aftsolve a program for multi kinetic modeling of apatite fission track data aftsolve is a computer program for deriving thermal history information from apatite fission track data it implements a new fission track annealing model that takes into account the known kinetic variability among different apatite species to fully utilize this model a fission track worker must obtain data that can be used to infer the kinetic characteristics of each apatite grain from which a measurement was taken such data can consist of etch figure lengths or chemical composition the benefit of this overall approach is that it allows useful information to be derived from previously unusable analyses extends the practical range of geological temperatures constrained by fission track analyses and increases overall confidence in model predictions aftsolve also incorporates the effects of fission track orientation relative to the apatite crystallographic c axis variation in initial track length and the biasing effect of 2 5 2 cf irradiation for enhancing confined horizontal track length detection aftsolve is written for windows operating systems and has a graphical interface that allows interactive input of thermal histories and real time generation of estimates for fission track length distributions and ages for up to six simultaneously modeled kinetic populations it also includes procedures for estimating the range of time temperature histories that are statistically consistent with a data set and constraints entered by the user

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

normal fault thermal regimes and the interpretation of low temperature thermochronometers abstract exhumation rates inferred from thermochronometers are dependent on spatial and temporal variations in temperature in active extensional mountain belts the temperature field is complicated by tectonic and surface processes including 1 lateral heat flow across large range bounding normal faults due to the juxtaposition of a cool hanging wall and a relatively warmer footwall 2 uplift and erosion of the footwall 3 sedimentation and burial of the hanging wall 4 lateral heat refraction around low thermal conductivity sediments deposited in the hanging wall basin and 5 3d temperature variations due to high relief topography developed on the footwall we explore these mechanisms through a series of 2d conductive thermal models designed to investigate the effect of tectonics and topography on apatite fission track aft and u th he thermochronometer data models were tuned to the geometry and kinematics of the wasatch mountains utah usa the principal parameters in our model are exhumation and burial rates ranging from 0 2 to 5 7 mm per year at the range front and decreasing with distance from the fault surface morphology taken from usgs digital elevation models and basin geometries inferred from seismic and gravity surveys predicted aft and u th he ages were generated using cooling rate dependent annealing and diffusion kinetic models results indicate after 10 million years of exhumation footwall u th he and aft closure temperature isotherms within 10 km of the fault are advected upward 500 and 1000 m respectively from their initial position the upward advection of isotherms and the 2d nature of the thermal regime can result in erroneous exhumation rates calculated from plots of sample elevation versus age using 1d thermal models for simulations with a uniform vertical uplift rate and canyon and ridge topography 1d and 2d exhumation rate differences were 20 70 for u th he and 10 for aft data samples collected perpendicular to fault strike up the range front are sensitive to the exhumation rate and footwall tilt differences between 1d and 2d range front exhumation rates were 10 95 for u th he and 10 40 for aft data furthermore 1d thermal models are incapable of deciphering footwall tilt at simulated exhumation rates of 3 4 and 5 mm per year predicted u th he and aft ages form closely spaced near vertical lines on a plot of elevation versus sample age therefore suggesting a decreased sensitivity of data to topography and surface processes at high exhumation rates despite large errors in previously collected aft data from the wasatch mountains our model suggests a revised fault adjacent exhumation rate of 0 5 0 6 mm per year compared to previous estimates of 0 7 mm per year based on 1d thermal models

preservation of miocene glacier ice in east antarctica antarctic climate during the pliocene has been the subject of considerable debate one view holds that during part of the pliocene east antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a 4 an alternative viewpoint argues for the development of a stable polar ice sheet by the middle miocene which has persisted since then5a 10 here we report the discovery of buried glacier ice in beacon valley east antarctica which appears to have survived for at least 8 1 million years we have dated the ice by 40ar 39ar analysis of volcanic ash in the thin overlying glacial till which we argue has undergone little if any reworking isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet we suggest that stable polar conditions must have persisted in this region for at least 8 1 million years for this ice to have avoided sublimation

cerro toledo rhyolite jemez volcanic field new mexico 40ar 39ar geochronology of eruptions between two caldera forming events the cerro toledo rhyolite comprises a group of domes and tephra which were erupted during the interval between two caldera forming ignimbrites the tshirege member and otowi member of the bandelier tuff in the jemez volcanic field new mexico to provide a chronologic framework for geochemical and isotopic studies on these rhyolites which record the evolution of the bandelier magma system during this interval a 40 ar 39 ar geochronology study was undertaken pumice from major pyroclastic fall deposits within the rhyolite tephra and samples from the rhyolite domes were dated as well as the stratigraphically bracketing bandelier tuff analyzed crystal populations range from being fairly homogeneous juvenile material to very heterogeneous mixed juvenile and xenocrystic assemblages in most cases dominant groups of juvenile sanidine crystals define 40 ar 39 ar ages which agree with stratigraphic constraints plagioclase analyses are distinctly more scattered and do not typically define reasonable ages the 40 ar 39 ar ages for the two members of the bandelier tuff yield an interval of 380 20 k y between these caldera forming eruptions during this interval nine major pyroclastic pumice units were deposited in the sections studied for which six yield isochron ages one a weighted mean age one a maximum age and one no reliable age due to lack of sanidine 40 ar 39 ar dates on pumice fall units within the cerro toledo rhyolite tephra indicate that eruptive activity occurred at 1 59 1 54 1 48 1 37 and 1 22 ma 40 ar 39 ar dating of cerro toledo rhyolite domes indicates these were erupted within the caldera at 1 54 1 45 1 38 1 34 and 1 27 ma the dates obtained indicate that eruptive activity occurred throughout the 380 k y interval between the two members of the bandelier tuff but suggest that eruptions producing both tephra and domes occurred during discrete intervals at ca 1 54 1 48 and 1 38 1 34 ma the interval from 1 34 to 1 38 ma was particularly active 7 of 18 units dated are these ages

long term rates of denudation in t he dry valleys transantarctic mountains southern victoria land antarctica bases on in situ produced cosmogenic 21ne abstract concentrations of cosmogenic 21 ne 21 ne c measured in quartz have been used to estimate long term rates of denudation for contrasting landscape components in the dry valleys area of the transantarctic mountains southern victoria land antarctica samples of beacon supergroup sandstones and granitic basement were collected from two contrasting landscape elements low relief high elevation surfaces and rectilinear slopes to assess variations in rates of denudation with topographic position the sample sites for rectilinear slopes were selected because of proximity to 40 ar 39 ar dated lavas and ash avalanche deposits all 21 ne 20 ne ratios are significantly greater than the atmospheric value and concentrations of 21 ne c were calculated from the measured 21 ne values assuming an atmospheric composition for the trapped component apparent exposure ages calculated from the concentrations of 21 ne c assuming no denudation since exposure range from 3 78 4 66 ma for samples from the high elevation plateau surface and 0 61 2 48 ma for samples from the rectilinear slopes exposure ages for 3 he c were 2 to 42 times lower than those derived from the abundances of 21 ne c because of preferential diffusive loss of 3 he c from the quartz lattice concentrations of 3 he c were therefore not used in the calculation of rates of denudation we interpret the abundances of 21 ne c as reflecting variations in the rates of denudation rather than exposure age in view of independent evidence for prolonged exposure 15 ma of bedrock surfaces at the sample sites calculated maximum rates of denudation range from 0 26 1 02 m ma 1 for the rectilinear slopes down to only 0 133 0 164 m ma 1 for the high elevation surface sites these rates are comparable to other estimates of denudation rates for the dry valleys derived from analyses of cosmogenic isotopes but are around two orders of magnitude lower than the long term mean rate over the past 50 ma estimated from fission track thermochronology combined with the preservation of volcanic deposits dating back to the mid miocene these 21 ne c data demonstrate that only minimal modification of the landscape has occurred in the dry valleys over at least the past 15 ma this significant conclusion supports the view that the east antarctic ice sheet has been essentially stable over this period rather than experiencing major fluctuations as late as the pliocene as has previously been suggested

solving the production diffusion equation for finite diffusion domains of various shapes part 2 application to cases with alpha ejection and non homogeneous distribution of the source abstract in this paper numerical methods derived in part i chem geol 2002 are expanded to accommodate the effects of emission and zonation of u and th as relevant for u th he thermochronology expressions for the required coefficients are derived for spheres finite and infinite cylinders and rectangular blocks for parent nuclide distributions which are either uniform or have stepwise zoning we show that the effects of emission and zonation on the he retention in a nonspherical crystal of modest aspect ratio can be approximated by a sphere of identical surface to volume ratio s v if the zonation is transposed on to the sphere the currently usual correction method is to perform first a diffusive calculation without considering emission and zonation and to multiply afterward with a correction factor that has been derived originally for nondiffusive calculations our results demonstrate that more accurate calculations yield different results and that the differences are often too large to be ignored in practice especially for thermal histories that include a protracted residence in the partial retention zone the difference between the usual correction and our correction can be as high as 20 for homogeneous sources for zoned crystals the difference can be even larger

quantifying the diffusion kinetics and spatial distributions of radiogenic 4he in minerals containing proton induced 3he apatite titanite and olivine samples were bombarded with a 150 mev proton beam to produce 10 8 atoms mg of spallation 3he high precision stepped heating experiments were then performed in which the artificial 3he and for apatite and titanite the natural radiogenic 4he were measured to characterize the diffusive behavior of each isotope helium 3 diffusion coefficients are in excellent agreement with concurrently and or previously determined he diffusion coefficients for each mineral our results indicate that proton induced 3he is uniformly distributed and that radiation damage associated with a proton fluence of 5 x 10 14 protons cm 2 does not cause noticeable changes in 4he diffusion behavior in at least apatite and titanite proton induced 3he can therefore be used to establish he diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural 4he distribution is inhomogeneous in addition step heating 4he 3he analysis of a mineral with a uniform synthetic 3he concentration provides a means by which a natural 4he distribution can be determined

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

thermochronology of lower cretaceous source rocks in the idaho wyoming thrust belt lower cretaceous organic rich source rocks that are thermally mature to postmature crop out on the absaroka darby and prospect plates in linear belts that run parallel to the trace of the thrusts in the idaho wyoming portion of the idaho wyoming utah thrust belt although the common assumption is that burial by thrust plates and the synorogenic sediments derived from them have been responsible for thermal maturation of the organic rich strata commercial amounts of hydrocarbons have not been found in structural traps in this portion of the thrust belt in a companion paper burtner and nigrini demonstrated that gravity driven fluid flow in the idaho wyoming portion of the thrust belt was responsible for moving large amounts of heat from the depths of the early cretaceous foreland basin eastward toward the stable platform in this paper we demonstrate through the application of organic maturation indicators and a new refinement of the apatite fission track technique that this process heated lower cretaceous organic rich source rocks to temperatures sufficient to generate hydrocarbons hydrocarbon generation and migration occurred prior to the development of the thrusts that are often assumed to have played a major role in the generation and entrapment of hydrocarbons in this portion of the thrust belt

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

influence of wildfires on apatite and zircon u th he ages low closure temperatures of the apatite and zircon u th he thermochronometers allow valuable constraints on timing and rates of bedrock exhumation through shallow crustal depths but raise the possibility that shallow level processes other than exhumation related cooling may also influence he ages a simple he diffusion model predicts that wildfires can completely or partially reset apatite he ages as much as 3 cm below rock surfaces and partially reset zircon he ages in the outermost 1 cm measured he ages in bedrock and sediments from the washington cascades that were exposed to extensive wildfires in 2001 show strong agreement with these model predictions apatite he ages decrease from a regionally consistent age of 19 5 1 2 ma at a distance 3 cm from the rock surface to as low as 1 9 ma in the outermost 1 cm whereas zircon he ages decrease from 65 to 55 ma over the same distance thin 3 cm flakes shed from a nearby boulder during or after the most recent fire have apatite he ages ranging from 9 7 0 6 to 17 2 1 0 ma the partial resetting profiles are best explained by model thermal histories involving at least one short duration 5 10 min high temperature 575 650 c event and at least one longer 30 40 min lower temperature 350 450 c event age depth profile data may be useful in determining wildfire intensities or locations and also suggest that he ages of detrital apatites from some environments may be subject to bias from the thermal effects of wildfires

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

backstacking apatite fission track stratigraphy a method for resolving the erosional and isostatic rebound components of tectonic uplift histories measuring tectonic uplift of the mean surface of earth using thermochronological data to document uplift histories is often plagued by confusion arising from the independent nature of the thermal and sea level frames of reference the technique of backstacking apatite fission track stratigraphy differentiates between vertical movement of rocks toward earth9s surface due to erosional or tectonic denudation and tectonic uplift of earth9s surface apatite fission track analysis can also be used to identify episodes of erosion where no other evidence for the erosional event exists in addition to providing information about the timing magnitude and geographic distribution of erosion

polar desert chronologies through quantitative measurements of salt accumulation we measured salt concentration and speciation in the top horizons of moraine sediments from the transantarctic mountains antarctica and compared the salt data to cosmogenic nuclide exposure ages on the same moraine because the salts are primarily of atmospheric origin and their delivery to the sediment is constant over relevant time scales a linear rate of accumulation is expected when salts are measured in a consistent grain size fraction and at a consistent position within the soil column a linear correlation between salt concentration and exposure age is evident this correlation is strongest for boron containing salts r2 0 99 but is also strong r2 0 9 for most other water extracted salt species the relative mobility of salts in the soil column does not correspond to species solubility borate is highly soluble instead the highly consistent behavior of boron within the soil column is best explained by the extremely low vapor pressure of boric acid at cold temperatures the environment is sufficiently dry that mobility of salt species within the soil column is controlled by vapor phase effects in other cold desert settings topsoil salts specifically boron may be employed as a proxy for relative sediment exposure age

post orogenic evolution of the dabie shan eastern china from u th he and fission track thermochronology the dabie shan of eastern china is a 200 kilometers wide mountain range with nearly 2 kilometers of relief and is an archetype of deep ultrahigh pressure metamorphic rock exhumation despite its regional and petrologic importance little is known about the low temperature and post orogenic evolution of the dabie shan here we present apatite and zircon u th he ahe and zhe respectively and apatite fission track aft cooling ages from the dabie shan that constrain the patterns and history of exhumation over the last 115 myr on the scale of the whole orogen zhe and ahe ages are inversely correlated with mean elevation and are systematically younger in the core of the range these cooling ages were converted to exhumation rates assuming steady state erosion and accounting for topographic effects these results indicate that since the eocene flanks of the range have eroded at rates as low as 0 02 km myr while the range core has eroded at about 0 06 km myr even in the core of the range these recent exhumation rates are at least 10 to 20 times slower than those estimated for the initial stages of exhumation in the triassic jurassic in a 1 4 kilometer vertical transect in the core of the range all ages are positively correlated with elevation with zhe ages increasing from 76 to 112 ma aft from 44 to 70 ma and ahe from 24 to 43 ma we present a simple model for topographic correction of thermochronometric ages in vertical transects using the admittance ratio ratio of isotherm relief to topographic relief applied to the ahe age elevation relationship this yields tertiary exhumation rates of 0 05 to 0 07 km myr in the core of the dabie shan in good agreement with regional exhumation rate patterns finally age elevation relationships for all three chronometers in the vertical transect are consistent with a constant exhumation rate of 0 06 0 01 km myr since the creta ceous with a possible modest increase in exhumation rates as high as 0 2 km myr between 80 to 40 ma these data show no evidence for significant variations in exhumation rates over the last 115 myr as might be expected for decay of old topography or tectonic reactivation of old structures

minimal pliocene pleistocene uplift of the dry valleys sector of the transantarctic mountains a key parameter in ice sheet reconstructions we propose that there has been little or no pliocene pleistocene uplift of the dry valleys sector of the transantarctic mountains on the basis of isotopic dating and mapping of in situ cinder cone deposits on the walls of taylor valley twenty seven 40ar 39ar incremental heating analyses on whole rock samples from subaerially erupted olivine basanite volcanic outcrops of known elevations define 14 eruptions ranging in age from 3 89 to 1 50 ma because taylor valley opens directly onto the ross sea these results show that any surface uplift during the past 2 57 m y was 300 m our conclusion of minimal uplift contradicts previous models of dramatic uplift up to 3000 m since 3 ma and associated hypotheses that uplift caused climatic cooling and growth of a polar east antarctic ice sheet

mesozoic and cenozoic exhumation history and magmatic hydrothermal events of the central tianshan mt range nw china evidence from u th he and 40ar 39ar dating abstract the thermal history of rocks in the awulale area central chinese tianshan is significant to understanding the formation and tectonothermal evolution of the tianshan orogen k feldspar 40ar 39ar and apatite u th he age data reveal at least four mesozoic cenozoic tectonothermal events in the central tianshan which occurred during the middle late triassic 244 4 216 5 ma early jurassic ca 180 3 ma early cretaceous 99 1 98 1 ma and early miocene 18 6 13 7 ma this thermal history is consistent with that throughout the tianshan orogenic belt and surrounding blocks modeling of the thermal history shows that intrusive rocks in the central tianshan have experienced four stages of cooling in the mesozoic cenozoic stage i occurred from ca 310 to 230 ma after magma emplacement and resulted in cooling to the 40ar 39ar closure temperature of k feldspar 200 180 c during this time tectonism in the central tianshan was not intense and the rapid cooling was due mainly to thermal exchange with a maximum cooling rate of 7 1 c myr stage ii occurred from ca 230 to 180 ma collision between the qiangtang microcontinent and the eurasia continental margin resulted in active tectonism in the central tianshan and cooling was caused mainly by tectonic uplift with the maximum uplift of intrusive rocks exceeding 9 km stage iii occurred from ca 180 to 45 ma when regional tectonism was weak and cooling was gradual during this stage the apatites in the syenite porphyry remained in the he partial retention zone for a long time this indicates that during this stage the central tianshan was only weakly influenced by the far field collisions of lassa and karakoram pamir with the southern margin of eurasia stage iv occurred from 45 ma to the present day and was related to the far field influence of the middle late himalayan orogeny during which tectonism in the central tianshan became more intense in this final stage many types of intrusive rock were exhumed to the surface with the maximum uplift exceeding 2 km furthermore the cooling history and regional structural trends indicate that since the mesozoic under the influence of tectonic compression there has been differential uplift on the northern and southern sides of the east awulale area the cooling and exhumation rates of intrusive rocks are much higher on the northern side of the area than the southern side this was related to reverse movement on the yuximolegai fault as exhumation rate of the hanging wall northern region was higher than that of the footwall southern region under n s compression these findings combined with the inversion model results indicate that this differential uplift occurred mainly from the late triassic to early jurassic

uplift and denudation of the central alaska range a case study in the use of apatite fission track thermochronology to determine absolute uplift parameters apatite fission track thermochronology aftt on granitic samples collected in the central alaska range in conjunction with geologic constraints from basins to the north nenana basin and south cook inlet of the range is used to constrain the timing amount rate and pattern of surface uplift rock uplift and denudation since the late miocene the conversion from a thermal frame of reference apatite fission track data to an absolute frame of reference with respect to mean sea level which requires constraining the paleoland surface elevation the paleomean annual temperature and the paleogeothermal gradient is evaluated and shown to be viable in the context of an exhumed apatite partial annealing zone paz apatite ages at denali mount mckinley range from 16 ma near the summit 6 km elevation to 4 ma at 2 km elevation a distinctive break in slope in the apatite age profile at an elevation of 4 5 km also marked by a change in confined track length distributions marks the base of an exhumed apatite paz rock uplift and denudation are greatest at denali decreasing southward away from the mckinley strand of the denali fault system as shown by progressively older apatite ages 7 35 ma from a suite of samples along the kahiltna glacier a correlative decrease in topography occurs southward from the fault the central alaska range lies within an arc defined by the denali fault with the highest peaks including denali concentrated at the arc apex patterns of rock uplift and denudation within the central alaska range mimic topography between early and late miocene and possibly earlier the central alaska range was most likely an area of relative tectonic and thermal stability rock uplift denudation and mean surface uplift of the denali region began by the late miocene 5 6 ma being 8 5 km 5 7 km and 2 8 km respectively at average rates of 1 5 km m y 1 km m y and 0 5 km m y the amount of rock uplift denudation and surface uplift decreases to 3 km 2 km and 1 km at little switzerland some 45 km south of the denali fault we conclude that the topographic and rock uplift patterns of the central alaska range the shape and proximity of the mckinley strand of the denali fault to these patterns the timing of the onset of rock uplift and denudation at 5 6 ma and a significant change in relative plate motion between north america and the pacific plates circa 5 6 ma are all inherently related

4he 3he thermochronometry using classical diffusion theory we present a mathematical technique for the determination of 4he concentration profiles in minerals this approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting u th he ages for partial diffusive loss the calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3he obtained by proton irradiation shuster et al earth planet sci lett 217 2004 19 32 in minerals devoid of natural helium this isotope allows measurement of he diffusion coefficients in minerals with measurable radiogenic he it permits determination of 4he profiles arising during ingrowth and diffusion in nature the 4he profile can be extracted from stepwise degassing experiments in which the 4he 3he ratio is measured the evolution of the 4he 3he ratio as a function of cumulative 3he released can be compared with forward models to constrain the shape of the profile alternatively we present a linear inversion that can be used to directly solve for the unknown 4he distribution the inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution using either approach we show that stepwise degassing data can yield robust and high resolution information on the 4he profile profiles of radiogenic he are a sensitive function of the time temperate t t path that a cooling sample experienced thus by step heating a proton irradiated sample it is possible to restrict the sample s acceptable t t paths the sensitivity of this approach was explored by forward modeling 4he profiles resulting from a range of realistic t t paths using apatite as an example results indicate that 4he profiles provide rich information on t t paths especially when the profiles are coupled with u th he cooling ages on the same sample samples that experienced only moderate diffusive loss have 4he concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain such profiles can be identified by nearly invariant 4he 3he ratios after the first few to few tens of percent of 3he have been extracted by step heating we show how such data can be used to correct u th he ages for partial diffusive loss

exhumation of the central wasatch mountains utah 2 thermokinematic model of exhumation erosion and thermochronometer interpretation 1 the wasatch fault is a 370 km long normal fault in utah that marks the boundary between the stable colorado plateau to the east and the extending basin and range to the west understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones e g east africa rift transantarctic mountains we explore the thermokinematics of footwall exhumation and erosion in the cottonwood intrusive belt of the central wasatch mountains emphasis is placed on using low temperature thermochronometers to quantify 1 the spatial and temporal variability of exhumation and erosion rates 2 the geometry of footwall tilt 3 the fault dip angle and 4 the magnitude and duration of exhumation these processes are investigated using two dimensional 2 d thermal kinematic models coupled with cooling rate dependent kinetic models which predict exhumed apatite fission track aft and u th he ages the range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0 2 and 2 0 mm yr 1 footwall tilt hinge positions between 15 and 40 km distance from the fault a single planar normal fault with dip angles of 45 and 60 and exhumation magnitudes of up to 15 km at the fault simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat producing layers erosion maintains a constant topographic profile the kinematic and exhumation history of the wasatch mountains is investigated by comparing model predicted thermochronometer ages to observed aft zft and u th he ages predicted and observed ages are compared using a reduced chi square analysis to determine a best fit kinematic model for the wasatch mountains the preferred model includes exhumation occurring on either a 45 or 60 dipping fault a footwall hinge located a minimum of 20 25 km from the fault and a step decrease deceleration in the footwall exhumation rate at the fault from 1 2 to 0 8 mm yr 1 at around 5 ma the model also suggests an exhumation duration of 12 myr 2 myr index terms 1035 geochemistry geochronology 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8109 tectonophysics continental tectonics extensional 0905 8130 tectonophysics heat generation and transport keywords exhumation erosion normal faults numerical modeling heat flow thermochronometers

influence of crystal size on apatite u th he thermochronology an example from the bighorn mountains wyoming near surface tectonic and geomorphic processes involve cooling of rocks through low temperatures 50 200 c because rates of helium diffusion in apatite titanite and zircon are sensitive to temperature variations in this range uranium thorium helium thermochronometry u th he dating is well suited to establishing the timing and rates of these processes in the geologic record however because fractional loss of he is controlled by crystal size such that larger crystals retain a larger fraction of radiogenic he u th he ages must vary not only with thermal history but also with crystal size here we present crystal size correlated he ages from co existing apatites from the bighorn mountains wyoming that range from 100 to 350 ma these correlations are a sensitive indicator of the rock s thermal history in a temperature range below the system s nominal closure temperature t c 70 c for apatite and are consistent with a thermal history involving residence in the upper 2 3 km of crust since the precambrian with maximum temperatures of 65 80 c just prior to laramide orogenic exhumation the influence of crystal size on he ages will be most apparent in rocks where temperatures have been in the range of partial he retention for long periods of time 30 70 c for 10 7 years in such cases accurate interpretation of u th he ages must incorporate the effect of crystal size and this method may provide insights to thermal histories of rocks in previously inaccessible low temperature ranges

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

an evaluation of low temperature apatite uth he thermochronometry abstract three studies have been conducted to test the usefulness of apatite u th he thermochronometry the first study compared u th he and fission track ages from quickly cooled plutonic rocks having concordant zircon and apatite fission track ages several samples analyzed show good helium retention the second experiment assessed the effect of compositional variations in apatite on closure temperature calculated closure temperatures agree with existing data and show no statistical dependence on composition arrhenius plots for all of the samples show linear trends at low temperatures representing volume diffusion that roll over to more shallowly sloping non linear paths at higher temperatures the roll over temperature appears to increase with increasing cl content in the apatite the third test focused on fluorapatites from the ktb borehole in germany to compare laboratory estimates with diffusion behavior in a natural setting the ktb apatites show a fair amount of scatter and record u th he ages that are younger than or equal to apatite fission track ages indicating a closure temperature less than or equal to that for the apatite fission track method closure temperatures predicted from the scattered data range from 80 120 c 1 c ma cooling rate or 95 135 c 10 c ma cooling rate the low end of this range agrees with our laboratory diffusion estimates for fluorapatite

apatite fission track and u th he data from fennoscandia an example of underestimation of fission track annealing in apatite abstract apatite fission track aft data from cratons have commonly been interpreted in terms of major episodes of sedimentary burial and subsequent denudation often in the absence of any remnant sedimentary rocks or other supporting geological evidence reconciling these interpretations with geologically documented denudation events or outcropping regional stratigraphy can be problematic also u th he ages from cratonic igneous rocks that are older than paired aft data are commonly discarded as too old even when the former are repeatable at the 2 confidence level for example aft data from central fennoscandia have been interpreted to reflect first burial by a caledonian foreland basin several kilometres in thickness and subsequently its denudation however throughout fennoscandia there is no direct evidence that such a deep extensive foreland basin ever existed and indeed some geological evidence points to the contrary additionally a significant number of u th he ages from fennoscandia are older than their aft pairs but have been interpreted by their authors as too old relative to the aft ages and not geologically meaningful despite repeatable results here we propose a new interpretation of aft data from fennoscandia that closely reflects the observed data yet is compatible with the generally accepted geological history discarding the hypothesis of a thick caledonian foreland basin we suggest that 1 aft ages in finland are too young rather than that the u th he ages are too old 2 repeatable sequences of u th he ages are more likely to record the true denudation history of cratons 3 the importance of radiation enhanced annealing at low temperatures has been overlooked and 4 fennoscandian aft data may be divided into an eastern domain strongly affected by radiation enhanced annealing where current aft annealing models are invalid and a western domain much less affected by radiation enhanced annealing where current modelling methods may appropriately be used to extract thermal histories in principle radiation enhanced annealing is also pertinent to the interpretation of young aft ages from apatites rich in emitter actinides the construction of a properly calibrated annealing model that fully incorporates the effects of radiation enhanced annealing is fundamentally important

estimating exhumation rate and relief evolution by spectral analysis of age elevation datasets extracting independent information on mean exhumation rate and the rate of surface relief change from thermochronometric datasets is essential to improve our understanding of the complex coupling between tectonics and surface erosion i e the time scale over which landforms react to changes in uplift rate and or climate a new method based on spectral analysis of age elevation data collected along one dimensional profiles is presented that provides independent estimates of the mean exhumation rate and the relative change in surface relief the results are shown to be independent of the assumed geothermal gradient the method is applied to an existing age dataset from the sierra nevada california and provides constraints on the evolution of the present day relief the spectral analysis demonstrates how current sampling strategies should be modified to optimize the tectonic and geomorphic information that can be retrieved from a thermochronometric dataset

magnetobiostratigraphic chronology of the eocene oligocene transition in the ciros 1 core victoria land margin antarctica implications for antarctic glacial history in 1986 cores were obtained to a depth of 702 m with 98 recovery from the ciros 1 drill hole beneath the ross sea on the victoria land margin glaciogene sediments identified near the base of the hole mark the earliest known record of antarctic glaciation initial biostratigraphic analysis indicated that the lower 336 m of the core is early oligocene in age and that the upper 366 m is of late oligocene early miocene age recently the chronology of the ciros 1 core has been questioned we developed a magnetostratigraphy for the lower 400 m of the ciros 1 core to clarify the chronology our magnetobiostratigraphic results indicate that the base of the ciros 1 core is early late eocene in age corresponding to chron c16r ca 36 5 ma we identify the eocene oligocene boundary at about 410 420 m within a 20 m thick poorly stratified bioturbated sandy mudstone this makes the ciros 1 core the highest latitude site 77 1 s from which this datum event has been recognized at 366 m a 4 m y hiatus which lies immediately beneath fluvial sediments accounts for most of chrons c11 and c12 we recognize three major climatic episodes in the ciros 1 core 1 the late eocene 34 5 36 5 ma 430 702 m when relatively warm conditions dominated and there were high sedimentation rates and some glacial activity 2 the late eocene early oligocene boundary interval 28 5 34 5 ma 340 430 m which was a transition from relatively warm to cooler conditions that coincided with glacial intensification sea level fall and subaerial erosion of the shelf and 3 the late oligocene early miocene 22 28 5 ma 50 340 m when large scale glaciation dominated the region and glaciers grounded across the continental shelf from correlation with global oxygen isotope and sea level records we infer that the antarctic climate and surrounding oceans cooled after separation of australia and antarctica and development of deep water circulation between them this marked the onset of the eocene oligocene transition at ca 34 5 ma a major east antarctic ice sheet did not develop until the early late oligocene boundary toward the end of the eocene oligocene transition ca 28 5 ma outlet glaciers did not breach the transantarctic mountains and ground across the ross sea shelf until 0 5 m y later ca 28 ma

the importance of apatite composition and single grain ages when interpreting fission track data from plutonic rocks a case study from the coast ranges british columbia abstract fission track results determined from granitic rocks exposed within the coast ranges of british columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples to test ages determined earlier by parrish using the population method of fission track dating new apparent apatite ages were completed using the external detector method so that single grain ages could be generated in most cases the new apatite ages were similar to the earlier ones within 2 however in many cases it was found that individual samples contained significant variations in single grain apatite ages microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry such that chlorine rich apatites retained older ages than fluorine rich grains examination of the single grain age data suggested that the spread in single grain ages provided information which constrained the thermal history of the sample and that distributions of single grain ages reflected the temperatures at which they resided during cooling these results suggest that it is important to generate more than a few single grain fission track ages when dating apatite from granitic rocks since variations in the chlorine concentration in apatite grains may exist in kg sized granitic rock samples therefore it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results

variability of apatite fission track annealing kinetics ii crystallographic orientation effects a method is presented that permits the length of any horizontal confined fission track inclined at a specified angle to the crystallographic c axis in apatite to be converted to an equivalent track length parallel to the crystallographic c axis the model is based on the results of annealing experiments for six selected apatites five calcian fluorapatites and durango apatite representing a subset of the 15 total apatite specimens studied an iterative process of calculation is required to project fission track lengths onto the c axis and computer source code implementing the solution to this problem is presented this method of projecting apatite fission track lengths onto the crystallographic c axis is shown to remove effectively fission track length variation within single fission track populations due to anisotropic track length reduction for all 15 apatites studied in addition a model is developed that offers predictions that closely reproduce published experimental data concerning the relationship between fission track density etched fission tracks per unit area of apatite surface and the arithmetic mean fission track length finally it is shown that natural fission track populations exhibit fission track length anisotropy similar to that of fission track populations created and annealed in the laboratory this observation implies that the same process by which apatite fission tracks anneal in the laboratory is responsible for annealing of apatite fission tracks in the geological environment

reconstruction of the differentiated long term exhumation history of fuerteventura canary islands spain through fission track and u th sm he data miocene intrusives and lower cretaceous siliciclastic sedimentary rocks from the basal complex in western fuerteventura were analyzed with low temperature thermochronometric methods such as fission track and u th sm he dating in order to reveal the evolution of the island s exhumation history the obtained thermochronometric data yields a very slow rate of cooling in the order of 1 5 3 c myr from 50 to 20 ma for the early cretaceous siliciclastic rocks these sedimentary units have never been heated significantly above 240 c after deposition and still record the submarine onset of the island s formation in the eocene intrusive bodies associated with the early miocene magmatic activity of the central volcanic complex of the island show rapid initial cooling rates of 50 70 c myr from 20 to 14 ma contemporaneous with the intrusions the cooling rate of the cretaceous sedimentary units increased to 25 35 c myr and it is inferred that this increase is associated with enhanced uplift and erosion of the central volcanic complex after 14 ma rates slowed down to 3 6 c myr palaeosols overlying the sedimentary units are themselves covered by pliocene basalt flows and reveal that the sedimentary rocks reached the surface before 5 ma the thermochronometric data obtained in this study for central fuerteventura is difficult to reconcile with the cooling history derived from previously obtained fission track and k ar data from the north western part of the island this inconsistency is likely to indicate that the exhumation history of fuerteventura is more complex and regionally subdivided than previously believed

pliocene paleoclimate and east antarctic ice sheet history from surficial ash deposits the preservation age and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in arena valley southern victoria land indicate that a cold desert climate has persisted in arena valley during the past 4 3 million years these data indicate that the present east antarctic ice sheet has endured for this time and that average temperatures during the pliocene in arena valley were no greater than 3 c above present values one implication is that the collapse of the east antarctic ice sheet due to greenhouse warming is unlikely even if global atmospheric temperatures rise to levels last experienced during mid pliocene times

some uncertainties in the derivation of rates of denudation from thermochronologic data low temperature thermochronology such as that provided by apatite fission track analysis provides a valuable means of establishing the timing of major denudational events and associated rates of denudation over geological time scales of 106 108 ma care must be taken however in deriving denudation rates from the crustal cooling histories documented by thermochronologic techniques especially in rapidly eroding terrains since in such cases apparent denudation rates derived from thermochronologic data will usually overestimate true rates if the advective effect of denudation is not included this is likely to be resolvable where the rate of denudation exceeds 300 m ma 1 and when the depth of denudation occurring at these rates exceeds several kilometres prior to the sample cooling below the appropriate closure temperature because the time at which a sample cools below a particular closure temperature is relatively insensitive to advection the initiation of denudation can be accurately established even given uncertainties in the estimation of depths and rates of denudation where thermal events originate from a source within or below the lower crust the cooling through denudation will dominate the low temperature history of the shallow crust if denudation occurs coevally with the subsurface heating 1997 by john wiley sons ltd

an empirical test of helium diffusion in apatite borehole data from the otway basin australia we have analyzed helium ages of apatites from several boreholes in the otway basin australia to evaluate whether laboratory helium diffusivity can be accurately extrapolated to conditions relevant in nature downhole apatite helium ages define a broad swath of values from 78 71 ma at the surface 15 c to nearly zero at depths corresponding to ambient temperatures of 80 c the width of the swath results from uncertainties in corrected borehole temperatures differences in the thermal history experienced by the various boreholes and possibly from slightly different helium diffusivities among the detrital apatite grains studied in the eastern otway basin the shape and position of the helium age profile is in good agreement with predictions based on the extrapolation of laboratory diffusivity data for durango apatite coupled with published thermal histories for this part of the basin in contrast helium ages are much younger than predicted in the western otway basin based on measured ages from otway sediments which have been essentially isothermal over the last few million years an empirical diffusivity d 2 of 2 10 15 s 1 can be assigned to apatites residing at downhole temperatures of 67 97 c this empirical diffusivity is consistent with laboratory diffusion measurements demonstrating that such measurements are reasonably accurate and can be applied with confidence to natural geologic settings given this confirmation of the laboratory diffusivity data the discrepancy between the observed and modeled helium age profiles in the western otway basin suggests that these sediments recently experienced higher temperatures than presently supposed

interpretation of u th he single grain ages from slowly cooled crustal terranes a case study from the transantarctic mountains of southern victoria land low temperature thermochronologic techniques e g apatite fission track aft thermochronology and u th he dating constrain near surface t t paths and are often applied to uplift denudation and landscape evolution studies samples collected in vertical profiles from granitic walls on either side of the ferrar glacier southern victoria land antarctica were analyzed using aft thermochronology and apatite u th he dating to further constrain the lowest temperature thermal history of this portion of the transantarctic mountains aft central ages vary systematically with elevation and together with track length information define a multi stage cooling denudation history in the cretaceous and early tertiary apatite u th he single grain age variation with elevation is not as systematic with considerable intra sample age variation although many complicating factors e g u and th rich micro inclusions fluid inclusions variation in crystal size particle ejection correction zonation and particle ejection correction implantation of he into a crystal or impediment of he diffusion out of a crystal and 147sm derived particles may contribute to age dispersion we found that variation in single grain ages correlated with cooling rate samples that cooled relatively quickly have less variation in single grain ages whereas samples that cooled relatively slowly 3 c m y or resided within an u th he partial retention zone heprz prior to more rapid cooling have a comparatively greater variation in ages decay of u and th via particle emission creates a 4he concentration profile dependent upon the initial parent u th within a crystal variation of single grain ages for samples with non homogeneous u th distributions will be enhanced with long residence time in the partial retention zone i e slow cooling because of the relative importance of loss via volume diffusion and loss via particle ejection with respect to the u th zonation and the grain boundary correction of ages for particle ejection ft correction factor typically assumes uniform u and th distribution within the crystal and when applied to a population of crystals with different u and th distributions will enhance the variation in ages most complicating factors listed above for apatite u th he ages result in ages that are too old we propose that if considerable variation in u th he single grain ages exists that a weighted mean age is determined once outlier single crystal ages are excluded using the criterion of chauvenet or a similar approach we suggest that the true age or most representative age for that age population lies between the minimum u th he age and the weighted mean age we apply this approach coupled with composite age profiles to better constrain the t t history of the profiles along the ferrar glacier significant intra sample variation in single crystal apatite u th he ages and other minerals dated by the u th he method should be expected especially when the cooling rate is slow the variation of u th he single crystal ages is therefore another parameter that can be used to constrain low temperature thermal histories

cenozoic tectonic evolution of the white mountains california and nevada the white mountains represent the westernmost range of the central northern basin and range province they are situated to the east of the unextended sierra nevada and represent a crustal block that is bounded along its western flank by the high angle white mountains fault zone the fault zone accommodates up to 8 km of total dip slip displacement investigation of the structural and thermal history of the white mountains indicates a two stage cenozoic tectonic evolution preextensional miocene volcanic rocks preserved along the eastern side of the range unconformably overlie mesozoic granitic basement and currently dip up to 25 to the east recording the total cenozoic tilt of the crustal block apatite fission track and u th he thermochronological data indicate that the white mountains underwent rapid exhumation and eastward tilting in the middle miocene starting at ca 12 ma geologic mapping 1 10 000 fault kinematic analysis and dating of younger volcanic sequences show that following middle miocene east west extension the white mountains have been dominated by right lateral transtensional deformation related to the walker lane belt the eruption of late miocene and pliocene volcanic sequences in the eastern white mountains postdates the majority of the uplift of the range as evidenced by infilling of paleodrainages and the presence of east directed flow fabrics fault kinematic indicators from the white mountains fault zone are characterized by apparent overprinting of dip slip fault motion indicators by right lateral slickenfibers and fault striations demonstrating that the range bounding fault system along the western side of the white mountains was reactivated as a dextral strike slip fault system at the northern and southern ends of the range pliocene right lateral transtension along this northwest southeast trending fault systems resulted in the formation of northeast trending pull apart basins that truncate the mountain range and transfer strike slip displacement eastward from the owens valley fault zone to the fish lake valley fault zone the inception of strike slip faulting in fish lake valley occurred at ca 6 ma as constrained by late miocene volcanic units right lateral faulting on the western side of the white mountains occurred at ca 3 ma and is distinctly younger than the faulting in the fish lake valley area indicating a westward migration of transcurrent deformation through time

lignin biomarkers and pollen in postglacial sediments of an alaskan lake abstract we analyzed a 12 000 yr sediment core from wien lake central alaska for a suite of phenolic products from cuo oxidation of lignin polymers and compared their composition with pollen data from the same core to assess lignin phenols as sedimentary biomarkers inferences of the gross taxonomic origin of sediment organic matter from lignin phenol composition agree with vegetational reconstructions based on pollen assemblages in particular the ratios of syringyl to vanillyl phenols are consistently higher before 6500 yr bp when angiosperms dominated or codominated the regional vegetation than after 6500 yr bp when gymnosperms dominated however the ratios of cinnamyl p coumaric and ferulic acids to total vanillyl phenols c v do not show patterns expected from the abundance of woody plants c v ratios are particularly high 0 7 0 85 after 6500 yr bp when pollen spectra suggest closed boreal forests dominated by picea and the stratigraphic patterns are strikingly similar between c v and picea pollen concentrations cuo oxidation of modern pollen of p glauca and p mariana yields exceptionally high amounts of cinnamyl phenols 8 90 and 6 41 mg 100 mg oc for p glauca and p mariana respectively in particular p coumaric acid is obtained in large amounts 8 87 and 6 41 mg 100 mg oc for p glauca and p mariana respectively versus vanillyl phenols 0 25 and 0 49 mg 100 mg oc for p glauca and p mariana respectively and ferulic acid 0 03 and 0 00 mg 100 mg oc for p glauca and p mariana respectively thus lignin phenols derived from fossil picea pollen preserved in sediments likely drive the c v profile of the wien lake core these data imply that if picea pollen concentrations are sufficiently high the amount of nonwoody tissue in sediments may be grossly overestimated when the lignin composition of gymnosperm needles is used as the end member of nonwoody tissues given that pollen grains are among the most resistant components of sedimentary organic matter and that p coumaric acid is labile it is important to consider pollen as a nonwoody tissue type when lignin biomarkers are used to determine the sources of vascular plant material in sediments

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

solving the production diffusion equation for finite diffusion domains of various shapes part i implications for low temperature u th he thermochronology abstract we propose an accurate fast and easy to use method to derive numerical solutions for production diffusion equations for finite diffusion domains of various shapes and arbitrary cooling histories previous studies provide solutions for spheres infinite cylinders and infinite sheets we extend this range and provide solutions for finite bodies i e finite cylinders and rectangular blocks of any aspect ratio this approach is important as recently it has become clear that for example the physical grain is the diffusion domain for he diffusion in apatite and titanite j geophys res 105 2000 2903 geochim cosmochim acta 63 1999 3845 we discuss the use of the new approach for forward modelling u th he production diffusion in apatite taking results with finite cylinders as a good approximation for apatite crystals it is found that approximating instead with spheres or infinite cylinders having the same radius yields differences in calculated ages that can easily be as large as 20 35 the relative differences are most pronounced in thermal histories that spend significant time at or near the closure temperature on the other hand reasonable agreement is found with spheres having the same surface to volume ratio

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

paleo geomorphology of the sierra nevada california from u th he ages in apatite new apatite u th he ages from the central sierra nevada california place limits on the morphology and evolution of longitudinal profiles of major transverse river drainages developed in the late cretaceous helium ages from a new orogen parallel constant elevation sample transect are relatively uniform 60 ma and not correlated with topography unlike those from a similar lower elevation transect approx 15 km to the west we interpret the marked difference in the two orogen parallel profiles to reflect a headward decrease in long wavelength transverse relief at the time of cooling consistent with either a concave up stream gradient typical of those observed near the headwaters of modern sierran trunk streams or a convex gradient like those found along slope breaks of eroding plateau edges from the san joaquin drainage we obtained a new helium age versus elevation profile which has a comparable slope to those previously reported for the yosemite valley kings canyon and mt whitney areas this new profile yields slightly older ages at a given elevation as expected from its position adjacent to the largest major drainage the age versus elevation profiles imply that the mean denudation rate of the region in the cenozoic was about 0 04 to 0 05 mm yr we show that long wavelength 70 km relief inferred from longitudinal helium age variations is a strong function of erosion rate such that even relatively subdued relief is detectable with helium age profiling provided that denudation rates were low using the rate implied by sierran age versus elevation profiles we infer long wavelength relief of 1500 500 m in the late cretaceous by analogy with modern orogenic plateaus this value of long wavelength relief suggests a cretaceous interior cordilleran plateau lay at an elevation of at least 3000 m