landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

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

convective storm life cycle and environments near the sierras de cordoba argentina abstractsatellite observations have revealed that some of the world s most intense deep convective storms occur near the sierras de cordoba argentina south america a c band dual polarization do

effects of bedrock landslides on cosmogenically determined erosion rates abstract the successful quantification of long term erosion rates underpins our understanding of landscape formation the topographic evolution of mountain ranges and the mass balance within active orogens the measurement of in situ produced cosmogenic radionuclides crns in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such long term erosion rates although deep seated bedrock landsliding is an important erosional process in active orogens its effect on crn derived erosion rates is largely unquantified we present a numerical simulation of cosmogenic nuclide production and distribution in landslide dominated catchments to address the effect of bedrock landsliding on erosion rates results of the simulation indicate that the temporal stability of erosion rates determined from crn concentrations in sediment decreases with increased ratios of landsliding to bedrock weathering rates within a given catchment area and that as the frequency of landsliding increases larger catchment areas must be sampled in order to accurately evaluate long term erosion rates in addition results of this simulation suggest that sediment sampling for crns is the appropriate method for determining long term erosion rates in regions dominated by mass wasting processes whereas bedrock surface sampling for crns may generally underestimate long term erosion rates response times of crn concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected but that complete equilibration of crn concentrations to new erosional conditions may take tens of thousands of years comparison of simulated crn derived erosion rates with a new data set of such rates from the nepalese himalaya underscore the conclusions drawn from the model results

mineralogical control on the magnetic anisotropy of lavas and ignimbrites a case study in the caviahue copahue field argentina

orogenic convection in subtropical south america as seen by the trmm satellite abstractextreme orogenic convective storms in southeastern south america are divided into three categories storms with deep convective cores storms with wide convective cores and storms containing broad stratiform regions data from the tropical rainfall measuring mission satellite s precipitation radar show that storms with wide convective cores are the most frequent tending to originate near the sierra de cordoba range downslope flow at upper levels caps a nocturnally enhanced low level jet thus preventing convection from breaking out until the jet hits a steep slope of terrain such as the sierra de cordoba mountains or andean foothills so that the moist low level air is lifted enough to release the instability and overcome the cap this capping and triggering is similar to the way intense convection is released near the northwestern himalayas however the intense storms with wide convective cores over southeastern south america are unlike their himalayan counterparts in that they exhibit leadi

late stage calcites in the permian capitan formation and its equivalents delaware basin margin west texas and new mexico evidence for replacement of precursor evaporites comparison of upper guadalupian fore reef reef and back reef strata from outcrops in the guadalupe mountains with equivalent subsurface cores from the northern and eastern margins of the delaware basin indicates that extensive evaporite diagenesis has occurred in both areas in both surface and subsurface sections the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite these evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution enlarged fractures and karstic sink holes into the underlying strata outcrop areas today however contain no preserved evaporites in reef and fore reef sections and only partial remnants of evaporites are retained in back reef settings in their place these rocks contain minor silica very large volumes of coarse sparry calcite and some secondary porosity the replacement minerals locally form pseudomorphs of their evaporite precursors and less commonly contain solid anhydrite inclusions some silicification dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km however no calcite replacements were noted from any subsurface core samples subsurface alteration has also led to the widespread late stage development of large and small scale dissolution breccias the restriction of calcite cements to very near surface sections petrographic evidence that the calcites post date hydrocarbon emplacement and the highly variable but generally light carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic telogenetic phenomenon evaporite dissolution and calcitization reactions have only taken place where permian strata were flushed with meteoric fluids as a consequence of tertiary uplift tilting and breaching of regional hydrological seals a typical sequence of alteration involves initial corrosion of anhydrite one or more stages of hydration dehydration during conversion to gypsum dissolution of gypsum and precipitation of sparry calcite such evaporite dissolution and replacement processes are probably continuing today in near outcrop as well as deeper settings this study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements the extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic carbonate rocks

catastrophic rock avalanche 3600 years bp from el capitan yosemite valley california large rock slope failures from near vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes particularly glacially steepened cliffs the morphology and age of a 2 19 106 m3 rock avalanche deposit beneath el capitan in yosemite valley indicates a massive prehistoric failure of a large expanse of the southeast face geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of 8 5 104 m2 the rock mass free fell 650 m reaching a maximum velocity of 100 m s 1 impacted the talus slope and spread across the valley floor extending 670 m from the base of the cliff cosmogenic beryllium 10 exposure ages from boulders in the deposit yield a mean age of 3 6 0 2 ka the 13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing the 3 6 ka age for the rock avalanche does coincide with estimated late holocene rupture of the owens valley fault and or white mountain fault between 3 3 and 3 8 ka the coincidence of ages combined with the fact that the most recent ad 1872 owens valley fault rupture triggered numerous large rock falls in yosemite valley suggest that a large magnitude earthquake m7 0 centered in the south eastern sierra nevada may have triggered the rock avalanche if correct the extreme hazard posed by rock avalanches in yosemite valley remains present and depends on local earthquake recurrence intervals published in 2010 by john wiley sons ltd

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

the influence of helicity on numerically simulated convective storms abstract a three dimensional numerical cloud model is used to investigate the influence of storm relative environmental helicity sreh on convective storm structure and evolution with a particular emphasis on the identification of ambient shear profiles that are conducive to the development of long lived strongly rotating storms eleven numerical simulations are made in which the depth and turning angle of the ambient vertical shear vector are varied systematically while maintaining a constant magnitude of the shear in the shear layer in this manner an attempt is made to isolate the effects of different environmental felicities on storm morphology and show that the sreh and bulk richardson number rather than the mean shear in the low levels determine the rotational characteristics and morphology of deep convection the results demonstrate that storms forming in environments characterized by large sreh are longer lived than those in less helical surroundings further it appears that the storm relat

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

late quaternary fault segmentation from analysis of scarp morphology a thorough knowledge of the lateral extent of individual prehistoric ruptures along a fault is necessary to assess the locations and geologic characteristics of boundaries between independent faultrupture segments paleoseismic studies employing only trenching may yield insufficient data to evaluate the lateral extent of individual rupture events systematic mapping of fault scarps and degradation equation modeling of relative scarp ages on the lemhi fault a range bounding normal fault in east central idaho provide a method to assess rupture extent and boundaries between rupture segments the youngest latest quaternary surface ruptures occurred at distinctly different times on adjacent segments of the fault the boundaries between rupture segments differ in characteristics although most are at intersections of the currently active range bounding fault and mid tertiary normal faults several major structural and tectonic discontinuities did not form segment boundaries in the most recent rupture cycle or probably in the previous cycle the lemhi fault illustrates both the importance of preexisting structures in controlling extent of ruptures and the need for detailed analysis along an entire fault zone

numerical modeling of cosmogenic deglaciation records front range and san juan mountains colorado 1 we use cosmogenic radionuclide crn exposure ages from polished striated bedrock to constrain numerical simulations of deglaciation in the middle boulder creek valley colorado front range and the animas river valley san juan mountains colorado in both valleys the cosmogenic ages suggest initiation of deglaciation 20 ka and ongoing retreat until 12 13 ka while the first order trend in crn concentrations in each valley suggests a monotonic glacial retreat we evaluate other retreat scenarios with different implications for post last glacial maximum regional climate we use a 2 d numerical glacier simulation with a crn layer to investigate how crn based deglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance the crn layer simulates both production during periods of exposure and reduction by glacial erosion during readvances we simulate glacial occupation of the valleys as they respond to equilibrium line altitude ela histories characterized by stepwise change gradual rise or a rise punctuated by short periods of lowering each scenario generates a distinct spatial pattern of concentrations in the crn layer these results and the spatial pattern of measured concentrations in bedrock constrain the range of ela histories that reproduce the crn pattern in each valley in the animas river valley the exposure ages are well explained by a linear ela rise from full glacial to deglacial conditions ages in middle boulder creek valley are best explained by a deglaciation history including a stillstand or partial readvance between 16 and 14 ka followed by rapid retreat

a mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution rift basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate the physical basis for this transition is not well understood although an increase in extension rate is commonly implied here a numerical fault growth model is used to investigate the influence of segment linkage on fault displacement rate patterns along an evolving normal fault array the linkage process we describe is controlled by a stress feedback mechanism which leads to enhanced growth of optimally positioned faults model results indicate that even with constant extension rates slow displacement rates prevail during an initial phase of distributed extension followed by an increase in displacement rates as strain becomes localized on linked fault arrays this is due to the dynamics of fault interactions rather than mechanical weakening comparison of model simulations with rift basin subsidence and stratigraphic patterns in the gulf of suez and north sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault zone evolution without invoking a change in regional extension rates

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

timing and extent of late quaternary paleolakes in the trans pecos closed basin west texas and south central new mexico abstract the trans pecos closed basin is a hydrographically closed region covering 20 000 km 2 centered on salt basin 160 km east of el paso texas geomorphic and limnetic evidence have been used to identify four major highstands for lake king during the last glacial maximum lgm additional geomorphic features from a second recently identified paleolake lake sacramento have been found in the beargrass subbasin a nested subbasin approximately 75 km northwest of salt basin radiocarbon ages of the organic material in lake king sediments date four abrupt climate changes and rapid lacustrine transgressions during the lgm with a quasi periodicity of 2000 yr geomorphic evidence in the beargrass subbasin identifies lake cycles contemporaneous with those in lake king the dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in new mexico the quasi periodicity of the events approximates that of dansgaard oeschger events identified from greenland ice cores the contemporaneity of the trans pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the lgm

three dimensional modelling and sequence stratigraphy of a carbonate ramp to shelf transition permian upper san andres formation this study highlights three dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate siliciclastic clinoforms in the permian san andres formation standard field techniques and mapping using ground based lidar reveal a high degree of architectural complexity in channellized scoured and mounded outer ramp stratigraphy development of these features was a function of location along the ramp profile and fluctuations in relative sea level deposition of coarse grained and fine grained turbidites in the distal outer ramp occurred through dilute and high density turbidity flows and was the result of highstand carbonate shedding within individual cycles in this setting high frequency cycles of relative sea level are interpreted on the basis of turbidite frequency lateral extent and composition submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation abundant siliciclastic material is preserved in the basin and distal outer ramp as point sourced lowstand wedges and line sourced early transgressive blankets in mounded topography of the outer ramp siliciclastic preservation is minimal to absent and rare incised channels offer the best opportunity for recognition of a sequence boundary growth of mounded topography in the outer ramp began with scouring followed by a combination of bioherm construction fusulinid mound construction and isopachous draping intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit mound growth was independent of high frequency cycles in relative sea level but was dependent on available accommodation dictated by low frequency cyclicity low angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence this example represents a ramp to shelf transition that is the result of forcing by relative sea level fluctuations rather than ecologic or tectonic controls

convective initiation near the andes in subtropical south america abstractsatellite radar and radiometer data indicate that subtropical south america has some of the deepest and most extreme convective storms on earth this study uses the full 15 yr trmm precipitation radar dataset in conjunction with high resolution simulations from the weather research and forecasting model to better understand the physical factors that control the climatology of high impact weather in subtropical south america the occurrence of intense storms with an extreme horizontal dimension is generally associated with lee cyclogenesis and a strengthening south american low level jet sallj in the la plata basin the orography of the andes is critical and model sensitivity calculations removing and or reducing various topographic features indicate the orographic control on the initiation of convection and its upscale growth into mesoscale convective systems mcss reduced andes experiments show more widespread convective initiation weaker average storm intensity and more rapid propagation

mantle convection and the recent evolution of the colorado plateau and the rio grande rift valley the colorado plateau contains late cretaceous marine strata that are at a mean elevation of 2 km the timing and amount of uplift since the cretaceous has generated considerable debate with the exception of a few studies topography supported by vertical stresses generated by viscous fl ow in the mantle has not been explicitly considered to contribute to the elevation of this region herein we compute the viscous fl ow beneath north america that is driven by density anomalies inferred from joint seismic geodynamic modeling we fithat the colorado plateau overlies a strong mantle upwelling that is coupled to the sinking farallon slab currently beneath the eastern united states consequently the colorado plateau is currently a focused dynamic topography high within the western u s cordillera moreover this strong upwelling impacts the base of the lithosphere at an oblique angle east of the plateau directly below the rio grande rift we attribute this fl ow as being responsible for some of the recent magmatic activity along the jemez lineament as well as contributing to the recent rifting process in the rio grande rift valley

elevated mixed layers and associated severe thunderstorm environments in south and north america abstractthis study presents a climatological and composite analysis of elevated mixed layers emls in south and north america derived from the ncep climate forecast system reanalysis the emls are identified based on objective criteria applied to the reanalysis data composite analyses of synoptic scale conditions and severe weather parameters associated with spring eml cases are presented emls are more frequent immediately to the east of the andes and the rockies the north american emls form by surface heating over the higher terrain of the rockies with peak frequency occurring in spring and summer emls in south america are generated by differential temperature advection due to ageostrophic circulations east of the andes as indicated by the temperature lapse rate tendency equation which relates to the higher frequency of emls during the cold season in south america emls over north america are about 100 hpa lower than over south america due to the lower height of the rockies in comparison to the a

a case study of terrain influences on upscale convective growth of a supercell abstractsatellite and ground based radar observations have shown that the northern half of argentina south america is a region susceptible to rapid upscale growth of deep moist convection into l

numerical modeling of spatially variable precipitation and passive margin escarpment evolution abstract passive margin escarpments are long lasting landforms which can remain steep and high over tens of millions of years precipitation variability has seldom been considered as a contributor to passive margin escarpment evolution however large and distinct variability in precipitation rates with elevation is observed in mountainous landscapes we assess the potential impacts of spatially variable precipitation on passive margin escarpment evolution with a numerical model that couples a simple elevation dependent precipitation rule with the cascade surface processes model and a simple flexural uplift model modeled topography is more sensitive to the elevation of the precipitation maximum than to the effective elastic thickness of the lithosphere if precipitation is maximized at the escarpment crest the escarpment decreases in elevation slope and proximity to the coast more rapidly than if precipitation is focused at low elevations low elevation precipitation maxima contribute to increasing escarpment slope over time and preservation of a high elevation escarpment using tropical rainfall measuring mission trmm precipitation radar data we establish precipitation elevation relationships at high elevation passive margins across the tropics and document the occurrence of patterns similar to those included in our model despite uncertainty as to the robustness of precipitation patterns and the simplified tectonic and geomorphic framework of our model we conclude that spatial variability in precipitation is a potentially significant contributor to passive margin escarpment evolution

distributed deformation across the rio grande rift great plains and colorado plateau we use continuous measurements of gps sites from across the rio grande rift great plains and colorado plateau to estimate present day surface velocities and strain rates velocity gradients from five east west profiles suggest an average of 1 2 nanostrains yr east west extensional strain rate across these three physiographic provinces the extensional deformation is not concentrated in a narrow zone centered on the rio grande rift but rather is distributed broadly from the western edge of the colorado plateau well into the western great plains this unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain rate deformation within continental interiors is accommodated

transverse ageostrophic circulations associated with elevated mixed layers abstract an elevated mixed layer is a principal component of the conceptual model recently proposed by carlson and others to explain the evolution of a severe storm environment over the southern plains of the united states during springtime elevated mixed layers are most likely to be found downwind of strongly heated arid land areas often plateaus which favor the growth of a deep mixing layer with high potential temperature the lower and lateral boundaries of elevated mixed layers are distinctly defined respectively by a statically stable layer referred to as a lid inversion and a midlevel front these boundaries mark the zonesof transition between the airstream defining the elevated mixed layer and other airstreams of differing geographical origin and consequently thermodynamic properties in this study the sawyer eliassen secondary circulation equation is used to diagnose the transverse ageostrophic circulations that are associated with the dynamical forcing implied by the above conceptual mod

serial upstream propagating mesoscale convective system events over southeastern south america abstract serial mesoscale convective system mcs events with lifetimes over 18 h and up to nearly 70 h are routinely observed over southeastern south america from infrared satellite imagery during the spring and summer these events begin over the southern la plata river basin with individual convective systems generally moving eastward with the cloud layer mean wind however an important and common subset of these serial mcs events shows individual mcss moving to the east or southeast yet the region of convective development as a whole shifts upstream to the north or northwest analyses of the composite mean environments from 10 of these upstream propagating serial mcs events using ncep ncar reanalysis data events indicates that the synoptic conditions resemble those found in mesoscale convective complex environments over the united states the serial mcs events form within an environment of strong low level warm advection and strong moisture advection between the surface and 700 hpa from the amazon

deciphering the rate of mountain growth during topographic presteady state an example from the ne margin of the tibetan plateau 1 we use field investigations topographic profiles and 10be exposure dating to constrain the growth rate of the yumu shan a thrust bounded mountain range at the northeastern margin of tibet wind gaps fault scarps at the mountain front and hanging paleocatchments indicate that the yumu shan is growing both laterally and vertically scarp profiles and 10be ages yield rock uplift rates of 500 and 800 mm ka 1 for the eastern and central parts of the range respectively the fact that the rock uplift rate in the high center of the yumu shan exceeds catchment wide 10be denudation rates which range from 180 to 280 mm ka 1 indicates that the mountain range continues to rise and has not yet reached a topographic steady state combining the total structural relief of the range with the rates of rock uplift and denudation suggests that the growth of the yumu shan started 3 7 0 9 ma ago

evaluating escarpment evolution and bedrock erosion rates in the western guadalupe mountains west texas and new mexico abstract the guadalupe mountains tower above surrounding plains and basins in western texas and southeast new mexico although this mountain range is extensively studied for the permian stratigraphy related to the petroleum reservoirs in the adjacent delaware basin little is known about the geomorphic history the western escarpment in particular rises abruptly from the salt basin to the west of the range and guadalupe peak rises 1600 m above the surrounding landscape this study investigates how geologic processes create and maintain the steep profile of the escarpment on the western boundary of the guadalupe mountain range in situ produced cosmogenic nuclides were used to quantify erosion rates across the steep western front of the range to assess how tectonic climatic and lithologic forces influence the escarpment profile erosion rates range from 1 to 73 m myr faster erosion rates in the southern area of the escarpment coincide with more frequently mapped faults higher elevations at the top of the rim and locations with colder temperatures and higher precipitation topographic profiles across the western escarpment surfaces indicate greater retreat from the original positions of fault surfaces at the north end of the mountain range faster erosion rates in the mountain range are linked to greater uplift and more recent activity in the southern end of the guadalupe mountains

quantifying passive margin denudation and landscape development using a combined fission track thermochronology and cosmogenic isotope analysis approach abstract a new approach to empirically constraining numerical models of denudation and landscape development across passive continental margins is presented involving a combination of apatite fission track thermochronology and cosmogenic isotope analysis this integrated strategy is applied to the namibian sector of the south west african margin where the conventional landscape evolution model of inland retreat of an escarpment initiated at the coast at break up implies a mean rate of retreat of 1 km ma 1 the thermochronological and cosmogenic isotope data are incompatible with a simple escarpment retreat model since they show that denudation rates oceanward of the present escarpment position have been low 1 since the end of the eocene and the estimated rate of escarpment retreat has been only 10 m ma 1 this low rate of retreat is however consistent with numerical landscape evolution models where the escarpment is pinned at an inland drainage divide

the accumulation of cosmogenic chlorine 36 in rocks a method for surface exposure dating because of the reaction of cosmic rays with elements in minerals chlorine 36 accumulates in rocks exposed at the earth s surface this suggests that the ratio of chlorine 36 to stable chloride can be used as a geochronometer reasonable agreement has been obtained between measurements of chlorine 36 in volcanic rocks of known age and calculations of chlorine 36 production based on rock composition and cosmicray flux the buildup of chlorine 36 should be a useful means for measuring the surface exposure time of young volcanic rocks and geomorphic features in the age range of 10 3 to 10 6 years

influence of rock strength properties on escarpment retreat across passive continental margins we examine the influence of bedrock strength properties imparted by jointing and fracturing on the retreat of erosional escarpments across passive continental margins two types of escarpment are identified and focus is placed on the gorge head knickpoint form of escarpment where upland streams drain across the escarpment along the macleay river system of eastern australia major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth despite differences in bedrock lithology and order of magnitude differences in drainage areas upstream of the gorge heads implying an average escarpment retreat rate of 2 km m y since the margin formed 85 m y ago field observations from apsley river gorge a macleay river tributary indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls where dominant jointing dips into hillslopes failure occurs by block and column toppling conversely where jointing dips out of hillslopes mass wasting occurs by joint parallel rock sliding allowing slopes to remain steep as material is shed from the gorge sidewalls the toppling slopes develop lower overall gradients the gorge becomes asymmetric in cross section and the channel remains nearer to the steeper sidewall affected by joint parallel sliding coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint gorge head and to facilitate mass wasting from the gorge sidewalls peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus to incise the stream channel and to erode the toes of the sidewalls if these conditions on fluvial processes are met then hillslope processes including rockslope failure mechanisms are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin

mesoscale convective systems over southeastern south america and their relationship with the south american low level jet abstract prior studies have shown that the low level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems mcss over the great plains of the united states the south american low level jet sallj over southeastern south america sesa has an analogous role advecting heat and moisture from the amazon basin southward into the central plains of southeastern south america generating ideal environmental conditions for convection initiation and growth into mcss this research has two purposes one is to describe the characteristics of a 3 yr mcs sample in south america south of the equator and its related geographical distribution of convection frequency the other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large mcss specifically those that mature under sallj conditions high horizontal and temporal resolution satellite images are used to detect mcss in the area for the

life cycle variations of mesoscale convective systems over the americas abstract using goes 7 isccp b3 satellite data for 1987 88 the authors studied the evolution of the morphological and radiative properties of clouds over the life cycles of deep convective systems cs over the americas at both tropical and middle latitudes a deep convective cloud system is identified by adjacent satellite image pixels with infrared brightness temperatures tir 245 k 28 c that at some time contain embedded convective clusters that are defined by pixel values of tir 218 k 55 c the first part of the analysis computes parameters for each convective system that describe the system areal size number of convective clusters fractional convective area average and maximum size of the convective clusters shape eccentricity and orientation mean tir variance of tir tir gradient and the mean variance and gradient of collocated visible reflectances when available the second part of the analysis searches a 5 5 region centered on each convective system but in the subsequent

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

decadal to millennial scale solar forcing of last glacial maximum climate in the estancia basin of central new mexico abstract lacustrine sediments from the estancia basin of central new mexico reveal decadal to millennial oscillations in the volume of lake estancia during last glacial maximum lgm time lgm sediments consist of authigenic carbonates detrital clastics delivered to the lake in stream flow pulses and evaporites that precipitated in mudflats exposed during lake lowstands and were subsequently blown into the lake variations in sediment mineralogy thus reflect changes in hydrologic balance and were quantified using rietveld analysis of x ray diffraction traces radiocarbon dates on ostracode valve calcite allowed the construction of mineralogical time series for the interval 23 600 to 18 300 ka which were subjected to spectral analysis using redfit schulz and mudelsee 2002 dominant periods of 900 375 and 265 yr are similar to cycles in holocene 14 c production reported for a variety of tree ring records suggesting that the lake estancia sediments record variations in solar activity during lgm time a prominent spectral peak with a period of 88 yr appears to reflect the solar gleissberg cycle and may help along with the 265 yr cycle to explain an ongoing mystery about how lake estancia was able to undergo abrupt expansions without overflowing its drainage basin

a diagnostic study of the isentropic zonally averaged mass circulation during the first garp global experiment abstract the mass circulation of the zonally averaged global circulation is investigated in isobaric and isentropic coordinates through the use of a fgge level iiia data set the isobaric mass circulation during the first garp global experiment is consistent with previous results i e direct hadley cells in low latitudes and indirect ferrel cells in midlatitudes of both hemispheres within the isentropic framework the combination of a geostrophic mean mode of mass transport within the baroclinic wave structure of extratropical latitudes and an ageostrophic mean mode in tropical latitudes yields an isentropic mass circulation spanning the hemisphere the asymmetric structure of midlatitude baroclinic waves provides the degree of freedom for a geostrophic mode of mass transport that is fundamental to the direct hadley type circulation in isentropic coordinates this direct hadley type circulation is explicitly linked to heat sources and sinks on a planetary scale the diabatic heating estimated from th