plio pleistocene time averaged field in southern patagonia recorded in lava flows 1 paleomagnetic directions were obtained from stepwise alternating field or thermal demagnetization of 53 lava flows from southern patagonia latitudes 49 5 52 1 s that include the pali aike volcanic field and the meseta viscachas plateau lavas in addition to previous miocene late quaternary ages of these flows 40ar 39ar dates spanning from 0 1 to 15 4 ma were obtained for 17 of the sites all except one of the magnetic polarities coincide with the expected polarities of the magnetic polarity timescale cande and kent 1995 for the obtained 40ar 39ar ages the mean direction from 33 sites eliminating sites 4 ma that pass a selection criteria of 95 5 is dec 358 7 inc 68 2 95 3 5 a value that coincides within the statistical uncertainty with the direction of the geocentric axial dipole for that area inc 68 1 likewise the mean virtual geomagnetic pole vgp coincides within the statistical uncertainty with the geographic north pole the secular variation described by the vgp angular standard deviation for these sites is 17 1 a value expected for that latitude according to model g of paleosecular variation mcfadden et al 1988 the characteristics of the data presented are optimum for time averaged field taf studies because of the good age control and good quality of the paleomagnetic data 1 primary components of magnetization were obtained using principal component analysis kirschvink 1980 from at least five points and maximum angular deviation 5 2 site means were calculated with fisher statistics using at least three samples and c 38 of the 53 flows had 95 5 no results five sites or high 95 values 5 were obtained primarily from sites affected by lightning

thermal history of the rio muni west africa ne brazil margins during continental breakup abstract we document the thermal record of breakup of the conjugate rio muni west africa and ne brazil margins using apatite fission track analysis vitrinite reflectance data and stratigraphic observations from both margins these results permit determination of the timing of four cooling episodes and the temperature of samples at the onset of each episode all samples are interpreted to have experienced higher temperatures in the geological past due to i elevated basal heatflow palaeogeothermal gradient in rio muni 1 well decaying from 58 c km during the mid cretaceous to 21 5 c km in the late cenozoic and ii progressive exhumation from formerly greater burial depth a well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section exhumation than if a constant heatflow is assumed cooling episodes identified from the palaeotemperature data at 110 95 ma both margins and 85 70 ma rio muni only coincide with major unconformities signifying respectively the cessation of rifting breakup and compressional shortening that affected the african continent following the establishment of post rift sedimentation drift the interval between these separate unconformities is occupied by allochthonous rafts of shallow water carbonates recording gravitational collapse of a marginal platform the rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent ascension fracture zone and thermal doming due to the coeval st helena and ascension plumes the data also reveal a c 45 35 ma cooling episode attributed to deep sea erosion at the onset of eo oligocene ice growth and a c 15 10 ma episode interpreted as the record of miocene exhumation of the west african continental margin related to continent wide plume development integration of thermal history methods with traditional seismic and stratigraphy based observations yields a dynamic picture of kilometre scale fluctuations in base level through the breakup and early drift phases of development of these margins major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy but also regional plate margin generated deformation local intra basinal reorganization and the amplifying effect of negative feedbacks between these processes

upper mantle s velocity structure of central and western south america we have used broadband seismograms recorded at portable and permanent seismic stations in south america to image the three dimensional s velocity structure beneath central and western south america we have interactively fitted the waveforms of s and surface wave trains in 500 seismograms from 90 earthquakes and 60 different stations the constraints on upper mantle s velocity structure provided by the waveform fits have been combined with independent estimates of depth to the mohorovicic discontinuity yielding model sa99 the s velocity structure of sa99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km s above 100 km the heterogeneity seems to be correlated with tectonic and geologic domains sa99 shows high lithospheric s velocities for the precambrian cratons the amazon basin is also underlain by high s velocities while three other intraplatform basins are underlain by low s velocities at various depths sa99 shows extremely low velocities in the mantle wedge beneath the volcanically active andes we explain these low s velocities as originating from realistic amounts of volatiles released by the subducting nazca crust and resulting amounts of partial melt within the peridotitic mantle wedge without requiring elevated temperatures while sa99 could be consistent with diffuse influx for corner flow in the mantle wedge it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low viscosity channel

late miocene high and rapid surface uplift and its erosional response in the andes of central chile 33 35 s 1 we address the question of the late cenozoic geomorphological evolution of the central chile andes 33 35 s using uplift markers river incision previous and new ages of volcanic bodies and new fission track ages the uplift markers consist of relicts of high elevated peneplains that evidence 2 km of regional surface uplift lasting 2 ma with variable amount along an e w transect the eastern coastal cordillera was uplifted 1 5 2 1 km at 33 34 s and 2 5 at 33 45 s and 1 5 km at 34 30 s erosional response to uplift was characterized by the retreat of a sharp knickpoint with celerities between 10 and 40 mm a 1 extrapolation using a stream power law shows that uplift began shortly before 4 ma or at 10 5 4 6 ma 7 6 ma central age depending on the morphostructural units involved the first alternative implies simultaneous uplift of the continental margin the second model the most reliable one implies that the uplift affected together the eastern coastal cordillera and the principal cordillera while the rest of the western fore arc subsided this regional uplift can be mostly balanced by crustal thickening resulting from coeval shortening related to the out of sequence thrusting event in the principal cordillera and the uplift of the frontal cordillera simultaneously emplacement of the southern edge of the flat slab subduction zone might have partially contributed to this uplift event

possible evidence for a connection between methyl iodide emissions and saharan dust methyl iodide has been measured during two field campaigns in the atlantic region in 2002 the first took place in july august at 2300 m on the island of tenerife while the second was a shipborne east west crossing of the tropical atlantic at 10 n in october november of the same year both campaigns were periodically impacted by dust advected from africa in trade winds unexpectedly during these dust events methyl iodide mixing ratios were observed to be high relative to other times backward calculations with the particle dispersion model flexpart show the origin of the dust storms as mauritania and southern algeria for the ground and ship based campaigns respectively the dust laden air traveled from its source above the marine boundary layer to the measurement region on the basis of the field data correlations and the simulations we suggest that dust stimulated emission of methyl iodide has occurred to test this hypothesis dust samples were collected from the identified source regions and added to filtered atlantic seawater this rapidly produced methyl iodide further tests established that the addition of iodide increased the yield and that iodide with h2o2 was greater still this was found for both sterilized and nonsterilized samples we conclude that there is an abiotic methyl iodide production mechanism that can occur via substitution analogous to those in soil rather than radical recombination this may occur when dust contacts seawater containing iodide or when marine water vapor condenses on dust containing iodide this hypothesis appears to be consistent with recent long term methyl iodide data sets from tasmania and may help resolve current uncertainties in the iodine cycle

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

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

methyl halides in surface seawater and marine boundary layer of the northwest pacific 1 the partial pressures of methyl halides ch3x x cl br or i and of chclf2 hcfc 22 which are all volatile organic compounds vocs were measured in the air of the marine boundary layer pvocair and in surface seawater pvocwater during a cruise from the subarctic to subtropical regions of the northwest pacific in summer of 2008 in the northern transition water twn with high biological activity high levels of the three ch3xs in surface seawater were frequently observed probably owing to their enhanced production by phytoplankton supersaturation of ch3br was only present in twn water with a saturation anomaly sch3br of 0 95 sch3x pch3xwater pch3xair pch3xair the highest saturation anomalies for ch3cl sch3cl 1 6 and ch3i sch3i 91 were found in the southern subtropical water sts with low biological production south of the subtropical front we found that the molar concentrations of ch3cl cch3cl and ch3i cch3i sharply increased with increasing sea surface temperature sst in the subtropical waters the maximum cch3cl 144 pmol l 1 was present in sts water at sst 30 c and is 1 5 times the value extrapolated from the previously reported relationship between cch3cl and sst photochemical production might have contributed to the production of ch3cl and ch3i in sts water

vertical tectonics at a continental crust oceanic plateau plate boundary zone fission track thermochronology of the sierra nevada de santa marta colombia 1 the topographically prominent sierra nevada de santa marta forms part of a faulted block of continental crust located along the northern boundary of the south american plate hosts the highest elevation in the world 5 75 km whose local base is at sea level and juxtaposes oceanic plateau rocks of the caribbean plate quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary and the driving forces of rock uplift along the active margin the sierra nevada province of the southernmost sierra nevada de santa marta exhumed at elevated rates 0 2 km my during 65 58 ma in response to the collision of the caribbean plateau with northwestern south america a second pulse of exhumation 0 32 km my during 50 40 ma was driven by underthrusting of the caribbean plate beneath northern south america subsequent exhumation at 40 25 ma 0 15 km my is recorded proximal to the santa marta bucaramanga fault more northerly regions of the sierra nevada province exhumed rapidly during 26 29 ma 0 7 km my further northward the santa marta province exhumed at elevated rates during 30 25 ma and 25 16 ma the highest exhumation rates within the sierra nevada de santa marta progressed toward the northwest via the propagation of nw verging thrusts exhumation is not recorded after 16 ma which is unexpected given the high elevation and high erosive power of the climate implying that rock and surface uplift that gave rise to the current topography was very recent i e 1 ma and there has been insufficient time to expose the fossil apatite partial annealing zone

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

climatic and ecologic changes during miocene surface uplift in the southern patagonian andes abstract the up to 4 km high southern patagonian andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies and cause one of the most drastic orographic rain shadows on earth geologic data imply that this climatic pattern has been established or significantly enhanced during miocene surface uplift of this andean segment we report evidence for important climatic and ecologic changes in the eastern foreland of the patagonian andes that appear to be the result of this uplift to provide constraints on miocene plant ecosystems and precipitation in the eastern leeward foreland of the patagonian andes we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a 500 m thick section of the continental santa cruz formation the age of these deposits was constrained by ar ar dating of intercalated tuffs which range from 22 to 14 ma at 16 5 ma the 13 c values increase by 3 the 18 o values decrease by 2 and the scatter in the oxygen isotope data increases significantly we interpret these changes as the consequence of 1 km surface uplift in this andean segment from the 18 o values and increased aridity to its east from the 13 c values and the increased scatter in the 18 o values sediments overlying the santa cruz formation are very limited in extent and volume and dominated by coarse conglomerates related to pleistocene and older glaciations it thus seems that by 14 ma deposition in the eastern foreland of the southern patagonian andes had essentially ceased as the result of rain shadow formation

oceanic bromoform sources for the tropical atmosphere oceanic bromoform chbr3 is the major source of organic br to the atmosphere and may be significant for ozone depletion through the contribution of reactive bromine to the upper troposphere and lower stratosphere of the midlatitudes and tropics we report the first analyses of boundary layer air surface and deep ocean waters from the tropical atlantic the data provide evidence of a source of chbr3 throughout the tropical open ocean associated with the deep chlorophyll maximum within the tropical thermocline equatorial upwelling carries the chbr3 to the surface adding to increased concentrations in the equatorial mixed layer and driving oceanic emissions that support locally elevated atmospheric concentrations in air masses that had crossed the coastal upwelling region off nw africa even higher atmospheric mixing ratios were measured the observations suggest a link between climate wind driven upwelling and the supply of br to the upper atmosphere of the tropics

in situ evaluation of air sea gas exchange parameterizations using novel conservative and volatile tracers measurements of air sea gas exchange rates are reported from two deliberate tracer experiments in the southern north sea during february 1992 and 1993 a conservative tracer spores of the bacterium bacillus globigii var niger was used for the first time in an in situ air sea gas exchange experiment this nonvolatile tracer is used to correct for dispersive dilution of the volatile tracers and allows three estimations of the transfer velocity for the same time period the first estimation of the power dependence of gas transfer on molecular diffusivity in the marine environment is reported this allows the impact of bubbles on estimates of the transfer velocity derived from changes in the helium sulphur hexafluoride ratio to be assessed data from earlier dual tracer experiments are reinterpreted and findings suggest that results from all dual tracer experiments are mutually consistent the complete data set is used to test published parameterizations of gas transfer with wind speed a gas ex change relationship that shows a dependence on wind speed intermediate between those ofliss and merlivat 1986 and wanninkhof 1992 is found to be optimal the dual tracer data are shown to be reasonably consistent with global estimates of gas exchange based on the uptake of natural and bomb derived radiocarbon the degree of scatter in the data when plotted against wind speed suggests that parameters not scaling with wind speed are also influencing gas exchange rates

paleosurfaces from northeastern amazonia as a key for reconstructing paleolandscapes and understanding weathering products the characterization and mapping of paleoweathering surfaces as well as their associated deposits throughout the northeastern amazonia can be a useful tool for inter basin stratigraphic correlations and for establishing the evolution of landscapes and climates through time this type of investigation has led to the recognition of five weathering surfaces developed from the upper cretaceous through the quaternary designated here as s1 to s5 surface s1 occurs at the top of late cretaceous deposits represented by the ipixuna cujupe succession as well as on older precambrian and paleozoic rocks surface s2 marks the top of an indurated massive semi flint1 unit for cases in which surfaces s1 and s2 are amalgamated a thick lateritic paleosol displaying hexagonal branched concretions is present surface s3 occurs at the top of the pirabas and lower portion of the barreiras formations of late oligocene early miocene age and it is also associated with paleosol but has no lateritization when surface s3 is amalgamated with surfaces s1 and s2 bauxite is present in association with the underlying laterites surface s4 bounds the top of the middle to upper barreiras formation of middle miocene age as well as correlatable deposits known as the belterra clay this surface is marked by a lateritic paleosol having columnar concretions surface s5 occurs between a lower unit consisting of massive light red sands of the plio pleistocene post barreiras 1 sediments and an upper unit of yellow eolian sands of the holocene post barreiras 2 sediments the paleoweathering surfaces present in the northeastern amazonia enable the reconstruction of several depositional episodes represented by thin only few meters thick sedimentary units from the late cretaceous to the quaternary the creation of new accommodation space on subsiding areas gave rise to sediment accumulation during transgressions intervening weathering surfaces formed during significant falls in relative sea level associated with tectonically stable episodes the overprinting of weathering surfaces formed during different times and thus under different climatic conditions seems to have been crucial for the development of the various weathering products observed throughout the region which include economically important laterites bauxites semi flint beds and kaolin deposits this paper shows that the genesis of these deposits was controlled by the complex interplay of tectonics climate sediment deposition and erosion resulting in a complex evolution of paleolandscapes

exhumation history of the southern altiplano plateau southern bolivia constrained by apatite fission track thermochronology 1 although the structural geometry of the cenozoic altiplano puna plateau in the central andes is well defined the temporal evolution of this contractile deformation is poorly constrained to address this shortcoming we used apatite fission track thermochronology aft to quantify the cooling and exhumation history along a transect at 21 s in southern bolivia through the deformed intermontane altiplano basin the doubly vergent thrust belt of the eastern cordillera and the inner foreland thrust belt east of the plateau interandean zone thermal history modeling combined with published balanced cross sections and stratigraphic data constrain exhumation histories exhumation started during the late eocene 40 36 ma in the central eastern cordillera possibly due to bivergent thrusting and cretaceous rift structure inversion during the early oligocene 33 27 ma exhumation spread across the study area as the current boundary thrusts of the eastern cordillera were activated the inner west vergent thrust system became active in irregular order until circa 20 ma whereas the east vergent interandean thrust belt formed by eastward propagating deformation since circa 30 ma plateau exhumation continued at 0 2 mm yr until shortening terminated by 11 7 ma shortening within the plateau since circa 30 ma did not evolve by lateral accretion of thrust wedges the propagating deformation style is spatially confined to the foreland thrust belt which initiated coeval to plateau deformation interandean zone but propagated mainly after circa 10 ma subandean zone early oligocene plateau wide tectonically driven exhumation suggests that subduction related processes had already thermally weakened the continental lithosphere prior to the 27 25 ma onset of volcanic activity

photometric anomalies of the lunar surface results from clementine data 1 we mapped the photometric characteristics of the lunar surface for several small areas using clementine uvvis camera images the maps of the phase function steepness showed several anomalous sites several small fresh impact craters have anomalous halos in these maps the phase function within the halos is less steep than for the surrounding mare surface we interpret these halos to be due to geologically recent impact caused alteration of the equilibrium millimeter scale regolith structure this equilibrium structure is established through micrometeoritic bombardment at a geologically short timescale an anomaly of the same signature was found at the apollo 15 landing site we interpret it as being a result of the regolith structure alteration with the lander jets a unique photometric anomaly not correlated with albedo was found within the reiner gamma formation we suggest that this anomaly is genetically related to the formation which indicates its young age this favors the impact hypothesis for the nature of the reiner gamma formation our study showed that mapping of photometric characteristics is a new powerful tool in studies of the surfaces of atmosphereless bodies future photometric studies of the moon with existing and new data sets are promising for a search for traces of recent seismic events studies of the recent population of meteoroids in the inner solar system an advance in the understanding of swirls etc

thermochronometer record of central andean plateau growth bolivia 19 5 s 1 quantifying the timing magnitude and rates of exhumation and deformation across the central andes is a prerequisite for understanding the history of plateau rise we present 23 new apatite and zircon fission track thermochronometer samples to chronicle the exhumation and deformation across the entire 500 km andean fold thrust belt at 19 5 s in bolivia exhumation and deformation are constrained with inverse thermal modeling of the thermochronometer data regional stratigraphy geothermal gradients and mass deficits inferred from a balanced section results suggest the following 1 initial exhumation of the eastern cordillera ec fore thrust and back thrust belts began in the late eocene to early oligocene 27 36 ma and continued in a distributed manner in the late oligocene to early miocene 19 25 ma interandean zone ia exhumation began 19 22 ma followed by a third pulse of exhumation 11 16 ma in the ec back thrust belt and initial cooling in the westernmost subandes sa 8 20 ma finally exhumation propagated eastward across the sa during the late mio pliocene 2 8 ma 2 exhumation magnitudes are spatially variable and range from maximums of 8 km in the ec fore thrust belt to average values of 4 7 km across the ec 2 5 3 km in the altiplano 4 6 km in the ia and 3 km in the sa 3 exhumation rates range from 0 1 to 0 2 mm a in the ec from 0 1 to 0 6 mm a in the ia and from 0 1 to 0 4 mm a to locally 1 4 mm a or more in the eastern sa we synthesize similar constraints with sediments throughout bolivia and characterize plateau development by 1 distributed deformation throughout the altiplano and ec regions from 20 to 40 ma with minor deformation continuing until 10 ma 2 contemporaneous cessation of most ec deformation and exhumation of the ia 20 ma implying establishment of the modern plateau width with significant but unknown crustal thickness and elevation shortly thereafter by 15 20 ma and 3 dominantly eastward propagation of deformation from the ia since 20 ma with minor out of sequence deformation in the central to eastern sa

uplift histories of africa and australia from linear inverse modeling of drainage inventories this is the final version it first appeared at http onlinelibrary wiley com wol1 doi 10 1002 2014jf003297 abstract

quantifying canyon incision and andean plateau surface uplift southwest peru a thermochronometer and numerical modeling approach incision we quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism furthermore the large magnitude of incision leads to additional threedimensional variations in the thermal field we address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with 0 2 km of incision in the forearc region prior to 14 ma and up to 3 0 km of incision starting between 7 and 11 ma incision starting at 7 ma requires incision to end by 5 5 to 6 ma however a 2 2 ma age on a volcanic flow on the current valley floor and 5 ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2 2 and 5 ma these additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 ma which must coincide with or postdate surface uplift

spatial and temporal patterns of exhumation across the venezuelan andes implications for cenozoic caribbean geodynamics 1 the venezuelan andes formed by complex geodynamic interaction between the caribbean plate the panama arc the south american plate and the continental maracaibo block we study the spatial and temporal patterns of exhumation across the venezuelan andes using 47 new apatite fission track aft ages as well as topographic analyses this approach permits the identification of at least seven tectonic blocks escalante cerro azul trujillo caparo sierra nevada sierra la culata and el carmen blocks with contrasting exhumation and cooling histories the sierra nevada sierra la culata and el carmen blocks located in the central part of the venezuelan andes and separated by the bocono fault system cooled rapidly but diachronously during the late miocene pliocene major surface uplift and exhumation occurred in the sierra nevada block since before 8 ma a second phase of uplift and exhumation affected the el carmen and sierra la culata blocks to the north of the bocono fault during the late miocene pliocene the highest topography and steepest relief of the belt coincides with these blocks the caparo and trujillo blocks located at the northeastern and southwestern ends of the orogen cooled more slowly from the oligocene to the late miocene these blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional caribbean geodynamics on the evolution of the venezuelan andes our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events e g collision of the panama arc rotation of the maracaibo block

dynamic reorganization of river basins river networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels we demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides assessing this metric throughout a landscape maps the dynamic states of entire river networks revealing diverse conditions drainage divides in the loess plateau of china appear stationary the young topography of taiwan has migrating divides driving adjustment of major basins and rivers draining the ancient landscape of the southeastern united states are reorganizing in response to escarpment retreat and coastal advance the ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape scale interactions among tectonics erosion and ecology

oceanic distributions and air sea fluxes of biogenic halocarbons in the open ocean 1 surface seawater and atmospheric concentrations of methyl iodide chloroiodomethane bromoform dichlorobromomethane and chlorodibromethane were measured during three open ocean cruises in the atlantic and southern oceans the measurements spanned a longitudinal range of 115 between 50 n and 65 s the saturation anomalies and the instantaneous air sea fluxes of the gases during one of these cruises ant xviii 1 are presented and discussed methyl iodide and chloroiodomethane were highly supersaturated 1000 throughout the temperate and tropical regions with calculated mean fluxes of 15 and 5 5 nmol m 2 d 1 respectively the oceanic emissions of the brominated compounds were less substantial and a significant area of the temperate atlantic ocean was found to be a sink for bromoform correlation analyses have been used to investigate possible controls on the concentrations of these gases in particular the relationship of ch3i with sea surface temperature and light is discussed with the tentative conclusion that this compound may be formed abiotically

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

solutions of the stream power equation and application to the evolution of river longitudinal profiles 1 erosion by bedrock river channels is commonly modeled with the stream power equation we present a two part approach to solving this nonlinear equation analytically and explore the implications for evolving river profiles first a method for non dimensionalizing the stream power equation transforms river profiles in steady state with respect to uniform uplift into a straight line in dimensionless distance elevation space second a method that tracks the upstream migration of slope patches which are mathematical entities that carry information about downstream river states provides a basis for constructing analytical solutions slope patch analysis explains why the transient morphology of dimensionless river profiles differs fundamentally if the exponent on channel slope n is less than or greater than one and why only concave up migrating knickpoints persist when n 1 at migrating knickpoints slope patches and the information they carry are lost a phenomenon that fundamentally limits the potential for reconstructing tectonic histories from bedrock river profiles stationary knickpoints which can arise from spatially varying uplift rates differ from migrating knickpoints in that slope patches and the information they carry are not lost counterparts to migrating knickpoints called stretch zones are created when closely spaced slope patches spread to form smooth curves in distance elevation space these theoretical results are illustrated with examples from the california king range and the central apennines

episodic orogenic front migration in the northern andes constraints from low temperature thermochronology in the eastern cordillera colombia 1 new thermochronometric data from the eastern cordillera of the colombian andes reveal diachronous exhumation associated with cenozoic contractional deformation in this sector of the northern andes we present a comprehensive account of exhumation patterns along a 150 km long across strike transect between 4 and 6 n by integrating 29 new apatite fission track aft ages and 17 new zircon fission track zft ages with sparse published thermochronological data from this area our data reveal episodic eastward migration of the orogenic front at an average rate of 2 5 2 7 mm a during the late cretaceous cenozoic we identify three major stages of orogen propagation 1 slow propagation 0 5 3 1 mm a until early eocene 2 rapid orogenic advance 4 0 18 0 mm a during middle late eocene which accounts for 86 of the orogen s present width and 3 slow orogen propagation 1 2 2 1 mm a from oligocene to holocene times our data demonstrate that in the course of changes in plate kinematics the presence of inherited crustal anisotropies such as the former rift bounding faults of the eastern cordillera favor a nonsystematic progression of foreland basin deformation through time by preferentially concentrating accommodation of slip and thrust loading along these zones of weakness

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

an uplift history of the colorado plateau and its surroundings from inverse modeling of longitudinal river profiles 1 it is generally agreed that a region encompassing the colorado plateau has been uplifted by sub crustal processes admittance calculations tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes notwithstanding advances in our understanding of present day setting uplift rate histories are poorly constrained and debated an improved history will aid discrimination between proposed models here we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles we assume that the shape of a river profile is controlled by uplift rate and moderated by erosion in our model uplift rate is allowed to vary smoothly as a function of space and time upstream drainage area is invariant with time simultaneous inversion of river profiles from the colorado rio grande columbia and mississippi catchments shows that three phases of regional uplift occurred the first phase occurred between 80 and 50 myrs when 1 km of uplift was generated at a rate of 0 03 mm yr a second phase occurred between 35 and 15 myrs when 1 5 km of uplift was generated at a faster rate of 0 06 mm yr a final phase of uplift commenced 5 myrs ago these distinct phases of late cretaceous and oligocene uplift are corroborated by stratigraphic considerations by thermochronometric data and by stratigraphic evidence of periodic clastic efflux delivered into the gulf of mexico an episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region which is currently centered on yellowstone

geometric kinematic and erosional history of the central andean plateau bolivia 15 17 s 1 latitudinal changes in topography climate and thrust belt geometry in the central andes have led to conflicting hypotheses that climate or tectonics exert a first order control on orogen evolution the relative roles of climate and tectonics in the evolution of the andean orogen are difficult to quantify because of a lack of detailed observations for both the long term deformation and erosion history of the andean fold thrust belt we contribute to the resolution of this problem by presenting a sequentially restored balanced cross section based on new mapping across the northern bolivia portion of the thrust belt 15 17 s the timing and magnitude of exhumation across the cross section are determined by synthesizing 10 new and 70 previously published mineral cooling ages once balanced and restored the section was sequentially forward modeled using stratigraphic and cooling age constraints results indicate the eastern cordillera ec records the highest magnitudes of shortening 123 km or 55 the interandean zone ia has shortened 48 km or 30 in both the ec and ia individual thrust sheets are tightly folded and have minor offsets of 1 5 km the subandes sa has multiple levels of detachments allowing for thrust sheets with relatively large offsets 6 17 km total shortening in the sa is 66 km or 40 total magnitude of shortening for the entire fold thrust belt in this region is 276 km 40 new apatite and zircon fission track cooling ages in conjunction with published ages indicate two phases of rapid exhumation an earlier phase from 40 to 25 ma in the ec and one prior to 25 ma in the ia followed by distributed exhumation of the entire fold thrust belt from 15 0 ma combined exhumation estimates from the balanced cross section and thermochronology suggest 9 11 km of exhumation in the ec 5 9 km in the ia and 3 4 km in the sa long term shortening rates are 7 mm a for the ec and ia and 4 8 mm a for the sa the sa shortening rates are based on a 15 0 ma or 8 0 ma deformation window by linking cooling ages to location and magnitude of shortening we suggest an 10 17 ma pause or a dramatic deceleration in the rate of deformation and propagation of the fold thrust belt between 25 and 15 or 8 ma

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

oceanic distributions and emissions of short lived halocarbons 1 using data from seven cruises over a 10 year span we report marine boundary layer mixing ratios i e dry mole fractions as pmol mol 1 or ppt degrees of surface seawater saturation and air sea fluxes of three short lived halocarbons that are significant in tropospheric and potentially stratospheric chemistry chbr3 c h2br2 and ch3i were all highly supersaturated almost everywhere all the time highest saturations of the two polybrominated gases were observed in coastal waters and areas of upwelling such as those near the equator and along ocean fronts ch3i distributions reflected the different chemistry and cycling of this gas in both the water and the atmosphere seasonal variations in fluxes were apparent where cruises overlapped and were consistent among oceans undersaturations of these gases were noted at some locations in the southern ocean owing to mixing of surface and subsurface waters not necessarily biological or chemical sinks the pacific ocean appears to be a much stronger source of chbr3 to the marine boundary layer than the atlantic the high supersaturations fluxes and marine boundary layer mixing ratios in the tropics are consistent with the suggestion that tropical convection could deliver some portion of these gases and their breakdown products to the upper troposphere and lower stratosphere

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding