sedimentary resurfacing and fretted terrain development along the crustal dichotomy boundary aeolis mensae mars 1 the evolution of the martian crustal dichotomy boundary which separates the southern cratered highlands from the northern lowland plains by 1 3 km of elevation remains among the fundamental outstanding issues in mars research for a study area at aeolis mensae we show that fretted terrain formed exclusively in a 2 km thick late noachian 3 7 ga sedimentary deposit that overlies the base of an older cratered dichotomy boundary slope in this equatorial study area fretted terrain does not exhibit the debris aprons or lineated valley fills that are attributed to ground ice in otherwise similar midlatitude fretted terrain in arabia terra the massive deposit of fine sand or loess was probably transported from the north by wind and trapped against the precursor dichotomy slope producing a similar initial form to the younger medusae fossae layered materials that occur east of aeolis mensae contemporary with the latest noachian to early hesperian decline in fluvial erosion the fretting process likely initiated as the massive layer s indurated surface was compromised by fracture cratering or collapse into possible voids in these depressions grain impact or contact with water disaggregated the fine sedimentary materials which were then largely deflated by wind the fretting process largely ended when liquid water was no longer widely available for weathering during the early hesperian period although some degradation of the region by aeolian and slope processes has continued to the present

morphology and inner structure of luanshibao rock avalanche in litang china and its implications for long runout mechanisms abstract large rock avalanches usually move rapidly and travel long distances making them extremely hazardous and often result in catastrophic damages the currently proposed hypotheses for the mechanisms causing their high mobility remain controversial petley 2013 the luanshibao ancient granite rock avalanche with a 10be exposure age of 3510 346 years b p and a deposit volume of 67 mm3 straddles on the litang active fault and was deposited on an open flat river terrace without confinement it travelled about 4000 m with an apparent coefficient of friction of 0 22 field investigation interpretation of satellite images and study of the morphology and inner structure of its deposit were used to analyze its kinetics and dynamics and its long runout mechanisms the study showed that there was probably a two stage movement transition from rock slope collapsing to debris spreading based on the remarkable high secondary scarps and a long belt of bare blocks the runout is closely related to the volume detached from the secondary scarp three different kinds of deposit morphologies were recognized i e parallel transverse ridge grooves x grooves and rhombic platforms and round elliptical hummocks from the proximal to the distal end indicating the strain response to stresses in the debris in motion due to compression shear and tension respectively these deposit morphologies as well as the inner structures such as homogeneous layers of blocks inner shear belts and jigsaw fractured blocks suggested that the extra runout debris moved as a pattern of laminar flow or as a flow sheet however the strong mixing of fluvial pebbles and avalanche debris in the distal hummocky zone suggested a pattern of turbulent flow the possible liquefaction of saturated fluvial sediments by a triggering strong earthquake the extra pore pressure generated by the high speed undrained shearing of debris in the substrate and the entrainment of a large amount of quaternary sediment could have contributed to the great reduction of friction of the basal layer the particle size decreasing with distance away from the secondary scarps and the disintegration of jigsaw structures suggest that rock fragmentation occurred during motion this implies that the long runout of the rock avalanche resulted from many factors

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

laboratory analogue of the formation of molards cones of rock avalanche debris debris cones or molards occur on the deposits of rockslide avalanches they are a few metres high conical in shape and contain material finer than the surrounding debris previously proposed formation mechanisms of molards were tested against field evidence from the frank slide and are found to be unsatisfactory laboratory experiments indicate that when a mass of cohesion less soil is vibrated particle formed molards therefore may form by vibration of the ground under rockslide avalanches this formation mechanism may bear on the problem of rockslide avalanche mobility

lithospheric flexure and the origin of the dichotomy boundary on mars the crustal dichotomy between the ancient highlands of the southern hemisphere and the young appearing northern lowlands is a defining feature of mars in the eastern hemisphere the dichotomy boundary is marked by a prominent scarp and extensional and compressional tectonic features topographic data across the boundary returned from the mars global surveyor indicate lithospheric flexure of the southern highlands the topography of the boundary can be fit by a universal lithospheric deflection profile that corresponds to an elastic thickness of 31 36 km flexure of the southern highlands may be due to late noachian early hesperian vertical loading of the northern lowlands fracturing and normal faulting along the boundary may be in response to bending stresses while thrust faulting may result from a combination of stresses due to flexure erosion and global contraction

mapping the u s west coast surface circulation a multiyear analysis of high frequency radar observations signals with phase speeds of o 10 and o 100 to 300 km day 1 and time scales of 2 to 3 weeks the signals with slow phase speed are only observed in southern california it is hypothesized that they are scattered and reflected by shoreline curvature and bathymetry change and do not penetrate north of point conception the seasonal transition of alongshore surfacecirculationforcedbyupwelling favorablewindsandtheirrelaxationiscapturedinfine detail submesoscaleeddies identifiedusingflowgeometry haverossbynumbersof0 1to3 diameters in the range of 10 to 60 km and persistence for 2 to 12 days the hfr surface currents resolve coastal surface ocean variability continuously across scales from

characterizing frontal eddies along the east australian current from hf radar observations the east australian current eac dominates the ocean circulation along south eastern australia however little is known about the submesoscale frontal instabilities associated with this western boundary current one year of surface current measurements from hf radars in conjunction with mooring and satellite observations highlight the occurrence and propagation of meanders and frontal eddies along the inshore edge of the eac eddies were systematically identified using the geometry of the high spatial resolution 1 5 km surface currents and tracked every hour cyclonic eddies were observed irregularly on average every 7 days with inshore radius 10 km among various forms of structures frontal eddies associated with eac meanders were characterized by poleward advection speeds of 0 3 0 4 m s migrating as far as 500 km south based on satellite imagery flow field kinematics show that cyclonic eddies have high rossby numbers 0 6 1 9 and enhance particle dispersion patches of intensified surface divergence at the leading edge of the structures are expected to generate vertical uplift this is confirmed by subsurface measurements showing temperature uplift of up to 55 m over 24 h and rough estimates of vertical velocities of 10s of meters per day while frontal eddies propagate through the radar domain independently of local wind stress upfront wind can influence their stalling and growth and can also generate large cold core eddies through intense shear such coherent structures are a major mechanism for the transport and entrainment of nutrient rich coastal or deep waters influencing physical and biological dynamics and connectivity over large distances

zooplankton distribution and dynamics in a north pacific eddy of coastal origin ii mechanisms of eddy colonization by and retention of offshore species abstract mesoscale anticyclonic eddies form annually in late winter along the eastern margin of the subarctic north pacific eddies that originate off the southern tip of the queen charlotte islands near 52 n 132 w are called haida eddies during the subsequent 1 3 years they propagate westward into the alaska gyre enroute the eddies are colonized by zooplankton originating from the central british columbia continental shelf the continental slope and along slope boundary current and the oceanic alaska gyre eddies also gradually lose kinetic energy water properties and biota to the surrounding ocean in this paper we analyze zooplankton samples from haida eddies obtained in late winter early summer and autumn of 2000 and in early summer and autumn of 2001 and compare the within eddy zooplankton distributions abundances and community composition of the oceanic origin species to observations from the continental margin and alaska gyre source regions most between region comparisons were consistent with a hypothesis that the eddy zooplankton are a mixture intermediate in abundance and community composition between the bc continental margin and offshore alaska gyre source regions although usually closer to the alaska gyre however about 30 of the comparisons showed within eddy abundances higher than in either source region this outcome cannot arise from mixing alone aggregation and retention appear to be linked to vertical distribution behavior most of the successful taxa spend much of their time below the surface mixed layer this minimizes their exposure to wash out by three physical processes slow upwelling and surface divergence that accompanies weakening of the anticyclonic geostrophic currents rapid but intermittent flushing of the surface layer by ekman transport during strong wind events exchange across the eddy margin geostrophic streamlines caused by temporary displacement by wind driven inertial currents

insights into the kinematics and dynamics of the luanshibao rock avalanche tibetan plateau china based on its complex surface landforms abstract complex topographic features on the surfaces of rock avalanche deposits have been regarded as important indicators for understanding avalanche kinematics and dynamics based on a combination of remote sensing data and in situ observations of surface landforms including toreva blocks transverse and longitudinal ridges ridges separated by conjugate troughs and hummocks a giant holocene rock avalanche on the tibetan plateau is recognized this work aims to understand the kinematics of this event in planview the tongue shaped rock avalanche exhibits a clear sequential distribution of various landforms the translation zone ii is characterised by toreva blocks originating from extension in subzone iii 1 of the accumulation zone iii compression related transverse ridges are well developed longitudinal ridges resulting from lateral velocity differences due to radial spreading are present in the front part of subzone iii 1 farther down ridges separated by conjugate troughs generated by a combination of compression and radial spreading are common in subzone iii 2 subzone iii 3 is characterised by abundant scattered hummocks with circular to oval shapes based on the distributions of these landforms this rock avalanche is proposed to have mainly experienced a motion featuring laminar flow with radial spreading in its distal and marginal parts during transport three distinct processes appear to have occurred in the avalanche mass from the proximal to the distal zones extension dominated sliding in zone ii compression dominated sliding in subzones iii 1 and iii 2 and rapid radial spreading in subzone iii 3

a western boundary current eddy characterisation study abstract the analysis of an eddy census for the east australian current eac region yielded a total of 497 individual short lived 7 28 days cyclonic and anticyclonic eddies for the period 1993 to 2015 this was an average of about 23 eddies per year 41 of the tracked individual cyclonic and anticyclonic eddies were detected off southeast queensland between about 25 s and 29 s this is the region where the flow of the eac intensifies forming a swift western boundary current that impinges near fraser island on the continental shelf this zone was also identified as having a maximum in detected short lived cyclonic eddies a total of 94 43 individual cyclonic eddies or about 4 5 per year were tracked in this region the census found that these potentially displaced entrained water by about 115 km with an average displacement speed of about 4 km per day cyclonic eddies were likely to contribute to establishing an on shelf longshore northerly flow forming the western branch of the fraser island gyre and possibly presented an important cross shelf transport process in the life cycle of temperate fish species of the eac domain in situ observations near western boundary currents previously documented the entrainment off shelf transport and export of near shore water nutrients sediments fish larvae and the renewal of inner shelf water due to short lived eddies this study found that these cyclonic eddies potentially play an important off shelf transport process off the central east australian coast

a slab on slab model for the flims rockslide swiss alps the flims rockslide is the largest landslide in the alps with an estimated volume of 12 km3 it resulted from a prehistoric high speed movement of a large limestone mass several main factors infl

decadal variability of east australian current transport inferred from repeated high density xbt transects a ctd survey and satellite altimetry a time series of the net geostrophic transport through the tasman sea representing the flow of the east australian current eac extension is determined from a full depth ctd section 15 years of high density xbt transects and satellite altimetry data a section between sydney and wellington px34 has been occupied four times per year since 1991 with high resolution xbt sampling two methods to infer baroclinic transport from proxy data along the section are presented the first uses shallow xbt transects to derive geostrophic transport relative to a deep 2000 m reference level in the second approach synts the subsurface temperature and salinity structure are inferred from satellite surface height and temperature fields using a model developed from historical in situ observations the baroclinic transport is then computed in the usual manner the methods are validated using both a full depth ctd occupation of the px34 section and further transects crossing the eac in the northern tasman sea there is close agreement between the 49 xbt and synts px34 transport estimates obtained between 1992 and 2006 the time series of transport through the sydney wellington section shows a range of temporal signals from eddyscale seasonal interannnual to decadal in particular we note that the net eac flow ranges from 5 sv in 1995 to a maximum of 16 sv in 2000 2001 this decadal variation confirms the eac response to a spin up of the south pacific circulation forced by changes in the basin wide winds and matches the changes in oceanic properties observed in the tasman sea

a numerical modeling study of the east australian current encircling and overwashing a warm core eddy 1 warm core eddies wces often form in the meanders of western boundary currents wbcs wces are frequently overwashed with less dense waters sourced from the wbc we use the regional ocean modelling system to investigate the ocean state during the overwashing of one such wce in october 2008 in the east australian current eac comparisons of model outputs with satellite sea surface temperature and vertical profiles show that the model provides a realistic simulation of the eddy during the period when the eac encircled and then overwashed the eddy during the encircling stage an eddy with closed circulation persisted at depth in the surface eac water entered from the north encircled the eddy and exited to the east the overwashing stage was initiated by the expulsion of cyclonic vorticity for the following 8 days after the expulsion waters from the eac washed over the top of the eddy transferring heat and anticyclonic vorticity radially inward after approximately one rotation period of overwashing the eddy separated the overwashing creates a two layer system that forms a subsurface maximum velocity at the interface of the two layers analysis of water mass properties eulerian tracer dynamics and lagrangian particle tracks show that the original eddy sinks 10 50 m during the overwashing period overwashing has been observed in many wbcs and occurs in most wces in the western tasman sea

characteristics and transport mechanism of the nyixoi chongco rock avalanche on the tibetan plateau china abstract for studying rock avalanches complex surficial and internal characteristics have been regarded as important indicators for understanding avalanche transport mechanism in the present study deposits of a massive holocene rock avalanche termed the nyixoi chongco rock avalanche located on the lhasa terrane of tibetan plateau in china are investigated based on a combination of remote sensing data and in situ observations it is revealed that abundant surface landforms including toreva blocks longitudinal and transverse ridges en echelon ridges and hummocks are commonly developed corresponding to these surface morphologies a series of internal sedimentary structures including jigsaw structures inner shear zones aligned clasts diapiric structures convoluted laminations and basal decollements are also well developed through an analysis of these surficial and internal characteristics we propose that a rapid motion similar to a laminar flow most likely occurred after the avalanche mass detaching from the source area in this process the detached avalanche mass was mainly emplaced by a frictional simple shear process characterized by a bulldozing effect that occurred between the sliding mass and the substrate from the proximal to the distal sections of the avalanche the transport of the rock mass can be classified into four major processes an extension dominated sliding process in the transition zone a compression dominated sliding process in subzones iii 1 and iii 2 of the accumulation zone iii a sliding process co dominated by compression and lateral spreading in subzone iii 3 and a rapid radial spreading process in subzone iii 4 the results of our study provided an improved understanding of the transport mechanisms of this rock avalanche

observations from the large rapid yigong rock slide debris avalanche southeast tibet the yigong rock slide debris avalanche yra which occurred on 9 april 2000 received worldwide attention as one of the largest nonseismic landslides in recent years with a volume of 0 3 109

internal structure and early thermal evolution of mars from mars global surveyor topography and gravity topography and gravity measured by the mars global surveyor have enabled determination of the global crust and upper mantle structure of mars the planet displays two distinct crustal zones that do not correlate globally with the geologic dichotomy a region of crust that thins progressively from south to north and encompasses much of the southern highlands and tharsis province and a region of approximately uniform crustal thickness that includes the northern lowlands and arabia terra the strength of the lithosphere beneath the ancient southern highlands suggests that the northern hemisphere was a locus of high heat flow early in martian history the thickness of the elastic lithosphere increases with time of loading in the northern plains and tharsis the northern lowlands contain structures interpreted as large buried channels that are consistent with northward transport of water and sediment to the lowlands before the end of northern hemisphere resurfacing

molards as an indicator of permafrost degradation and landslide processes molards have been defined in the past as conical mounds of debris that can form part of a landslide s deposits we present the first conclusive evidence that molards in permafrost terrains are cones of loose debris that result from thawing of frozen blocks of ice rich sediments mobilised by a landslide and hence propose a rigorous definition of this landform in permafrost environments we show that molards can be used as an indicator of permafrost degradation and that their morphometry and spatial distribution give valuable insights into landslide dynamics in permafrost environments we demonstrate that molards are readily recognisable not only in the field but also in remote sensing data surveys of historic aerial imagery allow the recognition of relict molards which can be used as an indicator of current and past permafrost conditions the triggering of landslides as a result of permafrost degradation will arguably occur more often as global atmospheric temperatures increase so molards should be added to our armoury for tracking climate change as well as helping us to understand landslide related hazards finally we have also identified candidate molards on mars so molards can inform about landscape evolution on earth and other planetary bodies

the nimbus 7 czcs experiment in the benguela current region off southern africa february 1980 2 interpretation of imagery and oceanographic implications three nimbus 7 czcs chlorophyll images of the southern benguela current region off the west coast of south africa on february 9 13 and 15 1980 during a surface truth experiment in the area are evaluated in the light of a time series of airborne radiation thermometry and wind data for this period and the validity of czcs chlorophyll estimates for the region is discussed there was good agreement between upwelling and relaxation in the system and changes in the distribution of chlorophyll the development of upwelling tongues with the subsequent increase in chlorophyll concentrations and the intensification of the oceanic front off capes was evident an s shaped band of high chlorophyll was present well offshore of the orange river mouth and lends support to the existence of a divergence zone at the shelf break in the area there being fair agreement between satellite buoy tracks topography and the position of the oceanic color front areas of elevated chlorophyll were evident in two bays on the south coast on february 13 1980 and these are ascribed to an earlier upwelling event in the region implications for future work in the area are discussed

an avenue of eddies quantifying the biophysical properties of mesoscale eddies in the tasman sea 1 the tasman sea is unique characterised by a strong seasonal western boundary current that breaks down into a complicated field of mesoscale eddies almost immediately after separating from the coast through a 16 year analysis of tasman sea eddies we identify a region along the southeast australian coast which we name eddy avenue where eddies have higher sea level anomalies faster rotation and greater sea surface temperature and chlorophyll a anomalies the density of cyclonic and anticyclonic eddies within eddy avenue is 23 and 16 higher respectively than the broader tasman sea we find that eddy avenue cyclonic and anticyclonic eddies have more strongly differentiated biological properties than those of the broader tasmansea as a result of larger anticyclonic eddies formed from coral sea water depressing chl a concentrations and for coastal cyclonic eddies due to the entrainment of nutrient rich shelf waters cyclonic eddies within eddy avenue have almost double the chlorophyll a 0 35 mg m 3 of anticyclonic eddies 0 18 mg m 3 the average chlorophyll a concentration for cyclonic eddies is 16 higher in eddy avenue and 28 lower for anticyclonic eddies when compared to the tasman sea with a strengthening east australian current the propagation of these eddies will have significant implications for heat transport and the entrainment and connectivity of plankton and larval fish populations citation everett j d m e baird p r oke and i m suthers 2012 an avenue of eddies quantifying the biophysical properties of mesoscale eddies in the tasman sea geophys res lett 39 l16608 doi 10 1029 2012gl053091

cyclonic entrainment of preconditioned shelf waters into a frontal eddy the volume transport of nutrient rich continental shelf water into a cyclonic frontal eddy entrainment was examined from satellite observations a slocum glider and numerical simulation outputs within the frontal eddy parcels of water with temperature salinity signatures of the continental shelf 18 19 c and 35 5 respectively were recorded the distribution of patches of shelf water observed within the eddy was consistent with the spiral pattern shown within the numerical simulations a numerical dye tracer experiment showed that the surface waters 50 m depth of the frontal eddy are almost entirely 95 shelf waters particle tracking experiments showed that water was drawn into the eddy from over 4 of latitude 30 34 5 s consistent with the glider observations the modeled particles entrained into the eddy sunk relative to their initial position particles released south of 33 s where the waters are cooler and denser sunk 34 m deeper than their release position distance to the shelf was a critical factor in determining the volume of shelf water entrained into the eddy entrainment reduced to 0 23 sv when the eddy was furthest from the shelf compared to 0 61 sv when the eddy was within 10 km of the shelf from a biological perspective quantifying the entrainment of shelf water into frontal eddies is important as it is thought to play a significant role in providing an offshore nursery habitat for coastally spawned larval fish

three dimensional structure of a swarm of the salp thalia democratica within a cold core eddy off southeast australia 1 swarms of the salp thalia democratica periodically occur off southeast australia following the austral spring bloom of phytoplankton in october 2008 a filament of upwelled water was advected south by the adjacent east australian current and formed a 30 km diameter cold core eddy cce the three dimensional structure of a subsurface swarm of t democratica within the eddy was examined using both oblique and vertical hauls and an optical plankton counter opc deployed on a towed body the cce displayed distinct uplift of the nutricline and elevated fluorescence net samples show the zooplankton community was dominated by t democratica comprising 73 88 of zooplankton abundance the size distribution of t democratica measured from net samples was 0 5 5 mm and was used to interpret the opc transects which showed the swarm formed a 15 km diameter disc located 20 40 m deep in the center of the eddy the maximum salp abundance was in the pycnocline and coincided with the subsurface fluorescence maximum the mean abundance of t democratica size particles within the disc was 5003 individuals m 3 ind m 3 contrasted with only 604 ind m 3 at the outer edge of the eddy the vertically concentrated and horizontally constrained disc shaped salp swarm occurred at the interface of salp bearing inner shelf water and nutrient rich upwelled water in a cce the physical processes that formed the cce on the inshore edge of the western boundary current led to the largest density of salps recorded

effects of the internal structure of a rock avalanche dam on the drainage mechanism of its impoundment flims sturzstrom and ilanz paleo lake swiss alps abstract the holocene sturzstrom of flims grisons switzerland mobilized 12 km3 of jurassic limestone making it one of the largest events of this kind on the earth the material dammed a 10 km long reach of the upper rhine river valley with thicknesses up to 400 m a 40 km long natural lake was formed upstream the body of the avalanche deposits displays a well preserved internal fabric five major sedimentary facies were identified related to the confinement of the deposits in a narrow valley that restricts the spreading of transported material in the deepest parts of the avalanche deposit a stratified facies attests that the movement was mainly non turbulent whereas a granular facies appears in the lateral and upper parts the relative position of these facies within the natural dam has played an important role in the way the water flow eroded the dam crest and emptied the lake as suggested by the present valley slope morphology ledges corresponding to the levels of the different facies on north and south slopes of the valley correspond to steps in the narrowing of the incision the outburst of the dam if it occurred only affected the upper part of the deposits that corresponds to the poorly cohesive granular facies one hundred kilometres downstream in lake konstanz bodensee lacustrine sediments the presence of two layers contrasting with the normal grey varved deposits and separated by six laminites are contemporaneous with the mass movement these layers which could correspond to the outburst of the dam point to not one but two major overflooding events

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

evidence for extensive denudation of the martian highlands high resolution topographic data from the mars orbiter laser altimeter reveal evidence for widespread denudation from margaritifer sinus to northern arabia terra an area of 1 107 km2 a major resurfacing event is indicated by 1 a heavily degraded landscape hosting numerous inliers 2 truncation or absence of valley networks and 3 a break in slope at the edge of early to middle noachian plateau materials geomorphic mapping was completed on the type locale of denudation 0 30 s 0 30 w superposition relations and crater counts for geomorphic units indicate that large scale resurfacing took place in the late noachian eroding the martian highlands and resulting in transportation and deposition of 4 5 106 km3 of sediment in the northern plains this is equivalent to a 120 m thick uniform layer of sediment on the surface of mars north of 30 n geomorphic mapping and crater counts limit the timing of denudation to the late noachian an interval of 350 500 m y using this limit we estimate a minimum rate of denudation of 2 0 m yr comparable to denudation of typical slopes in a temperate maritime climate on earth the morphology rate of denudation and extensive nature of upland degradation suggest that precipitation fed surface runoff is the most likely geomorphic agent capable of such a process indicative of a warm wet mars during the late noachian epoch

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

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

on the cross shelf exchange driven by frontal eddies along a western boundary current during austral winter 2007 western boundary currents are characterized by high mesoscale eddy activity including that of cyclonic frontal eddies cfes these form frequently adjacent to the shelf break export coastal water and affect biological productivity here we study the physical properties of eddies identified within the intensification zone of the east australian current eac through the analysis of remotely sensed data the application of an ocean circulation model and the evaluation of lagrangian simulations during austral winter 2007 we examine the spatial and temporal evolution of identified eddies investigate their formation mechanism and quantify the contribution made to cross shelf exchanges the mesoscale eddies are tracked from generation to decay using an eddy detection and tracking method in this case study we identify two cfes referred to as c1 and c2 with lifetimes of 11 and 38 days and radii of 45 and 70 km respectively and one anticyclonic eddy ace with a lifetime of 62 days and a radius of 60 km both cfes interact with the ace leading to quasi stationary dipole eddies located in the vicinity of the shelf this is the first time dipole eddies are reported for the intensification zone of the eac the location of cfes characterised by negative sea surface height anomaly ssha and clockwise rotation coincides with remotely sensed sea surface temperature sst and chlorophyll a chl a anomalies cfes c1 and c2 appear to contribute in different ways to the cross shelf transport along the shelf break cfe c1 drives a stronger onshore transport while cfe c2 appears to enhance the offshore transport the estimated daily cross shelf export of shelf water associated with cfe c2 is estimated with about 1 23 sv 106 km3 day 1 it renews the regional shelf water in about 7 days the lifetime of cfe c2 is about 38 days thus the eddy had the potential to flush the shelf at least five times the transient cfes were found for the first time to intensify the cross shelf exchange that is due to the wind driven cyclonic circulation of the fraser gyre this seasonally occurring gyre dominates the local shelf during austral autumn and winter the results from this study demonstrate the importance of cfes in exporting cooler and chl a enriched shelf water into the warmer and oligotrophic waters of the southward flowing eac

observed bottom boundary layer transport and uplift on the continental shelf adjacent to a western boundary current western boundary currents strongly influence the dynamics on the adjacent continental shelf and in particular the cross shelf transport and uplift through the bottom boundary layer four years of moored in situ observations on the narrow southeastern australian shelf in water depths of between 65 and 140 m were used to investigate bottom cross shelf transport both upstream 30 s and downstream 34 s of the separation zone of the east australian current eac bottom transport was estimated and assessed against ekman theory showing consistent results for a number of different formulations of the boundary layer thickness net bottom cross shelf transport was onshore at all locations ekman theory indicates that up to 64 of the transport variability is driven by the along shelf bottom stress onshore transport in the bottom boundary layer was more intense and frequent upstream than downstream occurring 64 of the time at 30 s wind driven surface ekman transport estimates did not balance the bottom cross shelf flow at both locations strong variability was found in bottom water transport at periods of approximately 90 100 days this corresponds with periodicity in eac fluctuations and eddy shedding as evidenced from altimeter observations highlighting the eac as a driver of variability in the continental shelf waters ocean glider and hf radar observations were used to identify the bio physical response to an eac encroachment event resulting in a strong onshore bottom flow the uplift of cold slope water and elevated coastal chlorophyll concentrations

satellite observations of chlorophyll phytoplankton biomass and ekman pumping in nonlinear mesoscale eddies nonlinear mesoscale eddies can influence biogeochemical cycles in the upper ocean through vertical and horizontal advection of nutrients and marine organisms the relative importance of these two processes depends on the polarity of an eddy cyclones versus anticyclones and the initial biological conditions of the fluid trapped in the core of the eddy at the time of formation eddies originating in the eastern south indian ocean are unique in that anticyclones typically associated with downwelling contain elevated levels of chlorophyll a enhanced primary production and phytoplankton communities generally associated with nutrient replete environments from analysis of 9 years of concurrent satellite measurements of sea surface height chlorophyll phytoplankton carbon and surface stress we present observations that suggest eddy induced ekman upwelling as a mechanism that is at least partly responsible for sustaining positive phytoplankton anomalies in anticyclones of the south indian ocean the biological response to this eddyinduced ekman upwelling is evident only during the austral winter during the austral summer the biological response to eddy induced ekman pumping occurs deep in the euphotic zone beyond the reach of satellite observations of ocean color

lagrangian and eulerian characterization of two counter rotating submesoscale eddies in a western boundary current in recent decades high spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies in the surface velocity temperature and chlorophyll a fields we use a suite of high resolution data to characterize two counter rotating short lived eddies formed at the front between the warm east australian current eac and temperate coastal waters 308s eastern australia in this region submesoscale filaments and short lived eddies are dynamically generated and decay at time scales of hours to days dominant cyclonic filaments of o 1 rossby number formed along frontal jets and eddy boundaries generating localized ageostrophic circulations at the submesoscale measurements of over ocean wind direction and surface currents from high frequency radars reveal the influence of the short term small scale wind forcing on the surface circulation enhancement of the horizontal shear frontal jet destabilization and the generation and decay of the cyclonic eddy by contrast the anticyclonic eddy formation was most likely associated with eac mesoscale instability and anticyclonic vorticity lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones limiting their transport mixing between eac derived and coastal waters was increased along the frontal regions and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales the cyclonic cold core eddy entrained high chlorophyll a shelf waters on its convergent side suggesting spiral eddy cyclogenesis

front eddy influence on water column properties phytoplankton community structure and cross shelf exchange of diatom taxa in the shelf slope area off concepcion 36 37 s in eastern boundary current systems ebcss submesoscale to mesocale variability contributes to cross shore exchanges of water properties nutrients and plankton data from a short term summer survey and satellite time series january february 2014 were used to characterize submesoscale variability in oceanographic conditions and phytoplankton distribution across the coastal upwelling and coastal transition zones north of punta lavapie and to explore cross shelf exchanges of diatom taxa a thermo haline front frn 1 flanked by a mesoscale anticyclonic intrathermocline eddy ite 1 or mode water eddy persisted during the study period and the survey was undertaken during a wind relaxation event at the survey time ite 1 contributed to an onshore intrusion of warm oceanic waters southern section and an offshore advection of cold coastal waters northern section with the latter forming a cold high chlorophyll a filament in situ phytoplankton and diatom biomasses were highest at the surface in frn 1 and at the subsurface in ite 1 whereas values in the coastal zone were lower and dominated by smaller cells diatom species typical of the coastal zone and species dominant in oceanic waters were both found in the frn 1 and ite 1 interaction area suggesting that this mixture was the result of both offshore and onshore advection overall front eddy interactions in ebcss could enhance cross shelf exchanges of coastal and oceanic plankton as well as sustain phytoplankton growth in the slope area through localized upward injections of nutrients in the frontal zone combined with ite induced advection and vertical nutrient inputs to the surface layer

dynamics of crustal compensation and its influences on crustal isostasy deviation from isostasy is commonly believed to be caused by the strength of the earth s lithosphere an analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin the analysis is based on analytic models that assume that 1 the medium is incompressible and has a layered and linear viscoelastic rheology and 2 the amplitude of topography is small compared with its wavelength the models can describe topographic relaxation of different density interfaces at both small e g postglacial rebound and large time scales the models show that for a simple crust mantle system with topography at the earth s surface and moho representing the only mass anomalies while the crust always approaches the isostatic state at long wavelengths 800 km crustal isostasy may not be an asymptotic limit at short wavelengths depending on crustal and lithospheric rheology for a crust with viscosity smaller than lithospheric viscosity at wavelengths comparable with widths of orogenic belts i e 300 km the crust tends to approach a state with significant overcompensation i e excess topography at the moho within a timescale of about 107 years and this characteristic time depends on wavelengths and crustal viscosity this overcompensation is greater for weaker crust and stronger lithosphere a thicker crust or lithosphere also enhances this overcompensation if crustal and lithospheric viscosities are both large and comparable the asymptotic state for the crust displays a slight undercompensation for an elastic and rigid upper crust the crust eventually becomes undercompensated after a characteristic decay time of topography at the moho the characteristic time is dependent on viscosity and thickness of the lower crust the deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the moho to that at the surface

cross shelf dynamics in a western boundary current regime implications for upwelling abstractthe cross shelf dynamics up and downstream of the separation of the south pacific ocean s western boundary current wbc are studied using two years of high resolution velocity and temperature measurements from mooring arrays the shelf circulation is dominated by the east australian current eac and its eddy field with mean poleward depth integrated magnitudes on the shelf break of 0 35 and 0 15 m s 1 up and downstream of the separation point respectively the high cross shelf variability is analyzed though a momentum budget showing a dominant geostrophic balance at both locations among the secondary midshelf terms the bottom stress influence is higher upstream of the separation point while the wind stress is dominant downstream this study investigates the response of the velocity and temperature cross shelf structure to both wind and eac intrusions despite the deep water up to 140 m the response to a dominant along shelf wind stress forcing is a classic two layer ekman structure du

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

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

momentum transfer and friction in the debris of rock avalanches when a mass of loose dry purely frictional material slides down an incline after release at a given velocity the runout the distance the centre of gravity of the displaced mass moves from its initial position depends on momentum transfer within the mass this can be estimated from the profile of the debris accumulation which also allows more accurate calculation of apparent angles of sliding friction in rockfall avalanches the apparent extreme mobility of the elm and frank slides typical rockfall avalanches is explained by momentum transfer in a loose dry purely frictional material with an angle of friction of 30 key words mass wasting momentum transfer landslide accumulation rock avalanche runout distance

ice driven creep on martian debris slopes 1 accumulations of rocky debris at the base of bedrock escarpments on mars have mean inclinations of 20 well below the angle of repose 35 these inclinations decrease with increasing latitude suggesting a climatic influence we present evidence that these low inclinations are the result of gravitational creep driven by repeated deposition and sublimation of ground ice we estimate the rates of bedrock erosion and ice driven creep on mars and use a simple model to show that the probable time scale for debris slope development is between 20 myr and 2 gyr our results suggest that water ice has shaped martian landscapes throughout the late amazonian period

observations of submesoscale variability and frontal subduction within the mesoscale eddy field of the tasman sea abstractsubmesoscale lenses of water with anomalous hydrographic properties have previously been observed in the east australian current eac system embedded within the thermocline of mesoscale a

florida current frontal eddies and the settlement of coral reef fishes we deployed three replicate larval light traps off the upper florida keys from june september 2001 to measure the delivery of settlement stage fish larvae to the coral reefs nightly measures of larval abundance were compared to water temperatures measured across the outer reef nearby wind records and the alongshore and cross shelf components of the currents measured at the seaward edge of the reef these time series together with satellitederived sea surface temperature and color fields indicate that a very large multi taxa larval pulse on 20 july was directly associated with the passage of a florida current fc sub mesoscale frontal eddy embedded within the elongated remnant of a mesoscale eddy a second large pulse of larvae occurred when a similar mesoscale eddy passed the upper keys in mid june periods of increased tidal bore activity occurred with the passage of these eddies semidiurnal internal tides caused near bottom onshore intrusions of cooler slope waters during periods of onshore meanders of the fc front when the downstream baroclinic flow and stratification increased at the reef margin the high abundance and similarity in larval ages within taxa on 20 july indicate a keys shelf origin although the temporal and spatial scales of entrainment cannot be resolved the passage of a third mesoscale eddy in september did not result in a larval pulse possibly because of a mismatch between biological and physical criteria several of which must be met for larval transport by mesoscale eddies to be successful

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

modeling of grabens extending above evaporites in canyonlands national park utah abstract grabens in canyonlands national park utah began extending above a layer of evaporites when the colorado river cut through the overburden two dimensional finite element models simulate the effects of geometry and rock properties on graben configuration and spacing only those models having a progressively increasing slope or no slope mimicked the natural upslope graben propagation typical rock properties produced the most realistic fault patterns an initial friction angle of 31 a cohesion of 1 mpa and strain weakening comprising cohesion loss and decrease of friction angle to 26 a tensile stress limit narrowed the grabens and reproduced the vertical upper portion of the natural faults the viscous salt resisted overburden spreading and controlled its rate modeled grabens spread at typical rates of 1 2 mm a 1 for a salt viscosity of 1 10 18 pa s and the entire system strained at rates from 6 0 10 14 s 1 to 0 5 10 14 s 1 the faults bounding a graben formed nearly simultaneously at the top surface and propagated downward salt rose beneath the grabens as reactive diapirs overburden adjacent to the canyon flexed as salt was expelled and formed an arching horst and graben a corresponding horst has been found in the field the model results scale to larger dimensions except for the steep upper part of the faults reduced dimensions create vertical or no faults

modification of caloris ejecta blocks by long lived mass wasting a volatile driven process abstract the caloris basin is the largest well preserved impact basin on mercury as such caloris ejecta afford us an opportunity to study material from mercury s deep interior with remote sensing we have made observations of the geomorphology colour distribution and flank slopes of the circum caloris knobs our observations suggest that these circum caloris knobs are modified ejecta blocks from the caloris impact high resolution messenger images show that knobs are conical and relatively uncratered compared with the surrounding plains which implies the knobs have undergone resurfacing we have observed material that has sloughed off knobs superposing impact craters that demonstrably postdate the caloris impact which requires some knob modification to have been more recent we have observed hollows depressions in mercury s surface generally believed to have been caused by volatile loss on and closely associated with several knobs which indicates that many knobs contain volatile material and that knob modification could extend into mercury s recent past our measurements show that knob flanks typically have slopes of 21 which is steep for a mound of unconsolidated material that was originally emplaced 3 8 ga the conical shape of knobs their steep slopes the dearth of superposed craters on knobs and knob superposition relationships with other landforms suggest that caloris ejecta blocks of arbitrary original shape were modified into their present shapes by long lived mass wasting mass wasting must have dominated over impact gardening which would have produced domal morphologies only we suggest that mass wasting was probably driven by volatile loss in a manner analogous to terrestrial landforms called molards if the circum caloris knobs are analogous to molards then they represent a landform and a process hitherto undocumented on mercury with implications for the volatile content of the planet s interior these knobs therefore are prime targets for bepicolombo which could search for fresh failures and volatile exposures in the knobs

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

liquefaction flowslides in rocky mountain coal mine waste dumps over the past 25 years there have been a large number of long runout flowslides from rocky mountain coal mine waste dumps the waste dumps are constructed as end dumped fills with an angle of repos

the cold core eddy and strong upwelling off the coast of new south wales in early 2007 abstract during the austral summer of 2006 07 a series of extreme oceanic events occurred in the tasman sea following a series of strong wind driven upwelling events an intense cold core eddy developed off sydney australia a data assimilating eddy resolving ocean model is used to create a three dimensional time varying reanalysis of these events the reanalysis indicates that the cold anomalies associated with the upwellings were in excess of 5 c near the coast where sea level decreased by as much as 0 2 m the reanalysed three dimensional structure of the cold core eddy shows the eddy leaning to the west north west in towards the continental shelf the diameter of the eddy is about 100 km and the sea level anomaly at the eddy centre peaks at around 1 m with an associated sub surface temperature anomaly in excess of 8 c at 200 m depth corresponding to an upward isotherm excursion of 600 m the circulation around the cyclonic eddy is ageostrophic with upwelling in the southern sector of the eddy where flow is onshore and climbing the continental slope and downwelling in the northern sector where flow is descending off the slope three dimensional trajectories of water parcels around the eddy involve 50 100 m vertical excursions based on the reanalysed circulation and composite satellite images of chlorophyll a we hypothesise that the circulation around the eddy led to significant nutrient enrichment in the euphotic zone around the perimeter of the eddy

a tale of two eddies the biophysical characteristics of two contrasting cyclonic eddies in the east australian current system mesoscale cyclonic eddies are known to be highly productive less well known are the dynamics and productivity of smaller cyclonic eddies known as frontal eddies that form on the landward side of western boundary currents in this study we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the east australian current eac the first murphy a mesoscale cyclonic eddy that formed at 28 s with a diameter of 160 km and high surface chlorophyll a concentrations which lived 47 days the second freddy a smaller frontal eddy 35 km diameter that formed from a shelf water billow 7 days prior to sampling at 31 5 s and was advected off the shelf along the eac front from 200m to 4000m of water both eddies were at least 1000m deep with a similar steric height anomaly we introduce and employ the method of closest approach using shipboard adcp velocities to estimate the eddy centers which reveals significant tilting through the water column we estimate rotation rates of 4 10 days and 1 9 days and rossby numbers 0 25 0 1 and 0 6 0 1 from the surface to 600m for murphy and freddy respectively high resolution altimetry measurements from the saral altika satellite provide estimates of the ageostrophic component of rotation our results show that the frontal eddy is significantly more ageostrophic energetic and productive than the mesoscale cyclone despite its small size and short life 4 weeks we suggest that frontal eddies have potential to contribute significantly to the net productivity of the tasman sea region this article is protected by copyright all rights reserved

long wavelength topographic relaxation for self gravitating planets and implications for the time dependent compensation of surface topography the support of planetary surface topography is controlled by the thickness of the elastic lithosphere and thus has implications for the thermomechanical structure of shallow planetary interiors previous analyses of the support of long wavelength topography have generally utilized elastic formulations which preclude consideration of the time evolution of lithospheric stresses and thermal state here we formulate a viscoelastic model for the support of topography on a spherical planet our stress relaxation model employs a multilayer linear viscoelastic rheology and includes the effect of self gravitation in these models we approximate internal rheology with an olivine flow law and we estimate the internal thermal structure assuming simple conductive cooling we analyze how relief at the surface and internal density interfaces evolves with time in response to a surface or internal load our analysis demonstrates that crustal compensation is strongly dependent on planetary radius with a tendency for a large terrestrial planet like earth to reach complete airy isostasy at long wavelengths within a time scale controlled by the mantle viscosity the correlation of degree of compensation with planetary radius is consistent with previous work turcotte et al 1981 which showed that membrane stress can support significant long wavelength gravity anomalies applied to the moon the degree of compensation associated with loading of 100 ma lithosphere is less than 0 3 and the load love numbers are greater than 0 6 for all harmonics including degree 2 even after 109 years of relaxation this conclusion does not change significantly even with a moderately low viscosity for the crust the tendency for a small body like the moon to support long wavelength stresses may be relevant in understanding the anomalously large degree 2 terms in the lunar shape the results also suggest that major impact basins would not be compensated throughout lunar geological history 4 b y even if the surfaces on which the basins were initially formed were relatively young i e 100 300 ma the fact that the south pole aitken basin is largely compensated suggests that the lateral variations in thermal and or mechanical structure induced by impact processes play significant roles in the compensation of this structure

admittance estimates of mean crustal thickness and density at the martian hemispheric dichotomy 1 admittance estimates from line of sight los acceleration profiles of the mars global surveyor spacecraft are used to constrain the mean crustal thickness and surface density centered on the hemispheric dichotomy from 110 e to 220 e 40 s to 20 n models with uniform crustal properties predict lower than expected bulk crustal densities two layer models with loading only at the surface and the moho produce satisfactory fits to the data the best fit surface density crustal thickness tc and elastic thickness te are 2 5 mg m 3 27 km and 61 km respectively higher elastic thicknesses require lower crustal thicknesses and vice versa the best fit ratio of moho to surface loading f is close to 1 models with no bottom loading f 0 provide a poor fit to the data underestimates in f result in underestimates of both tc and te the surface density is lower than that measured from martian meteorites and by admittance analyses of young volcanoes but is well constrained by the short wavelength admittance values for misfits up to 1 5 times the minimum value and a fixed surface density of 2 5 mg m 3 the ranges of tc te and f are 1 75 km 37 89 km and 0 4 2 6 respectively the apparently compensated nature of the large impact basins has been used to infer a lower bound on southern hemisphere crustal thickness of 45 km if this estimate is correct the likely mean crustal thickness in the area considered is 55 20 km

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

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

the relationship between martian gravity and topography the relationship between gravity and topography of various regions of mars is used to estimate their effective elastic thicknesses te using direct measurements of line of sight velocity rather than spherical harmonic coefficients estimates of te vary from 70 km for tharsis 29 km for elysium to 14 5 km for the southern hemisphere and show that the thickness of the martian lithosphere increases with age as the radioactive isotopes of k th and u decay a simple parameterised model of the convective thermal history is used to estimate the temperature structure of the lithosphere and shows that the base of the elastic layer has a temperature of 300 50 c or similar to the value for terrestrial continents in both cases the rheology is probably affected by the presence of water the short wavelength behaviour of the gravity field allows the density of the rocks that form the topography to be estimated and gives values of about 3 0 mg m3 for tharsis and elysium this value is substantially greater than that of 2 7 mg m3 obtained for earth and is in agreement with estimates from snc shergottites nahklites chassigny meteorites of 3 3 mg m3 the density of the topography of valles marineris is only 2 35 mg m3 and suggests that ice may be present below the surface in the heavily bombarded southern hemisphere isostatic compensation occurs at wavelengths as short as 700 km which requires the effective compensation depth to be no more than 10 km the gravity field with wavelengths greater than 1500 km may be supported dynamically by a plume rising beneath the tharsis region the difference in temperature between the solidus and the present areotherm is less than 250 c so melt generation can occur in rising plumes

revisiting the circulation of the east australian current its path separation and eddy field abstract the traditional view of the east australian current eac as depicted in many schematics is of a continuous boundary current that flows along the shelf off eastern australia between approximately 18 s and about 32 5 s where it separates from the coast and continues either towards new zealand along the tasman front or towards tasmania as the eac extension additionally it is widely recognised that eddies are prevalent in the eac region particularly south of the eac separation we revisit this long standing paradigm and suggest that the eac is perhaps better viewed as a continuous meandering stream flowing adjacent to the coast that feeds a field of mesoscale eddies observations show that eac eddies are prevalent over a broad region of the western tasman sea as far north as 25 s typically with maximum intensity between 30 and 35 s at any instant in time the eac jet is usually evident as a poleward stream adjacent to the continental shelf edge other commonly accepted components of the eac system including the tasman front and eac extension are rarely seen as distinct identifiable features rather these features are evident only in time mean fields when the eddy variability is filtered out it is also common for the eac to be spatially discontinuous due to the presence of eddies often with multiple short streams that sometimes separate and re attach to the coast recognition of the eac as an eddy dominated current system has seen many recent studies focus on various aspects of eddies in the eac system providing new insights into mesoscale ocean dynamics recent studies of individual eddies have shown that the circulation within eddies including tilting and vertical motion is more complex than previously understood a summary of these studies along with a review of the eac system particularly its path separation and eddy field is presented here

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