in that the edifice is cut by a series of large normal faults that appear to penetrate the entire volcano northeast trending narrow graben also cut the flank the large normal faults may be caused by loading of a ductile subsurface layer allowing failure of the edifice the narrow graben are typical tensional faults the flank is heavily mantled by aeolian material despite the bulbous appearance the overall morphology of tharsis tholus suggests it is a basaltic shield crater counts indicate an age of early hesperian placing tharsis tholus in the middle of the period of activity that built the other small tharsis volcanoes

global variations of stress drop for moderate to large earthquakes 1 we investigate the global variation of earthquake stress drops using spectra of about 2000 events of mb 5 5 between 1990 and 2007 we use an iterative least squares method to isolate source displacement spectra from travel path and receiver contributions based on a convolutional model the observed p wave source spectra are corrected with a globally averaged empirical correction spectrum and estimates of near source attenuation assuming a brune type source model we estimate corner frequencies and compute stress drops stress drop estimates for individual earthquakes range from about 0 3 to 50 mpa but the median stress drop of about 4 mpa does not vary with moment implying earthquake self similarity over the mw 5 2 to 8 3 range of our data a comparison of our results with previous studies confirms this observation over most of the instrumentally observable magnitude range while the absolute values of our estimated stress drops depend upon the assumed source model we identify relative regional variations of stress drop that are robust with respect to the processing parameters and modeling assumptions which includes an inherent assumption of constant rupture velocity we find a dependence of median stress drop on focal mechanism with a factor of 3 5 times higher stress drops for strike slip earthquakes and also find a factor of 2 times higher stress drops for intraplate earthquakes compared to interplate earthquakes

thermal anomaly at the earth s surface associated with a lava tube abstract lava tubes are frequently encountered in volcanic areas the formation of lava tubes has strong implications on the volcanic hazard during effusive eruptions the thermal dissipation of lava flowing in a tube is reduced in respect to the lava flowing in an open channel so the lava may threaten areas that would not be reached by flows in open channels for this reason it is important to detect the presence of lava tubes in this work we propose a model to detect the presence and the characteristics of lava tubes by their thermal footprint at the surface we model numerically the temperature distribution and the heat flow both in the steady and the transient state and we take into account the principal thermal effects due to the presence of an active lava tube i e the conduction to the ground and the atmosphere the convection and the radiation in the atmosphere we assume that lava fluid is at high temperature in motion inside a sloping tube under the gravity force the thermal profile across the tube direction in particular the width of the temperature curve allows to evaluate the depth of the tube the values of maximum temperature and of tube depth allow to estimate the area of the tube section the shape of the temperature curve and its asymmetry can give information about the geometry of the tube if we observe volcanic areas at different times by thermal cameras we can detect anomalies and evaluate their causes during an eruption in particular we can evaluate whether they are due to active lava flows or not and what is their state for lava tubes we can connect thermal anomalies with lava tube position characteristics and state

young lava flows on the eastern flank of ascraeus mons rheological properties derived from high resolution stereo camera hrsc images and mars orbiter laser altimeter mola data 1 we report on estimates of the rheological properties of late stage lava flows on the eastern flank of ascraeus mons mars from previous studies it is known that the dimensions of flows reflect rheological properties such as yield strength effusion rates and viscosity our estimates are based on new high resolution images obtained by the high resolution stereo camera hrsc on board the european space agency s mars express spacecraft in combination with mars orbiter laser altimeter mola data compared to earlier studies the high spatial resolution of the hrsc and mola data allowed us to map 25 late stage lava flows and to measure their dimensions as well as their morphological characteristics in greater detail our estimates of the yield strengths for these flows range from 2 0 102 pa to 1 3 105 pa with an average of 2 1 104 pa these values are in good agreement with estimates for terrestrial basaltic lava flows and are comparable to previous estimates derived for a small number of lava flows on ascraeus mons our investigation indicates that the effusion rates for the studied ascraeus mons flows are on average 185 m3 s 1 ranging from 23 m3 s 1 to 404 m3 s 1 these results are higher than earlier findings that indicate effusion rates of 18 60 m3 s 1 with an average of 35 m3 s 1 however our effusion rates are similar to terrestrial effusion rates of kilauea and mauna loa and other martian volcanoes on the basis of our estimates of the effusion rates and the measured dimensions of the flows we calculated that the time necessary to emplace the flows is on average 26 days viscosities were estimated on the basis of yield strengths and effusion rates yielding average values of 4 1 106 pa s and ranging from 1 8 104 pa s to 4 2 107 pa s on the basis of newly available data sets e g hrsc mola we are now able not only to identify possible differences in eruptive behavior between ascraeus mons and elysium mons but also to study such differences over time

directly estimating earthquake rupture area using second moments to reduce the uncertainty in stress drop author posting the author s 2018 this article is posted here by permission of oxford university press for personal use not for redistribution the definitive version was published in geophysical journal international 214 2018 2224 2235 doi 10 1093 gji ggy201

major episodes of geologic history of isidis planitia on mars abstract we have mapped the area of isidis planitia 1 27 n 75 103 e in order to assess the geologic history of this region using modern data sets such as mola topography and the high resolution images provided by the hrsc ctx and hirise cameras results of our mapping show that the geologic history of isidis planitia consists of three principal episodes 1 impact dominated episode noachian until 3 8 ga during this time the oldest materials in the study area were formed mostly by impact reworking and mass wasting other processes e g volcanism and fluvial glacial activity likely operated at this time but played a subordinate role 2 an episode related to volcanic and fluvial glacial activities late noachian early amazonian 3 8 2 8 ga volcanism appears as the most important process at the beginning of this episode 3 8 3 5 ga and was responsible for the formation of a large circum isidis volcanic province by the early hesperian epoch volcanic materials covered large portions of the isidis rim almost completely buried the previous crater record on the floor of the isidis basin and probably were the major contributors to the filling of the basin fluvial glacial processes prevailed closer to the end of the episode early hesperian early amazonian 3 5 2 8 ga and were responsible for widespread resurfacing in the isidis planitia region mostly at 3 1 3 4 ga glaciers and or ice sheets probably resulted in a massive glaciation of the rim and the floor of the isidis basin the total volume of material eroded from the isidis rim by glacial and fluvial activity is estimated to be about 35 000 50 000 km 3 which is equivalent to a composite layer about 40 60 m thick on entire floor of the basin more important however is that the eroded materials were likely saturated with ice water and could form wet deposits on the floor 3 wind dominated episode since early amazonian 2 8 ga wind activity dominated the later geologic history of isidis planitia but resulted only in minor modification of the surface

density of the mantle of mars the range of zero pressure densities of the martian mantle which are compatible with the mean density and moment of inertia factor of mars has been explored by 1 considering the full range of plausible core compositions and densities and 2 investigating the sensitivity of computed mantle densities to uncertainties in the input parameters ni fe fe3o4 and fes cores were considered corresponding zero pressure mantle densities range from 3 47 to 3 41 g cu cm uncertainty in the mantle density of about 0 04 g cu cm for the fe core model and about 0 05 g cu cm for the fes core model results from uncertainties in the input parameters the zero pressure density of the martian mantle is estimated to be 3 44 plus or minus 0 06 g cu cm

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

a fault constitutive relation accounting for thermal pressurization of pore fluid 1 the heat generated in a slip zone during an earthquake can raise fluid pressure and thereby reduce frictional resistance to slip the amount of fluid pressure rise depends on the associated fluid flow the heat generated at a given time produces fluid pressure that decreases inversely with the square root of hydraulic diffusivity times the elapsed time if the slip velocity function is crack like there is a prompt fluid pressure rise at the onset of slip followed by a slower increase the stress drop associated with the prompt fluid pressure rise increases with rupture propagation distance the threshold propagation distance at which thermally induced stress drop starts to dominate over frictionally induced stress drop is proportional to hydraulic diffusivity if hydraulic diffusivity is 0 02 m2 s estimated from borehole samples of fault zone material the threshold propagation distance is 300 m the stress wave in an earthquake will induce an unknown amount of dilatancy and will increase hydraulic diffusivity both of which will lessen the fluid pressure effect nevertheless if hydraulic diffusivity is no more than two orders of magnitude larger than the laboratory value then stress drop is complete in large earthquakes

dynamic shear rupture interactions with fault bends and off axis secondary faulting 1 on the basis of elastodynamic stress fields for singular crack and nonsingular slip weakening models of propagating rupture we develop preliminary answers to such questions as follows if a rupturing fault is intersected by another providing a possible bend in the failure path when will stressing be consistent with rupture along the bend what secondary fault locations and orientations in a damaged region bordering a major fault will be stressed to failure by the main rupture stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed especially near the limiting speed rayleigh for mode ii shear for mode iii whether a bend path once begun can be continued to larger scales depends on principal stress directions and ratios in the prestress field conditions should often be met in mode ii for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger scale stressing a basis for intermittent rupture propagation and spontaneous arrest secondary failure in the damage zone likewise increases markedly as the limiting speed is approached such may make the fracture energy much greater than for slip on a single surface the extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength for mode ii prestress controls whether activation occurs primarily on the extensional side which we show to be the typical case or on the compressional side too natural examples are consistent with the concepts developed

seismic slip record in carbonate bearing fault zones an insight from high velocity friction experiments on siderite gouge pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip we report from high velocity friction experiments on siderite bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite calcite and quartz initially at room temperature under normal stresses of 0 6 1 3 mpa and at seismic slip rates of 1 3 2 0 m s in all cases gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and co 2 gas as confirmed by co 2 measurement x ray diffraction analyses and transmission electron microscopy the weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude those changes can be recognized in natural fault zones and black gouge in the chelungpu fault zone in taiwan is perhaps such an example thus our results suggest that thermal decomposition in shallow crustal faults can be an important co seismic process not only for dynamic fault weakening but also for leaving seismic slip records

can observations of earthquake scaling constrain slip weakening summary we use observations of earthquake source parameters over a wide magnitude range m w 0 7 to place constraints on constitutive fault weakening the data suggest a scale dependence of apparent stress and stress drop both may increase slightly with earthquake size we show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes we select 30 earthquakes well recorded at 2 5 km depth at cajon pass california we use individual and empirical green s function spectral analysis to improve the resolution of source parameters including static stress drop and total slip s we also measure radiated energy e s w ecompare the cajon pass results with those from larger california earthquakes including aftershocks of the 1994 northridge earthquake and confirm the results of abercrombie 1995 e s m 0 where rigidity and both e s m 0 and increase as m 0 and s increases uncertainties remain large due to model assumptions and variations between possible models and earthquake scale independence is possible within the resolution assuming that the average trends are real we define a quantity g 2 es m 0 s 2w hich is the total energy dissipation in friction and fracture minus 1 s w here 1 is the final static stress if 1 d the dynamic shear strength during the last increments of seismic slip then g g the fracture energy in a slip weakening interpretation of dissipation

elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate and state dependent friction we present an efficient and rigorous numerical procedure for calculating the elastodynamic response of a fault subjected to slow tectonic loading processes of long duration within which there are episodes of rapid earthquake failure this is done for a general class of rate and state dependent friction laws with positive direct velocity effect the algorithm allows us to treat accurately within a single computational procedure loading intervals of thousands of years and to calculate for each earthquake episode initially aseismic accelerating slip prior to dynamic rupture the rupture propagation itself rapid post seismic deformation which follows and also ongoing creep slippage throughout the loading period in velocity strengthening fault regions the methodology is presented using the two dimensional 2 d antiplane spectral formulation and can be readily extended to the 2 d in plane and 3 d spectral formulations and with certain modifications to the space time boundary integral formulations as well as to their discretized development using finite difference or finite element methods the methodology can be used to address a number of important issues such as fault operation under low overall stress interaction of dynamic rupture propagation with pore pressure development patterns of rupture propagation in events nucleated naturally as a part of a sequence the earthquake nucleation process earthquake sequences on faults with heterogeneous frictional properties and or normal stress and others the procedure is illustrated for a 2 d crustal strike slip fault model with depth variable properties for lower values of the state evolution distance of the friction law small events appear the nucleation phases of the small and large events are very similar suggesting that the size of an event is determined by the conditions on the fault segments the event is propagating into rather than by the nucleation process itself we demonstrate the importance of incorporating slow tectonic loading with elastodynamics by evaluating two simplified approaches one with the slow tectonic loading but no wave effects and the other with all dynamic effects included but much higher loading rate

clues to the lithospheric structure of mars from wrinkle ridge sets and localization instability 1 wrinkle ridges are a manifestation of horizontal shortening in planetary lithospheres for which deformation is localized on faults that underlie individual ridges in ridged plains of mars such as solis planum or lunae planum wrinkle ridges are spaced 40 km apart whereas in the martian northern lowlands where ridges are identified only in mars observed laser altimeter mola altimetric data the ridge spacing is at least 80 km we attribute ridge spacing to an instability of the lithosphere under horizontal compression the localization instability which results in periodically spaced faults montesi and zuber 2003a links the difference of ridge spacing in the northern lowlands and in the highland ridged plains to the difference of crustal thickness via the depth of the brittle ductile transition bdt in solis and lunae plana where the crust is 50 to 60 km thick the crust may be ductile at depth limiting faulting to the bdt of crustal rocks in the lowlands the crust is only about 30 km thick and may be brittle throughout thus the depth of faulting may be controlled by the bdt of mantle rocks which is roughly a factor of two deeper than that of crustal rocks the geotherm can be identical in both regions at 12 3 k km 1 although differences of a few k km 1 can be accommodated within this model the heat flux implied by this geotherm is similar to the heat produced by radiogenic decay 3 gyr ago our analysis provides a rheological explanation for the difference in spacing between ridges in the highlands and the lowlands in contrast to the suggestion of head et al 2002 who proposed that alternating lowlands ridges are buried by sediments in addition finite element models that use the lithospheric structure deduced from ridge spacing show that modest gradients of crustal thickness or heat flux across a ridged plains favor the formation of faults dipping toward high elevation areas as may be the case in solis planum golombek et al 2001

rootless cones on mars a consequence of lava ground ice interaction fields of small cratered cones on mars are interpreted to have formed by rootless eruptions due to explosive interaction of lava with ground ice contained within the regolith beneath the flow melting and vaporization of the ice and subsequent explosive expansion of the vapour act to excavate the lava and construct a rootless cone around the explosion site similar features are found in iceland where flowing lava encountered water saturated substrates the martian cones have basal diameters of c 30 1000 m and are located predominantly in the northern volcanic plains high resolution mars orbiter camera images offer significant improvements over viking data for interpretation of cone origins a new model of the dynamics of cone formation indicates that very modest amounts of water ice are required to initiate and sustain the explosive interactions that produced the observed features this is consistent with the likely low availability of water ice in the martian regolith the scarcity of impact craters on many of the host lava flows indicates very young ages suggesting that ground ice was present as recently as less than 10 l00 ma and may persist today rootless cones therefore act as a spatial and temporal probe of the distribution of ground ice on mars which is of key significance in understanding the evolution of the martian climate the location of water in liquid or solid form is of great importance to future robotic and human exploration strategies and to the search for extraterrestrial life

upper cretaceous campanian phosphorites in jordan implications for the formation of a south tethyan phosphorite giant a record of sedimentary authigenic and biological processes are preserved within the upper cretaceous campanian alhisa phosphorite formation ap in central and northern jordan the ap formed near the eastern extremity of the south tethyan phosphorite province stpp a carbonate dominated upper cretaceous to eocene phosphorite giant that extends from colombia north africa to the middle east multidisciplinary research of the ap and associated cherts chalks and oyster buildups indicate that phosphatic strata formed on a highly productive storm dominated east west trending epeiric platform along the south tethyan margin the onset of phosphogenesis and the accumulation of economic phosphorite coincided with a rise in relative sea level that onlapped peritidal carbonates of the ajlun group pristine phosphates are associated with well developed micrite concretionary horizons and contain abundant non keeled spiral planktic foraminifera and a low diversity benthic assemblage of buliminacean foraminifera suggesting that pristine phosphates are a condensed facies and phosphogenesis was stimulated by the effects of a highly productive surface ocean and the suboxic diagenesis of sedimentary organic matter the bulk sediment composition and absence of fe bearing authigenic phases such as glauconite pyrite including pyrite molds siderite and goethite within pristine phosphates suggests that deposition and authigenesis occurred under conditions of detrital starvation and that iron pumping played a minimal role in phosphogenesis authigenic precipitation of phosphate occurred in a broad array of sedimentary environments herein termed a phosphorite nursery that spanned the entire platform this is a non uniformitarian phenomenon reflecting precipitation of sedimentary apatite across a wide depositional spectrum in a variety of depositional settings wherever the conditions were suitable for phosphogenesis sedimentologic data indicate that pristine phosphates were concentrated into phosphatic grainstones through storm wave winnowing and storm generated shelf parallel geostrophic currents economic phosphorites formed through the amalgamation of storm induced event beds stratigraphic packaging of phosphatic strata indicates that temporal variations in storm frequency were a prerequisite for the formation of economic phosphorite syndepositional phosphogenesis reworking and amalgamation to form phosphorites contrasts sharply with the principles of baturin cycling a transgressive systems tract coupled with high surface productivity created detritally starved settings favourable for phosphogenesis storm reworking of pristine

a constitutive law for rate of earthquake production and its application to earthquake clustering seismicity is modeled as a sequence of earthquake nucleation events in which the distribution of initial conditions over the population of nucleation sources and stress ing history control the timing of earthquakes the model is implemented using solutions for nucleation of unstable fault slip on faults with experimentally derived rate and state dependent fault properties this yields a general state variable constitutive formulation for rate of earthquake production resulting from an applied stressing history to illustrate and test the model some characteristics of seismicity following a stress step have been explored it is proposed that various features of earthquake clustering arise from sensi tivity of nucleation times to the stress changes induced by prior earthquakes the model gives the characteristic omori aftershock decay law and interprets aftershock parameters in terms of stress change and stressing rate earthquake data appear to support a model prediction that aftershock duration defined as the time for rates to return to the back ground seismicity rate is proportional to mainshock recurrence time observed spatial and temporal clustering of earthquake pairs arises as a consequence of the spatial depen dence of stress changes of the first event of the pair and stress sensitive time dependent nucleation applications of the constitutive formulation are not restricted to the simple stress step models investigated here it may be applied to stressing histories of arbitrary complexity the apparent success at modeling clustering phenomena suggests the possibil ity of using the formulation to estimate short to intermediate term earthquake probabil ities following occurrence of other earthquakes and for inversion of temporal variations of earthquake rates for changes in driving stress

assumptions about the presence of natural glasses on mars natural glasses occur not only on earth but also in lunar rocks and in some meteorites on geological grounds natural glasses could be expected on the surface of mars and their presence would have important implications for the weathering and alteration of the near surface layer ultramafic and mafic magmatism rich in iron and some volatiles appears to have been important on mars as has been impact cratering volcanism and possibly hydrothermal alteration available data indicate that we can presume the past or present existence of volcanic basaltic glasses impact glasses formed by thermal fusion of meteorite impact targets and diaplectic mainly maskelynite glasses conditions appear unsuitable for the formation of tektites on the martian surface and the production of measurable amounts of fulgurite or combustion glass seems improbable prospects for the remote and in situ identification of martian glasses and their subsequent weathering products are discussed and include orbital reflectance and thermal emission spectroscopic measurements and mossbauer spectroscopy from a surface lander

microseismicity simulated on asperity like fault patches on scaling of seismic moment with duration and seismological estimates of stress drops observations show that microseismic events from the same location can have similar source durations but different seismic moments violating the commonly assumed scaling we use numerical simulations of earthquake sequences to demonstrate that strength variations over seismogenic patches provide an explanation of such behavior with the event duration controlled by the patch size and event magnitude determined by how much of the patch area is ruptured we find that stress drops estimated by typical seismological analyses for the simulated sources significantly increase with the event magnitude ranging from 0 006 to 8 mpa however the actual stress drops determined from the on fault stress changes are magnitude independent and 3 mpa our findings suggest that fault heterogeneity results in local deviations in the moment duration scaling and earthquake sources with complex shapes of the ruptured area for some of which stress drops may be significantly 100 1 000 times underestimated by the typical seismological methods

lava flows on mars analysis of small surface features and comparisons with terrestrial analogs small scale surface features on lava flows provide information on the eruption parameters and the volcanic history of the flows in which they occur high resolution images of four sites on mars reveal lava flows with a different type or combination of features including pressure ridges tumuli festoon ridges and ring ridges pressure ridges tumuli and pressure plateaus are spaced irregularly and tend to be oriented parallel to flow they are formed by inflation of small flow lobes and are indicative of compound flows emplaced by sporadic eruptions with low effusion rates festoon ridges are oriented transverse to flow direction mimic the flow margin and have a regular spacing using a model by fink and fletcher 1978 and fink 1980 a interior viscosity values of approximately 108 pa s were calculated for festoon ridges tens to hundreds of meters apart and a few meters high on two basalt flows in iceland and a flow on mars this viscosity is comparable to results from similar studies of more silicic flows and suggests that high viscosities are necessary to form large festoon ridges by analogy to the icelandic flows martian flows containing festoon ridges are considered to be large single unit basalt flows emplaced through flood style eruptions high viscosity of the basaltic flows may result from cooling a high crystal content or low effusion temperatures disruption of the surface of some of these large sheet flows possibly by lava water interaction is suggested by the presence of ring ridges and rugged terrain consisting of mesa like mounds and channel like troughs

martian cratering revisited implications for early geologic evolution improved crater statistics from varied martian terrains are compared to lunar crater populations the distribution functions for the average martian cratered terrain and the average lunar highlands over the diameter range 8 2000 km are quite similar the martian population is less dense by approximately 0 70 from 8 to 256 km diameter and diverges to proportionally lower densities at greater diameters crater densities on martian pure terra give a lower limit to the mars moon integrated crater flux of 0 75 since the last stabilization of the respective planetary crusts the crater population 8 km diameter postdating the martian northern plains is statistically indistinguishable from that population postdating the lunar maria monte carlo simulations were performed to constrain plausible mechanisms of crater obliteration the models demonstrate that if the crater density difference between the lunar and martian terra has been due to resurfacing processes random intercrater plains formation cannot be the sole process if plains preferentially form in and obliterate larger craters then the observed martian distribution retains its shape as the crater density decreases this result is consistent with the morphology of martian intercrater plains

an in situ record of major environmental transitions on early mars at northeast syrtis major the noachian hesperian transition on mars was a period marked by changes in volcanic processes and styles of aqueous alteration understanding the timing and nature of environmental change requires the exploration of units recording both sets of processes herein we report the compositional stratigraphy of distinctive noachian to hesperian units along the northeastern margin of the syrtis major volcanic flows a layered polyhydrated sulfate bearing unit with jarosite ridges has been discovered beneath the syrtis major lava flows and above the regionally extensive stratigraphy of noachian plains units reported previously sequential clay carbonate and sulfate bearing units formed in situ and record a transition from alkaline ph to acidic ph waters the sequence is chronologically bookended by the isidis impact and syrtis major flows and is one of the most temporally constrained and well preserved stratigraphic sections from early mars available for landed exploration

three dimensional boundary integral modeling of spontaneous earthquake sequences and aseismic slip 1 fault processes involve complex patterns of seismic events and aseismic slip this work develops a three dimensional 3 d methodology for simulating long term history of spontaneous seismic and aseismic slip on a vertical planar strike slip fault subjected to slow tectonic loading our approach reproduces all stages of earthquake cycles from accelerating slip before dynamic instability to rapid dynamic propagation of earthquake rupture to postseismic slip and to interseismic creep including aseismic transients we use the developed 3 d methodology to study interaction of fault slip with a small patch of higher normal stress over long term slip history for uniform initial prestress dynamic rupture is significantly affected by the stronger patch in the first simulated event but not in subsequent ones the change in behavior is due to redistribution of shear stress by prior slip which demonstrates that distributions of fault strength and stress are related and illustrates the importance of simulating long slip histories even in studies of dynamic rupture despite no long term effect on dynamic rupture the small patch of higher normal stress influences nucleation processes and hence long term slip patterns in the model comparison of the fully dynamic simulations and a widely used quasi dynamic approach shows that the quasi dynamic approach modifies long term slip patterns in addition to resulting in much smaller slip velocities and rupture speeds during dynamic events we show that the response of quasi dynamic formulations with reduced radiation damping terms can be scaled to match the results of the standard quasi dynamic formulation and hence cannot improve the comparison