cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides abstract scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field these scaling factors are analogous to those determined in a similar work dunai t j earth planet sci lett 2000 176 157 169 that were accurate for field strengths equivalent to present day field strength but cannot accommodate field strength variations strategies to derive the relevant geomagnetic field parameters are discussed and are tested with existing production rate calibrations for 3 he a production rate of 103 4 2 atoms g olivine yr sea level and for zero cutoff rigidity was derived from calibration sites from hawaii western usa and canary islands covering the time range of 2 281 kyr

jupiter s great red spot as a shallow water system most current models of jupiter s great red spot grs are cast in terms of a two layer model where a thin upper weather layer which contains the vortex overlies a much deeper layer which is meant to represent the neutrally stratified deep atmosphere any motions in the deep layer are assumed to be zonal and steady this two layer model is dynamically equivalent to a one layer model with meridionally varying solid bottom topography called the reduced gravity model specifying the motions or lack thereof in the lower layer of the two layer model is equivalent to specifying the bottom topography and hence the far field potential vorticity in the reduced gravity model current models of the grs start by guessing the deep motions and then proceed to study vortices using the implied bottom topography here using the grs cloud top velocity data we derive the bottom topography up to a constant that depends on the unknown radius of deformation or equivalently the product of the reduced gravity and the mean thickness of the upper layer the bottom topography is inferred from three quantities derived from the velocity data bernoulli streamfunction kinetic energy per unit mass and absolute vorticity all of which are functions only of horizontal position in the reference frame of the vortex far from the vortex potential vorticity versus latitude is calculated from the observed cloud top zonal velocity and the derived bottom topography the results show that the deep atmosphere is in differential motion and that the far field potential vorticity gradient changes sign at several latitudes numerical shallow water experiments are performed using both the derived bottom topography and the bottom topographies prescribed by current models the results of three published studies are reproduced in our numerical experiments each of these models is successful in maintaining a long lived isolated vortex but only the present model yields absolute vorticity profiles along streamlines that agree with those observed for the grs by dowling and ingersoll in all the models large vortices form by merging with smaller vortices in the present observationally based model and in one other published model the smaller vortices arise spontaneously because the observed cloud top zonal velocity profile is unstable these two models require an additional momentum source to maintain the upper layer zonal velocity profile in the other two models the bottom topography stabilizes the zonal velocity profile if dissipation is present the latter two models require an additional source of smaller vortices to maintain the larger one a crucial unanswered question for the present model and for jupiter itself is how the cloud top zonal velocity profile is maintained in its present unstable state

tectonomagmatic characteristics of the back arc portion of the calama olacapato el toro fault zone central andes 1 post 20 ma magmatism in the central andes is either localized in the magmatic arc or distributed east of it on the altiplano puna plateau here there is a distinct concentration of magmatic centers on nw se trending lineaments such as the calama olacapato el toro cot that extends into the eastern cordillera to the east of the puna understanding the possible genetic relationship between prominent structures and magmatic centers on these lineaments is important to elucidate the tectonomagmatic evolution of the central andes we investigated the back arc area of the cot using remote sensing geological structural and petrochemical data our study demonstrates that this portion of the cot consists of nw se striking faults formed under overall left lateral transtension that decreases in activity toward the cot termini deformation on the cot occurred during and after activity of prominent n s striking transpressive fault systems and is coeval with magmatism which is focused on the central cot the most evolved magmatic rocks with an upper crustal imprint are exposed on the central cot whereas more primitive mantle derived mafic to moderately evolved magmatic rocks are found toward the cot termini this points to a genetic relationship between upper crustal deformation and magmatic activity that led to enhanced magma storage in the central cot cot magmas may result either from slab steepening or episodic delamination of the asthenospheric mantle

a quasi geostrophic circulation model of the northeast pacific part i a preliminary numerical experiment abstract a limited area quasi geostrophic numerical model with mesoscale resolution is developed to study the circulation in the northeast ne pacific ocean the model domain extends from the british columbia alaska coast out to 170 w and down to 45 n and incorporates the coastline geometry and bottom topography of the region a long term integration was conducted using a steady climatological wind stress curl field to drive the circulation several statistical properties of the solution are determined and compared with observations a cyclonic circulation develops in the model basin with a meandering alaska current feeding at the head of the gulf of alaska into an intense boundary current corresponding to the alaskan stream the head of the gulf is a region where anticyclonic closed streamline features are occasionally generated with characteristics resembling those of the sitka eddy in the downstream region the boundary current separates and is subject to lateral meandering due to topographic wave

life cycles of spots on jupiter from cassini images using the sequence of 70 day continuum band 751 nm images from the cassini imaging science system iss we record over 500 compact oval spots and study their relation to the large scale motions the 100 spots whose vorticity could be measured the large spots in most cases were all anticyclonic we exclude cyclonic features chaotic regions because they do not have a compact oval shape but we do record their interactions with spots we distinguish probable convective storms from other spots because they appear suddenly grow rapidly and are much brighter than their surroundings the distribution of lifetimes for spots that appeared and disappeared during the 70 day period follows a decaying exponential with time constant mean lifetime of 3 5 days for probable convective storms and 16 8 days for all other spots extrapolating the exponential beyond 70 days seriously underestimates the number of spots that existed for the entire 70 day period this and other evidences size shape distribution in latitude suggest that these long lived spots with lifetime larger than 70 days are from a separate population the zonal wind profile obtained manually by tracking individual features this study agrees with that obtained automatically by correlating brightness variations in narrow latitude bands porco et al 2003 some westward jets have developed more curvature and some have developed less curvature since voyager times but the number of westward jets that violate the barotropic stability criterion is about the same in the northern hemisphere the number of spots is greatest at the latitudes of the westward jets which are the most unstable regions according to the barotropic stability criterion during the 70 day observation period the great red spot grs absorbed nine westward moving spots that originated in the south equatorial belt seb where most of the probable convective storms originate although the probable convective storms do not directly transform themselves into westward moving spots their common origin in the seb suggests that moist convection and the westward jet compose a system that has maintained the grs over its long lifetime

geomorphic evidence for post 10 ma uplift of the western flank of the central andes 18 30 22 s 1 the western andean mountain front forms the western edge of the central andean plateau between 18 5 and 22 s latitude the mountain front has 3000 m of relief over 50 km horizontal distance that has developed in the absence of major local neogene deformation models of the evolution of the plateau as well as paleoaltimetry estimates all call for continued large magnitude uplift of the plateau surface into the late miocene i e younger than 10 ma longitudinal river profiles from 20 catchments that drain the western andean mountain front contain several streams with knickpoint bounded segments that we use to reconstruct the history of post 10 ma surface uplift of the western flank of the central andean plateau the generation of knickpoints is attributed to tectonic processes and is not a consequence of base level change related to pacific ocean capture eustatic change or climate change as causes for creating the knickpoint bounded stream segments observed minor valley filling alluvial gravels intercalated with the 5 4 ma carcote ignimbrite suggest uplift related river incision was well under way by 5 4 ma the maximum age of river incision is provided by the regionally extensive approximately 10 ma el diablo altos de pica paleosurface the river profiles reveal that relative surface uplift of at least1 km occurred after 10 ma

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

a comparative study of jovian cyclonic features from a six year 1994 2000 survey abstract this work presents a six year study aimed at characterizing the morphology and properties of the atmospheric features present in jovian cyclonic regions it complements our previous analysis for the same period on the anticyclonic vortices morales juberias et al 2002 icarus 157 76 90 the main difference between cyclonic and anticyclonic regions in jupiter is that a variety of organized morphologies are present in the cyclonic areas although they can be grouped consistently into five different types filamentary turbulence related to the highest speed jets organized folded filamentary regions elongated areas with contours closed by a ribbon like feature discrete closed brown cyclones barges and peculiar transient structures such as short wave trains and cyclonic cells we present data on their color contrast size aspect ratio distribution lifetimes relation to the jet system and to the anticyclones and motions global and internal to the features most cyclonic features show a rapid evolution compared to the anticyclones and tend to be dispersed zonally although some survive for a few years we used the barges that are the most representative long lived and extended type of cyclonic features to show that there exists a linear relationship between their relative velocity and the mean zonal flow speed similar to that found in our previous work on the anticyclones there is also some evidence that the drift rate of barges is related to the planetary minus flow vorticity gradient

jupiter new estimates of the mean zonal flow at the cloud level abstract previous estimates of the mean zonal flow on jupiter from voyager images by ingersoll et al a p ingersoll r f beebe j l mitchell g w garneau g m yagi and j p mueller 1981 j geophys res 86 8733 8743 and by limaye et al s s limaye h e revercomb l a sromovsky r j krauss v e suomi s a collins and c c avis 1982 j atmos sci 39 1413 1432 showed good agreement in the locations of the easterly and westerly jets but differed somewhat in magnitude recent measurements of the high speed jet located near 24 north plane topographic latitude by t maxworthy 1984 planet space sci 32 1053 1058 from high spatial and temporal resolution voyager images indicate that both ingersoll et al and limaye et al underestimated the magnitude of the jet by more than 30 40 msec 1 in an attempt to examine the differences in the magnitude of the jovian jets determined from voyager 1 and 2 images a new approach to determine the zonal mean east west component of motion was investigated the new technique based on simple digital pattern matching approach and applied on pairs of mapped images cylindrical mosaics yields a profile of the mean zonal component that reproduces the exact locations of the easterly and westerly jets between 60 latitude not only do the jet magnitudes but also the wings of the jets agree remarkably well from mosaic pair to pair further the latitudinal resolution is five midlatitudes to eight times equatorial greater than previous results results have been obtained for all of the voyager 1 and 2 cylindrical mosaics the correlation coefficient between voyager 1 and voyager 2 average mean zonal flow between 60 latitude determined from violet filter mosaics is 0 998 a slight latitude offset possibly due to navigation errors is detectable 0 2 latitude average in the voyager 1 data independent cloud motion measurements in two high resolution image pairs orange and violet acquired from voyager 1 cameras agree well with the average mean zonal flow for the fastest jovian jet at 23 8 n latitude comparison with maxworthy s 1984 results suggests longitudinal variations in cloud motions approaching about 20 msec 1 in particular the jet magnitude is about 163 7 rms msec 1 which compares well with 182 10 msec 1 reported by maxworthy there is excellent agreement in the location of the peak magnitude as well as its shape the time average voyager 2 mean zonal flow profile with latitude is presented in a table

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

sequential open boundary control by data assimilation in a limited area model the feasibility of sequential open boundary control by data assimilation in a regional ocean model has been investigated using a barotropic wind driven ocean circulation model a simple open boundary scheme has been constructed based on the idea of optimal boundary control of a diagnostic equation and illustrated with the problem of modeling the subpolar gyre subject to an open southern boundary the results show that use of such a scheme in conjunction with traditional radiation boundary conditions allows for a longer model integration that would otherwise be unstable when only the radiation boundary conditions are imposed due to presence of dispersive waves 39 refs 8 figs

oscillations and velocity structure of a long lived cyclonic spot dark brown cyclonic spots barges at 14 n were studied by using voyager 1 and 2 images of jupiter movie sequences were made to study the spots behavior over intervals of 50 days and longer these movies revealed that the length and width vary by 9 with a period of about 15 days while the area remains approximately constant the horizontal velocity field was investigated for an interval of about 1 day flow around the largest barge feature 6 occurs as a ring current the vorticity inferred is about 2 times that of the ambient cyclonic zonal circulation and about one half the value of the local planetary vorticity length and width variations appear to be associated with a nonzero horizontal divergence field if the oscillations are a natural mode of the system the 15 day period will provide an important datum for testing models of stable closed vortices

tectonic geomorphometrics of the western united states speculations on the surface expression of upper mantle processes the topography of the western united states provides a classic field laboratory for investigations of the relationship between surface features and sub crustal dynamic processes the interpretation of recently collected high resolution seismic images of the upper mantle beneath the central colorado rocky mountains substantiates the notion that much of the high elevation coincides with thin or attenuated continental crust with respect to predicted airy crustal thicknesses necessitating topographic support by anomalously buoyant mantle this is highly suggestive that broad scale topographic features may be correlated with buoyancy variations in the upper mantle in an attempt to sharpen our understanding of the underlying geodynamics we evaluate the correlation between the surface topographic character and data sets that provide information about density variations indicative of buoyancy in the upper mantle including the lithospheric geoid upper mantle seismic velocity anomalies and crustal lg q our general conclusion is that mantle buoyancy is driving differential surface uplift throughout the western united states and this driver of topography is manifested by measureable anomalies in the topographic roughness at short wavelengths tens of kilometer and elevated spectral power in the topography at longer several hundred kilometers wavelengths a provocative conclusion is that the long recognized physiographic provinces of the colorado plateau rocky mountains and rio grande rift are also neotectonic provinces that are related to convective processes and related buoyancy in the upper mantle

a study of the stability of jovian zonal winds from hst images 1995 2000 abstract we present a five year study of the temporal stability of jupiter s zonal wind velocity profile at cloud level based on hubble space telescope images obtained from 1995 to 2000 we used the correlation of east west albedo scans in pairs of images separated by one jovian rotation to retrieve the zonal winds the resolution in the hst images ranged from 140 to 190 km pixel 1 independent measurements of the motions of individual cloud tracers were used to control the above profiles three wavelengths were used in this analysis 410 nm violet continuum 892 nm methane absorption band and 953 nm red continuum our study indicates that globally jupiter s zonal flow did not change during this five year interval and it does not depend on the observed wavelengths when comparing our mean profile with that obtained 16 years ago using voyager data we find a reasonable agreement we note however a small shift in the latitude of the jets poleward of latitude 30 probably due to image navigation uncertainties however true changes in the intensity of the jets at 24 n and 32 n are found results also indicate that the 7 s jet has remained stable since the voyager era in addition we extend in this paper our previous study of the 24 n with new data from year 2000 the new data confirm the jet s stability during the 1995 2000 period finally we present the average profile for this period including error estimation in a table that extends the latitudinal coverage of limaye s 1986 icarus 65 335 352 voyager profile up to latitudes 68 s and 77 n

cassini imaging of jupiter s atmosphere satellites and rings the cassini imaging science subsystem acquired about 26 000 images of the jupiter system as the spacecraft encountered the giant planet en route to saturn we report findings on jupiter s zonal winds convective storms low latitude upper troposphere polar stratosphere and northern aurora we also describe previously unseen emissions arising from io and europa in eclipse a giant volcanic plume over io s north pole disk resolved images of the satellite himalia circumstantial evidence for a causal relation between the satellites metis and adrastea and the main jovian ring and information on the nature of the ring particles

the plio pleistocene evolution of the southern middle atlas fault zone smafz front of morocco the south middle atlas front constitutes a northeast trending shear zone located north of the neogene missour basin and east of the taza guercif basin this paper analyses the southern middle atlas fault zone smafz deformation since the pliocene the set of structures observed suggests that reverse and thrust faulting along the central part of the smafz are combined with left lateral slip along n s striking faults of its south western termination and right lateral faulting along e ne striking faults of the east northeast termination thrusts and oblique thrust related anticlines of the two lateral ramps partly accommodate north west directed motion of the african plate the thrusts probably resulted from rejuvenation of jurassic normal faults they were active during the upper miocene pliocene and the pleistocene the geometries of positive inversion structures and buttressing effects are clearly dependent on the geometry and sedimentology of the original basin controlling fault system and on the presence of a decollement level field mapping is integrated with landsat imagery and a digital elevation model to investigate the morphotectonic evolution of the south eastern range front of the middle atlas geomorphological features provide significant information on the processes that govern lateral propagation of active anticlines both suggest that the deformation front may have been active since pliocene

prediction of a global climate change on jupiter jupiter s atmosphere as observed in the 1979 voyager space craft images is characterized by 12 zonal jet streams and about 80 vortices the largest of which are the great red spot and three white ovals that had formed1 in the 1930s the great red spot has been observed2 continuously since 1665 and given the dynamical similarities between the great red spot and the white ovals the disappearance3 4 of two white ovals in 1997 2000 was unexpected their longevity and sudden demise has been explained5 however by the trapping of anticyclonic vortices in the troughs of rossby waves forcing them to merge here i propose that the disappearance of the white ovals was not an isolated event but part of a recurring climate cycle which will cause most of jupiter s vortices to disappear within the next decade in my numerical simulations the loss of the vortices results in a global temperature change of about 10 k which destabilizes the atmosphere and thereby leads to the formation of new vortices after formation the large vortices are eroded by turbulence over a time of 60 years consistent with observations of the white ovals until they disappear and the cycle begins again

rock slope failure and the river long profile this study examines the geomorphic effects of large 10 6 m 3 rock slope failures on long profiles of rivers in the swiss alps and the new zealand southern alps regression of channel slope versus drainage basin area objectively highlights knickpoints separating incised from aggraded reaches that often correspond to locations of large rock slope failures for a fixed concavity index the highest values of the steepness index and erosion index along a given profile spatially coincide with breach channels cut into formerly river damming rockslide debris as old as 10 k y assuming that the knickpoints do not predate slope failure data show that high profile steepness and inferred specific stream power are not always the cause but often a result of large river blocking rock slope failure in mountain basins omission of rockslide data from slope area plots lowers the steepness index and increases the concavity index on average yet only in few cases more than one standard deviation

evidence for surface uplift of the atlas mountains and the surrounding peripheral plateaux combining apatite fission track results and geomorphic indicators in the western moroccan meseta coastal variscan paleozoic basement this work represents an initial attempt to link the evolution of the topography in relation to the general tectonic framework of western morocco for this purpose in a section of the western moroccan meseta different tools are combined in order to attain the general objective apatite fission track aft data of granitic rocks of the rabat khenifra area give ages around 200 ma with track length distributions which are compatible with the thermal models already established for the area an inverse correlation between aft ages and elevation is observed which is compatible with previous models indicating northward tilting of the whole western moroccan meseta which is younger than 20 25 ma in order to test this possibility a detailed analysis of the topography at different scales in the estern moroccan meseta has been performed results indicate that two open folds with different amplitudes are recognized and that the one with wider wavelength could correspond to a lithospheric fold as previously stated by other authors on the basis of independent geological arguments the northward tilting proposed based on the aft data agrees with the results obtained in the analysis of the topography which reinforces the presence of a very open fold with a wavelength of 200 300 km in the north western limb of the western moroccan meseta

timing and nature of quaternary fluvial incision in the ouarzazate foreland basin morocco the history of alluvial fan and terrace formation within a stretch of the ouarzazate basin along the southern margin of the central high atlas is reconstructed using geomorphological and 10 be terrestrial cosmogenic nuclide tcn methods alluvial fan and terrace incision was controlled partially by a drop in base level during the pliocene or early pleistocene as the outlet channel the draa river progressively cut through the anti atlas to the south of the ouarzazate foreland basin the drainage of which started to become external after a long period of internal drained conditions the alluvial fans and terrace surfaces have abandonment ages that date to at least the past four glacial cycles their formation was strongly modulated by climate on glacial interglacial time scales as base level dropped this demonstrates a strong climatic control on sediment transfer and landscape denudation during the quaternary and provides a model for understanding sediment transfer in other intracontinental mountain belts furthermore these data show that mean rates of fluvial incision in this region range between 0 3 and 1 0 mm a 1 for the latter part of the quaternary this study provides the first comprehensive tcn chronology for the atlas mountains and it illustrates the applicability and limitations of tcn methods

crustal shortening and vertical strain partitioning in the middle atlas mountains of morocco the ne sw trending middle atlas mountains of morocco are obliquely oriented within the late cenozoic regional stress field resulting in deformation that is partitioned into strike slip faulting and thrust related folding in the central middle atlas thrusting is confined to a 20 km wide fold belt between two relatively rigid crustal blocks that are obliquely converging we suggest that in addition to strain partitioning observed in plan view a partitioning of deformation between the upper and lower crust may be necessary to reconcile estimated crustal thickening and horizontal shortening within the fold belt cross section balancing based on field observations demonstrates a relatively modest amount of cenozoic horizontal shortening 4 7 km normal to the fold belt producing 800 m of structural relief yet geophysical data suggest this contraction has not produced a significant crustal root beneath the fold belt that is the belt does not appear to be isostatically compensated assuming all horizontal shortening was accommodated by crustal thickening beneath the fold belt implies much greater thickening than is suggested by constraints on the preshortened crustal thickness it thus appears that thickening does not accommodate all of the contraction we suggest one possible solution the upper crust shortens by thickening faulting and folding whereas the lower crust deforms laterally

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

merging circulations on jupiter observed differences between cyclonic and anticyclonic mergers abstract the dynamics of mergers of large circulations in jupiter s atmosphere may permit different models of the atmosphere to be tested we report well resolved observations of such events at visible wavelengths three anticyclonic and three cyclonic events a merger of anticyclonic white ovals in the south south temperate domain 2002 march is compared with the previously reported merger of ovals be and fa in the south temperate domain 2000 march in each case the two similar sized ovals converged rapidly once they were separated by less than the sum of their diameters they orbited around each other anticyclonically during the merger the merged oval initially had the same rapid drift as the western parent and in an unexpected similarity a cyclonic oval emerged westward from the point of merger evidence suggests that a merger of smaller ovals in the north north temperate domain 2002 february had similar dynamics in contrast mergers of cyclonic ovals in the north equatorial belt barges 2001 november 2005 may proceeded in a different manner the two parent barges showed no consistent acceleration towards each other as they converged on contact there was no obvious sign of mutual circulation and the low albedo regions had almost passed each other before they finally merged and the resulting barge had a drift rate intermediate between the two parents and a length that was greater than either parent again a third such event involving a smaller barge 2002 december showed many of the same characteristics these observations define different dynamical behaviour during anticyclonic and cyclonic mergers

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

deep resistivity cross section of the intraplate atlas mountains nw africa new evidence of anomalous mantle and related quaternary volcanism 1 the atlas mountains are characterized by high elevations and quaternary volcanism long period magnetotelluric data acquired along a nnw sse transect reveal the presence of a conductive anomalous mantle below the high atlas data dimensionality analyses show a preferent n80 e strike of the deep resistivity structure in agreement with the induction vector alignment at long periods accordingly a 2d inversion of the data set was carried out large resistive bodies at the crustal basement most likely correspond to batholiths emplaced in more conductive metapelites they are covered by outcropping conductive sedimentary detritic and carbonate rocks lithospheric thinning producing anomalous mantle and basin development in the atlas probably started during triassic jurassic rifting inversion tectonics since the oligocene produced low shortening on previous lithospheric weak zones with thrusting of the atlas above the stable african plate melting at the top of the anomalous mantle is connected with quaternary basaltic volcanism in the middle atlas

explicit treatment of inheritance in dating depositional surfaces using in situ 10be and 26al we describe a new strategy for dating depositional landscape surfaces using in situ produced cosmogenic radionuclides crns that removes the complication of nuclide inheritance by clasts prior to depositon two amalgamated samples each consisting of 30 clasts one from the surface and one from a fixed depth in the subsurface constrain this crn inheritance and date the surface the inheritance may be used to estimate minimum exhumation rates and maximum transport times within the geomorphic system we test the technique using sup 10 be and sup 26 al to date the third fr3 of five terraces along the fremont river utah and the third wr3 of 15 along the wind river wyoming whereas effective ages based solely upon the surface samples yield 118 138 ka fr3 and 93 ka wr3 the subsurface samples reveal that inheritance accounts for 22 wr3 to 43 fr3 of the total crn concentration taking this into account yeilds terrace ages of nearly 61 81 ka for fr3 and nearly 67 76 ka for wr3 we explore the dependence of age estimates on the accumulation history of the terrace silt caps 22 refs 3 figs 1 tab

structural development of the taza guercif basin as a constraint for the middle atlas shear zone tectonic evolution abstract the neogene taza guercif basin in north eastern morocco is located in correspondence of the eastern termination of the rifian arc and is aligned along with the middle atlas shear zone since tortonian the tectonic evolution of the taza guercif basin was tied to repeated advancements of the rifian front and to the transpressive tectonics of the middle atlas shear zone the major structures in this basin consist of thrust related anticlines aligned along with the middle atlas structures and second order normal faults elongated according to the rifian trend the latter are well developed mainly in the north western sector of the basin bab stout area apart from the bab stout area the basin is dominated by compressive and transpressive structures guercif transpressive area the shortening directions responsible for the development of the major and minor compressive structures in the basin vary from nnw to nne but no superposition relationships have been found this change of the paleo stress field orientations is here ascribed to a progressive deformation under the constant push between africa and europe an evolutionary model for the development of these two structural areas in relation to the tectonics of the rif and of the middle atlas is here proposed in this model the sinistral transpressive middle atlas shear zone is regarded as the boundary between the moroccan meseta microplate to the west and the african indenter to the east which is moving faster to the north

fitting a regional ocean model with adjustable open boundaries to topex poseidon data topex poseidon altimeter data referenced to geoid jgm2 are assimilated into an eddy resolving quasi geostrophic ocean model the location of the model domain is the african sector of the antarctic circumpolar current all lateral boundaries of the model domain are open the model has three layers a spatial resolution of approximately 20 km and realistic bottom topography the adjoint method 4dvar is used to optimize control variables which are the initial state of the stream function and the time dependent boundary values of the stream function and the relative vorticity both the numerical implementation of the model and the assimilation method are formulated such that the model can be placed in any ocean location without difficulties a two step procedure is applied where first the initial values are optimized in the second step the time dependent boundary values are adjusted for assimilation periods which are several times longer than the decorrelation times of the initial field a number of experiments are reported with assimilation periods of different length up to 60 days for short assimilation periods the error can be reduced from a first guess error of 34 cm root mean square to 5 cm however our results indicate that in a long time limit the lowest achievable error lies between 10 and 12 cm this seems to be due to the model s inability to represent the dynamics of the data sufficiently well

atmospheric scaling of cosmogenic nuclide production climate effect 1 absorption of cosmic rays by atmospheric mass varies temporally due to a redistribution of atmospheric pressure by ice sheets during glaciations the compression and expansion of the atmosphere due to cooling and warming and changes in katabatic winds near large ice masses these atmospheric processes can result in changes in production rates of cosmogenic nuclides which when integrated over long exposure durations may result in 0 to 5 adjustments in site production rates depending on location combining a ccm3 model with imbedded ice sheets for 20 ka we show that production rates changes relative to today are greatest at high elevations 6 7 at 5 km altitude due to atmospheric compression from decreased temperature production rates at specific times for sites near ice sheet margins can be reduced more than 10 due to a combination of katabatic winds draining off the ice sheet margins and atmospheric cooling nunatak settings may be significantly affected by the climate effect due to persistent glacial atmospheric conditions atmospheric variability may explain some of the disparities among cosmogenic nuclide production rate calibrations

dating fluvial terraces with and profiles application to the wind river wyoming abstract fluvial strath terraces provide a record of river incision and the timing of climatic perturbations to the fluvial system dating depositional surfaces like terraces that are older than the range of 14 c however is difficult we employ a cosmogenic radionuclide crn profile technique that addresses a major problem of crn dating on such surfaces nuclide inheritance by measuring 10 be and 26 al profiles we constrain the exposure age and the mean crn inheritance for the deposit the crn profile also yields a self check on the assumptions underlying the method we report our attempts to date terraces along the wind river wy like many sequences of western north american fluvial terraces these are inferred to reflect oscillation between glacial and interglacial conditions in the headwaters previous dating of some of these terraces and the associated terraces and glacial deposits makes this a unique location to compare dating methods dates from five sites along the bull lake glacial correlative terrace wr 3 are 118 125 ka which agrees with dates on bull lake age moraines and independent age estimates on the terrace and is consistent with the model of terrace glacial relationship crn inheritance is significant and highly variable requiring it be considered despite the additional sampling complexity assuming all inheritance in wr 3 deposits arises during exhumation in the headwaters we obtain minimum mean rates of exhumation of 13 130 m my for the source rocks alternatively assuming the crns are inherited during clast transport the time of fluvial transport from source to terrace is 10 ka it increases downstream and is lower for sand than cobbles the crn ages for older terraces wr 7 300 ka and wr 15 510 ka are lower by 50 than previous estimates based on tephrochronology the most plausible explanation is eolian deflation of a once thicker loess cover on the terrace surfaces mean thicknesses of loess of 0 5 1 5 m are required to reconcile these concentrations of crn with the previous estimates of age difficulty in dating the older terraces emphasizes that geologic caution independent estimates of age and multiple sample sites should still be part of dating depositional surfaces with crns even when employing the inheritance correction technique

late neogene evolution of the taza guercif basin rifian corridor morocco and implications for the messinian salinity crisis abstract magnetostratigraphic and biostratigraphic results are presented from neogene deposits in the taza guercif basin located at the southern margin of the rifian corridor in morocco this corridor was the main marine passageway which connected the mediterranean with the atlantic during messinian times correlation of the biostratigraphy and polarity sequence of the taza guercif composite section to the astronomical time scale allows an accurate dating of three subsequent events in the rifian corridor 1 the oldest marine sediments marking the opening of the rifian corridor were deposited at 8 ma at this age a deep 600 m marine basin developed in the taza guercif area marked by deposition of precession controlled turbidite marl cycles 2 paleodepth reconstructions indicate that a rapid 5 m ka shallowing of the marine corridor took place at the tortonian messinian boundary at an age of 7 2 ma this shallowing phase is primarily related to active tectonics although a small glacio eustatic sea level lowering also took place 3 the taza guercif basin was emergent at an age of 6 0 ma and subsequently continental sedimentation continued well into the early pliocene we suggest that shallowing and restricting the marine passageway through the rifian corridor actually initiated the messinian salinity crisis well before the deposition of the messinian evaporites in the mediterranean

stochastic forcing of sediment routing and storage in channel networks the stochastic field of sediment supply to the channel network of a drainage basin depends on the large scale interactions among climatically driven processes such as forest fire and rainstorms topography channel network topology and basin scale during infrequent periods of intense erosion large volumes of colluvium are concentrated in parts of a channel network particularly near tributary junctions the rivers carry bed material and wash load downstream from these storage sites at different rates the bed material travels slowly creating transient patterns of sediment transport sediment storage and channel morphology along the channel network as the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion interference at tributary junctions and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion which converts bed material to wash load we investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215 km 2 drainage basin within the oregon coast range over a simulated 3000 year period with a climate typical of the late holocene we used field measurements and a simulation model to illustrate interactions between the major controls on large scale processes functioning over long periods of time in complex drainage basins

extracting dynamic topography from river profiles and cosmogenic nuclide geochronology in the middle atlas and the high plateaus of morocco abstract the moulouya river system has intensely eroded the arhbalou missour and guercif neogene foreland basins in northeastern morocco having changed from net aggradation during the miocene early pliocene to net incision punctuated by alluvial fan deposition at late pliocene or early quaternary time this region as a whole has experienced mantle driven surface uplift dynamic topography since the late cenozoic being locally affected by uplift due to crustal shortening and thickening of the middle atlas too knickpoints located along the major streams of the moulouya fluvial network appear on both the undeformed margins of the missour and guercif foreland basins high plateaus as well as along the thrust mountain front of the southern middle atlas where they reach heights of 800 1000 m 500 550 m of the knickpoint vertical incision might be explained by long wavelength mantle driven dynamic surface uplift whereas the remaining 450 500 m in the southern middle atlas front and 200 300 m in the northeastern middle atlas front seem to be thrust related uplift of the jebel bou naceur be 10 terrestrial cosmogenic nuclides have been used to date two quaternary river terraces in the chegg ard valley at 62 14 ka and 411 55 ka the dated terraces allow the incision rates associated with the frontal structures of the middle atlas to be estimated at 0 3 mm yr 1 furthermore these ages have served to evaluate mantle driven regional surface uplift since the middle pleistocene in the central missour basin yielding values of 0 1 0 2 mm yr 1

variational estimation of the wind stress drag coefficient and the oceanic eddy viscosity profile abstract a variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic ekman layer model an identical twin experiment is discussed first in which the observations are created by the dynamic model the field measurements from the lotus 3 long term upper ocean study 3 buoy are then analysed by fitting the model results to the data the unknown boundary condition the wind stress drag coefficient and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data and an optimal estimate of the current field is obtained though the model is simple the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution