evidence for muon induced production of 10be in near surface rocks from the congo distributions of 10be in vein quartz from a lateritic weathering profile from the congo provide evidence for significant in situ production of 10be by reactions other than neutron induced spallation after consideration of geomorphological and nuclear processes which might contribute to the observed depth variability we conclude that cosmic ray muon induced reactions are the most likely mechanism and that such reactions lead to 1 to 3 of 10be production at the surface this small muon induced component has only minor implications for the use of 10be for quantification of exposure histories of surficial rocks nevertheless the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes overcoming some of the limitations associated with the use of the neutron produced component

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

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

cosmogenic 3he in himalayan garnets indicating an altitude dependence of the 3he 10be production ratio to investigate the potential of using cosmogenic helium in garnet as a dating tool 3hec and 10bec have been measured within garnets and coexisting quartz respectively sampled in the himalayan range at various elevations comparison and correlation of 3hec and 10bec concentrations demonstrate that cosmogenic 3he is well retained in himalayan garnets and can thus be used in this mineral to quantify precisely earth s surface processes this in the case of recently cooled orogenic samples which contain limited amounts of non cosmogenic helium for these high elevation samples the exposure ages derived from 3hec using classical scaling factors lead however to an overestimation of 3hec ages compared to 10bec ages 0 1 to 28 ky by up to a factor of 2 additionally over the samples altitude range 3000 4622 m the theoretically invariant polar 3hec 10bec production ratio 22 5 increases with increasing elevation challenging classical models of production rate evolution to explain this observation we propose a new 3hec production mechanism based on the altitudinal dependence of the energy spectrum of cosmic rays and according to which neutrons or protons resulting from a first spallation reaction within the rock tertiary particles may have sufficient energy to induce an additional in situ spallation reaction producing a second 3h or 3he but insufficient energy to produce 10be the probability of such a mechanism increasing with altitude according to these observations we propose for future and ongoing studies to use the empirically determined attenuation length of 121 g cm2 for helium production at high altitude

rates and timing of earth surface processes from in situ produced cosmogenic be 10 beryllium 10 is the longest lived of the seven known unstable isotopes of be it results mostly from the interaction of cosmic radiation primarily neutrons with a variety of target atoms by spallation the splitting of nuclei lal 1988 it can also be produced at very low levels by radio disintegration of u and th sharma and middleton 1989 although all radioactive be isotopes are produced in the atmosphere via cosmic ray reactions arnold 1956 lal and peters 1967 morris et al chapter 5 this volume in this paper we are most interested in 10be produced in rock which we refer to as in situ produced such in situ produced 10be is a cosmic ray dosimeter functioning as a quantitative monitor of near surface residence time in some geologic situations 10be can be used as a chronometer allowing one to estimate the duration of surface exposure in other settings 10be can be used as a tracer allowing one to estimate the rate distribution and behavior of earth surface processes rates of 10be production are very low in rocks exposed at earth s surface 102 atoms g 1 y 1 resulting in typical in situ produced 10be concentrations termed activities of 103 to 107 atoms g 1 the development of accelerator mass spectrometry ams in the 1980s allowed routine measurement of 10be at levels typical of rocks exposed near earth s surface elmore and phillips 1987 analytical refinements in the 1990s lowered detection limits by an order of magnitude allowing dating of exposure periods 103 years in favorable geologic and geographic situations simple exposure histories at high altitude and latitude most in situ 10be is measured in quartz mineral separates where the primary spallation target is o lal and arnold 1985

teleseismic time functions for large shallow subduction zone earthquakes broadband vertical p wave records are analyzed from 63 of the largest shallow subduction zone earthquakes which have occurred in the circum pacific in the last 45 yr most of the records studied come from a common instrument the pasadena california benioff 1 90 seismometer propagation and instrument effects are deconvolved from the p wave records using a damped least squares inversion to obtain the teleseismic source time function the inversion has the additional constraint that the time function be positive everywhere the period band over which the time functions are considered reliable is from 2 5 to 50 sec fourier displacement amplitude spectra computed for each of the 1 90 p wave trains indicate spectral slopes measured between 2 and 50 sec of 1 0 to 2 25 with an average value of 1 5 these values assume an average attenuation of t 1 0 the seismic moments derived from the p wave time functions compare well with other published values for earthquakes having moments smaller than 2 5 10 28 dyne cm m w 8 2 because the 1 90 seismometer has little response at very long periods this technique underestimates the moments of the very largest events the time functions are characterized using five parameters 1 spectral slope between 2 and 50 sec 2 roughness of the time function 3 multiplicity of sources 4 pulse widths of individual sources and 5 overall signal duration the 63 earthquakes studied come from 15 subduction zones with a wide range in the ages of subducted lithosphere convergence rates and maximum size of earthquakes comparing the time function parameters with age rate and m w of the subduction zone does not yield obvious global trends however most of the subduction zones do behave characteristically and can be grouped accordingly

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

modes of active intraplate deformation flinders ranges australia 1 the flinders ranges form one of the most seismically active zones within the australian continent with seismogenic strain rates over the last 30 years of 10 16 s 1 active deformation in the region reflects late neogene increases in stress levels in the indo australian plate as a response to increased plate boundary forcing from collision zones with the neighboring asian and pacific plates geological and geophysical observations suggest two modes of active deformation in operation in the flinders ranges over the last several million years 1 low amplitude 200 500 m long wavelength 200 km lithospheric flexure and 2 active reverse faulting along the range front with fault slip rates of up to 50 m myr 1 numerical models are developed to explore the contribution of each of these deformation modes to the observed geophysical signals an elastic mode of deformation is suggested by a distinctive long wavelength positive correlation between gravity and topography in which the flinders ranges are bordered by anomalous topographic and gravity lows now occupied by playa lake systems centered some 50 km outboard of range bounding faults numerical models show that flexural instabilities localized by vertical loads arising from older tectonic structuring produce a first order match with observed topography and gravity numerical models are also used to illustrate how the localized failure evident in the contemporary seismicity and quaternary faulting record within the flinders ranges reflects thermal weakening associated with extraordinary concentrations of heat producing elements in the crust as reflected in modern day heat flows of 90 mw m 2

on the consistency of earthquake moment rates geological fault data and space geodetic strain the united states new and dense space geodetic data can now map strain rates over continental wide areas with a useful degree of precision stable strain indicators open the door for space geodesy to join with geology and seismology in formulating improved estimates of global earthquake recurrence in this paper 174 gps vlbi velocities map united states strain rates of 30 0 10 8 yr 1 with regional uncertainties of 5 to 50 per cent kostrov s formula translates these strain values into regional geodetic moment rates two other moment rates m seismic and m geologic extracted from historical earthquake and geological fault catalogues contrast the geodetic rate because m geologic m seismic and m geodetic derive from different views of the earthquake engine each illuminates different features in california the ratio of m geodetic to m geologic is 1 20 the near unit ratio points to the completeness of the region s geological fault data and to the reliability of geodetic measurements there in the basin and range northwest and central united states both m geodetic and m seismic greatly exceed m geologic of possible causes high incidences of understated and unrecognized faults probably drive the inconsistency the ratio of m seismic to m geodetic is everywhere less than one the ratio runs systematically from 70 80 per cent in the fastest straining regions to 2 per cent in the slowest although aseismic deformation may contribute to this shortfall i argue that the existing seismic catalogues fail to reflect the long term situation impelled by the systematic variation of seismic to geodetic moment rates and by the uniform strain drop observed in all earthquakes regardless of magnitude i propose that the completeness of any seismic catalogue hinges on the product of observation duration and regional strain rate slowly straining regions require a proportionally longer period of observation characterized by this product gamma distributions model statistical properties of catalogue completeness as proxied by the ratio of observed seismic moment to geodetic moment i find that adequate levels of completeness should exist in median catalogues of 200 to 300 year duration in regions straining 10 7 yr 1 comparable to southern california similar levels of completeness will take more than 20 000 years of earthquake data in regions straining 10 9 yr 1 comparable to the southeastern united states predictions from this completeness statistic closely mimic the observed m seismic to m godetic ratios and allow quantitative responses to previously unanswerable questions such as what is the likelihood that the seismic moment extracted from an earthquake catalogue of x years falls within y per cent of the true long term rate the combination of historical seismicity fault geology and space geodesy offers a powerful tripartite attack on earthquake hazard few obstacles block similar analyses in any region of the world

identifying fault heterogeneity through mapping spatial anomalies in acoustic emission statistics 1 seismicity clusters within fault zones can be connected to the structure geometric complexity and size of asperities which perturb and intensify the stress field in their periphery to gain further insight into fault mechanical processes we study stick slip sequences in an analog laboratory setting analysis of small scale fracture processes expressed by acoustic emissions aes provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events the present work connects x ray computer tomography ct scans of faulted rock samples with spatial maps of b values slope of the frequency magnitude distribution seismic moments and event densities our current experimental setup facilitates the creation of a series of stick slips on one fault plane thus allowing us to document how individual stick slips can change the characteristics of ae event populations in connection to the evolution of the fault structure we found that geometric asperities identified in ct scan images were connected to regions of low b values increased event densities and moment release over multiple stick slip cycles our experiments underline several parallels between laboratory findings and studies of crustal seismicity for example that asperity regions in lab and field are connected to spatial b value anomalies these regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

cosmogenic neon in recent lavas from the western united states the absolute cosmogenic production of 21ne 21nec and 3he 3hec in olivine fo81 is measured for the well dated tabernacle hill flow values scaled to sea level and high latitude 60 are 45 atoms g yr for 21nec and 109 atoms g yr for 3hec for the past 17 800 years production rates for 21nec are about 50 higher than the theoretical values given by lal 1991 a ratio of 21nec 3hec of 0 41 0 05 is measured in olivines that have a range of 25 in cosmogenic exposure there is no evidence of 3hec loss in samples older than 100 000 years volcanic plagioclase is shown to be a suitable mineral for cosmogenic 21nec dating of surfaces 21nec in co existing plagioclase and olivine phenocrysts have a constant ratio of 0 42 04 in samples with more than 106 atoms of 21nec using all of the 21nec data and the 3hec as a cosmic ray flux monitor production of 21nec in ab36 plagioclase is calculated at 16 8 1 7 atoms g yr for the past 17 800 years the production of 21nec from 23na is estimated at 160 50 atoms g yr at 60 and sea level or equivalent to that on 24mg

episodic intraplate deformation of stable continental margins evidence from late neogene and quaternary marine terraces cape liptrap southeastern australia the waratah fault is a northeast trending high angle reverse fault in the late paleozoic lachlan fold belt at cape liptrap on the southeastern australian coast it is susceptible to reactivation in the modern intraplate stress field in southeast australia and exhibits late pliocene to late pleistocene reactivation radiocarbon optically stimulated luminescence osl and cosmogenic radionuclide crn dating of marine terraces on cape liptrap are used to constrain rates of displacement across the reactivated waratah fault six marine terraces numbered qt6 tt1 youngest to oldest are well developed at cape liptrap with altitudes ranging from 1 5 m to 170 m amsl respectively on the lowest terrace qt6 barnacles in wave cut notches 1 5 m amsl yielded a radiocarbon age of 6090 5880 cal bp and reflect the local mid holocene sea level highstand qt5 yielded four osl ages from scattered locations around the cape ranging from 80 ka to 130 ka it formed during the last interglacial sea level highstand mis 5e at 125 ka inner edge elevations approximate paleo high tide line for qt5 occur at distinctly different elevations on opposite sides of the waratah fault offsets of the inner edges across the fault range from 1 3 m to 5 1 m with displacement rates ranging from 0 01 mm a to 0 04 mm a the most extensive terrace tt4 yielded four early pleistocene cosmogenic radionuclide crn ages two apparent burial ages of 0 858 ma 0 16 ma and 1 25 ma 0 265 ma and two apparent exposure ages of 1 071 ma 0 071 ma 10be and 0 798 ma 0 066 ma 26al allowing for muonic production effects from insufficient burial depths the depth corrected crn burial ages are 1 8 ma 0 56 ma and 2 52 ma 0 88 ma or late pliocene a late pliocene age is our preferred age offsets of tt4 across the waratah fault range from a minimum of 20 m for terrace surface treads to a maximum of 70 m for terrace bedrock straths calculated displacement rates for tt4 range from 0 01 mm a to 0 04 mm a using a late pliocene age 2 ma identical to the rates calculated for the last interglacial terrace qt5 this indicates that deformation at cape liptrap has been ongoing at similar time averaged rates at least since the late pliocene the upper terraces in the sequence tt3 110 m amsl tt2 140 m and tt1 180 m are undated but most likely correlate to sea level highstands in the neogene terraces tt1 tt4 show an increasing northward tilt with age the waratah fault forms a prominent structural boundary in the lachlan fold belt discernible from airborne magnetic and bouger gravity anomalies seismicity and deformation are episodic episodic movement on the waratah fault may be coincident with sea level highstands since the late pliocene possibly from increased loading and elevated pore pressure within the fault zone this suggests that intervals between major seismic events could be on the order of 100 ka

cosmogenic 3he in terrestrial rocks the summit lavas of maui we have identified terrestrial cosmic rayproduced 3he in three lava flows on the crest of haleakala volcano on maui 3 km above sea level and 0 5 million years old although these lavas like all oceanic basalts contain primordial 3he from the mantle the cosmogenic component 3hec can be identified unambiguously because it is extractable only by high temperature vacuum fusion in contrast a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing leaving a residual component with 3he 4he ratios as high as 75 those in the atmosphere which can be liberated by melting the crushed grains cosmogenic 3he is present in both olivines and clinopyroxenes at 0 8 1 2 10 12 ml stp g and constitutes 75 5 of the total 3he present the observed 3hec levels require a cosmic ray exposure age of only some 64 000 years much less than the actual age of the lavas if there is no erosion using a model that includes effects of uplift or submergence as well as erosion we calculate an apparent erosion rate of the order of 8 5 m 106 years for the western rim of the summit crater as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems

using pyroxene microphenocrysts to determine cosmogenic 3 he concentrations in old volcanic rocks an example of landscape development in central gran canaria abstract determinations of cosmogenic 3he exposure ages and erosion rates in volcanic rocks older than a few hundred thousand years are complicated by the presence of radiogenic he in addition to the magmatic and cosmogenic he in phenocryst minerals however by analysing microphenocrysts that crystallised on or immediately prior to eruption that have not trapped magmatic he the three component problem can be eliminated and accurate determinations of cosmogenic 3he made in this study we perform three experiments using pyroxene microphenocrysts in basaltic clasts in the pliocene ayacata formation breccias gran canaria that demonstrate they are free of magmatic he exposure ages and erosion rates calculated from the cosmogenic 3he concentrations are combined with a geomorphological study to produce a tentative interpretation of landscape evolution in the mountainous interior of gran canaria long term steady state erosion rates of 14 24 mm ka 1 are recorded from bedrock erosional surfaces on a high plateau headwall retreat rates for a major drainage system of 1 6 m ka 1 have been constrained from the ca 225 ka exposure age of a boulder emplaced on slopes beneath the headwall strath terraces and boulders in a small canyon system yield much younger exposure ages of 47 43 ka

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

cosmic ray produced 10be and 26al in antarctic rocks exposure and erosion history we have measured cosmic ray produced t1 2 1 5 million years 10be and t1 2 0 705 million years 26al in purified quartz fractions of selected rock samples from antarctic mountains from these data we calculate 1 mean erosion rates for the limiting case of steady state surface exposure to cosmic rays and 2 minimum exposure ages for the limiting case of no erosion calculated mean erosion rates are very low on the order of a few times 10 5 cm yr we believe the sampling to be sufficient to generalize this result to exposed bedrock in antarctica in favorable cases it is possible to distinguish between the limiting cases steady state erosion seems a better description in such cases most samples including some taken a few meters above the present ice level seem to have been exposed for millions of years without major episodes of burial or abrasion by ice

plate boundary forces are not enough second and third order stress patterns highlighted in the world stress map database 1 the world stress map project compiles a global database of contemporary tectonic stress information of the earth s crust early releases of the world stress map project demonstrated the existence of first order plate scale stress fields controlled by plate boundary forces and second order regional stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound the 2005 release of the world stress map project database provides for some areas high data density that enables us to investigate third order local stress field variations and the forces controlling them such as active faults local inclusions detachment horizons and density contrasts these forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic we present and discuss examples for venezuela australia romania brunei western europe and southern italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns

the april 2007 eruption and the dolomieu crater collapse two major events at piton de la fournaise la reunion island indian ocean abstract after 10 years of high activity and 24 eruptions piton de la fournaise pdf produced on april 2 to may 1 2007 one of its most voluminous and intense eruption since at least one century the eruption focused at 590 m elevation in the grand brule on the south east flank of piton de la fournaise 7 km away from summit it was located close to the southern cliff of the grand brule volcano tectonic collapse the rempart du tremblet and to the nearby village called le tremblet and it highly rattled and incommoded the inhabitants there eleven hours after the beginning of the eruption the lava flows reached the sea and created 0 45 km 2 of new land until the end of eruption in the night of april 5 the rock column beneath dolomieu crater started to collapse with a piston like mechanism into the magma chamber forming within less than 24 h the most recent and well documented caldera we report in this paper the chronology of these two related major events in the recent activity of pdf seen by the scientific networks of the volcanological observatory ovpf ipgp and field observations which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes

the cosmic ray produced he 21ne ratio in ultramafic rocks although production of noble gases in meteorites are well studied since many years e g bogard and cressy 1973 the first analysis of cosmic ray produced helium in terrestrial rocks were performed only in 1986 by kurz and by craig and poreda later marti and craig 1987 reported cosmic ray produced neon in summit lavas from maui more data on cosmogenic helium and neon were reported by staudacher and allegre 1991 on ultramafic nodules from asia the absolute production rate of cosmic ray produced helium has been determined by kurz 1986 1987 in olivine rich basalts from hawaii with known k ar or 14c ages the resulting hec production rate is 3 72 10 18cm stp g a this allows cosmogenic he to be used to determine cosmic ray exposure ages of surface samples for neon no such absolute production rate has yet been determined in order to develop a complementary dating method using cosmogenic 21ne staudacher and allegre 1991 measured the cosmogenic he 21ne ratio in ultramafic nodules their value was 1 4 which is significantly smaller than the value of 3 5 obtained in this work here we discuss the cosmogenic he 21ne ratio in view of new noble gas data on olivine separates of oceanites from reunion island and on ultramafic nodules from mongolia

effective attenuation lengths of cosmic rays producing 10be and 26al in quartz implications for exposure age dating we have measured cosmic ray produced 10be t 1 5 my and 26al t 0 72 my as functions of depth in a core of quartz sandstone bedrock collected in south victoria land antarctica these data were used to place limits on the exposure age and erosion rate of the exposed surface and to calculate the effective attenuation lengths of cosmic rays producing these nuclides these latter results indicate that the production attenuation length for 26al is comparable to that for 10be this suggests that the production ratio 26al 10be does not vary substantially with depth in a rock and that its value at the surface of exposed rock is not strongly affected by erosional losses allowing wider use of the ratio in determining exposure ages

attenuation length for fast nucleon production of 10be derived from near surface production profiles abstract we have measured the fast nucleon production of 10 be to depths of 2 37 m 659 g cm 2 in a granodiorite core of uniform density and composition from the mt givens pluton on the western side of the sierra nevada california at 37 14 7 n 118 53 7 w and 2286 m the data are well fit with a simple exponential decrease yielding an apparent attenuation length of 10 be production of 177 4 2 g cm 2 the shallowest data point is from a depth of 1 24 cm 3 45 g cm 2 and shows no indication of a near surface flattening of the radionuclide inventory and hence the 10 be production rate

fossil cosmogenic 3 he record from k ar dated basaltic flows of mount etna volcano sicily 38 n evaluation of a new paleoaltimeter this study investigates the reliability of a paleoaltimeter based on the elevation dependency of the cosmogenic nuclides production rate the proposed method relies on fossil cosmogenic record indeed measuring the cosmogenic nuclide concentration in an exposed and then buried lava flow combined with an independent estimate by k ar or 40ar 39ar dating of the past exposure duration theoretically allows recording fossil cosmogenic production rates which can thus be converted in paleoelevations this approach was tested by measuring cosmogenic 3he 3hec in olivines and clinopyroxenes of quaternary k ar dated basaltic flows of mount etna volcano sicily 38 n the agreement within uncertainties 1 500 m between current and cosmogenic derived elevations demonstrates the paleoaltimeter reliability however dating imprecision earth s paleomagnetic fluctuations and erosion contemporary to the flow exposure are parameters that may affect the accuracy and the precision of the method on older geological scales their respective influences were thus carefully evaluated and discussed to conclude that the paleoaltimeter can potentially reach resolutions better than 1000 m even for cenozoic records

the robustness of seismic moment and magnitudes estimated using spectral analysis we present an assessment of how microseismic moment magnitude mw e stimates vary with the method and parameters used to calculate seismic moment this is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity it is therefore imperative that these parties know and understand what errors exist in given magnitude values something that is poorly reported this study concentrates on spectral analysis techniques and compares mw computed in the time and frequency domains using recordings of mw 1 5e vents at cotton valley east texas the maximum discrepancy between mw estimated using the different methods is 0 6 units a significant variation by adjusting parameters in the mw calculation we find that the radiation pattern correction term can have the most significant effect on m w generally up to 0 8 units following this investigation we make a series of recommendations for estimating microseismic m w using spectral methods noise should be estimated and removed from recordings and an attenuation correction should be applied the spectral level can be measured by spectral fitting or taken from the low frequency level significant factors in obtaining reliable microseismic mw estimates include using at least four receivers recording at 1000 hz and making radiation pattern corrections based on focal mechanism solutions not average values

pebble population ellipsoid a three dimensional algebraic description of sedimentary fabric orientation and shape of clasts can be approximated by an ellipsoid such ellipsoids are easily transformed into a fixed geographic coordinate system and provide a 3 x 3 matrix description of the pebble these matrices can be summed and an average matrix determined eigenvector calculations produce the principal axes of the population ellipsoid paleocurrent interpretations based on these ellipsoids agree with other geological data other potential uses include the calculation of average clast dimension in any direction the calculation of volume changes associated with erosion and the characterization of various sedimentary environments deformed clasts are analyzed to illustrate the application of the population ellipse to strain studies

cosmic ray exposure dating with in situ produced cosmogenic 3he results from young hawaiian lava flows in an effort to determine the in situ production rate of spallation produced cosmogenic 3 he and evaluate its use as a surface exposure chronometer we have measured cosmogenic helium contents in a suite of hawaiian radiocarbon dated lava flows the lava flows ranging in age from 600 to 13 000 years were collected from hualalai and mauna loa volcanoes on the island of hawaii because cosmic ray surface exposure dating requires the complete absence of erosion or soil cover these lava flows were selected specifically for this purpose the 3 he production rate measured within olivine phenocrysts was found to vary significantly ranging from 47 to 150 atoms g 1 yr 1 normalized to sea level although there is considerable scatter in the data the samples younger than 10 000 years are well preserved and exposed and the production rate variations are therefore not related to erosion or soil cover data averaged over the past 2000 years indicate a sea level 3 he production rate of 125 30atoms g 1 yr 1 which agrees well with previous estimates the longer record suggests a minimum in sea level normalized 3 he production rate between 2000 and 7000 years 55 15atoms g 1 yr 1 as compared to samples younger than 2000 years 125 30 atoms g 1 yr 1 and those between 7000 and 10 000 years 127 19atoms g 1 yr 1 the minimum in production rate is similar in age to that which would be produced by variations in geomagnetic field strength as indicated by archeomagnetic data however the production rate variations a factor of 2 3 0 8 are poorly determined due to the large uncertainties in the youngest samples and questions of surface preservation for the older samples calculations using the atmospheric production model of o brien 1979 35 and the method of lal and peters 1967 11 predict smaller production rate variations for similar variation in dipole moment a factor of 1 15 1 65 because the production rate variations archeomagnetic data and theoretical estimates are not well determined at present the relationship between dipole moment and production rate will require further study precise determination of the production rate is an important uncertainty in the surface exposure technique but the data demonstrate that it is feasible to date samples as young as 600 years of age providing that there has been no erosion or soil cover therefore the technique will have important applications for volcanology glacial geology geomorphology and archaeology

using in situ produced cosmogenic isotopes to estimate rates of landscape evolution a review from the geomorphic perspective the application of in situ produced cosmogenic isotopes to problems in geomorphology has increased rapidly over the past decade at least 57 papers and numerous abstracts have been published since the mid 1980s when the first mass spectrometric measurements of terrestrially produced cosmogenic isotopes were made taken at face value these studies provide quantitative information about rates of landscape evolution and landform age however the significance of calculated erosion rates and exposure ages depends strongly on the models used to interpret isotopic data the validity of assumptions inherent to these models and the geologic surroundings in which the samples were collected this paper attempts to place cosmogenic isotope studies in a geomorphic context by reviewing fundamentals of the method and evaluating the validity of assumptions under which these data have been interpreted at present the establishment of high precision cosmogenically based glacial and alluvial chronologies is stymied by the evolution of geomorphic surfaces the erosion of rock from sampled boulders the potential for isotope inheritance from previous exposure and the uncertainty of isotopic measurements uncertainties in isotope production rates and the observed variability of exposure ages on individual geomorphic surfaces limit the confidence with which cosmogenic ages can be correlated reliably with those obtained by other techniques estimation of erosion rates at single points on the landscape gives useful small scale information extrapolation of these rates over longer time and larger spatial scales is less sure and most likely biased toward lower erosion rates by die inadvertent selection of resistant sample sites however because erosion rates are so poorly constrained at present even estimates to within a factor of 2 may be of significant value to geomorphologists and tectonicists

time variable deformation in the new madrid seismic zone new geodetic measurements show that the new madrid is currently deforming too slowly if at all to account for large earthquakes in the region over the past 5000 years this result together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicates that either tectonic loading rates or fault properties vary over a few thousand years

middle to late pleistocene uplift rate of the hungarian mountain range at the danube bend pannonian basin using in situ produced 3he topography of the terraced danube bend area indicates fast incision of the danube river which was followed by its tributaries dissecting deeply the former terrace levels these surfaces are vertically bended along the river course indicating antecedent incision of the danube into the sw ne trending hungarian mountain range hmr timing and rate of the incision of the danube into the hmr and consequently the rate of vertical motions have remained unresolved so far this study aims at quantifying the landscape evolution and neotectonic deformation of the central part of the hmr we used terrace levels along the antecedent section of the danube river to constrain its incision rate which is a measure for the uplift rate of the hmr here we use 3 he a terrestrial in situ produced cosmogenic nuclide tcn to date uplifted geomorphologic levels along in the danube bend gorge this method first applied in the carpathian pannonian system in the framework of present study proved to be suitable for the quantification of landscape evolution in this area our 3 he exposure age data suggest a maximum incision rate of 2 7f0 1 mm y for the last 170 ky considering likely effect of erosion a more conservative value of 1 6 mm y for the last 270 ky was obtained both rates are significantly higher than the incision rate of 0 41 mm y of the danube derived from previous geologic and geomorphic data for the last 360 ky the formation of the terrace levels in the danube bend probably occurred during the last two glacial cycles ois 1 8 according to the exposure age data there is no direct relationship between the terrace formation and climate in the danube bend incision of the danube appears to be connected to the uplift of the hmr obtained incision rate values can be taken as valid approximations of the uplift rate in the danube bend area d 2005 elsevier b v all rights reserved

cosmic ray produced neon in antarctic rocks we studied three antarctic rocks and quartz separates thereof for their records of cosmic ray produced 21ne all three samples reveal an excess of 21ne and in two of them a cosmic ray origin is documented by the spallation ratios 22ne 21ne 3he is shown to be incompletely retained in quartz 10 previous studies have shown the suitability of quartz as a monitor for cosmic ray produced radionuclides 10be and 26al and therefore as a tracer for geological processes 26al 10be ratios allow the calculation of minimum exposure ages and maximum erosion rates and measured ratios 21ne 10be and 21ne 26al add significant information to exposure histories especially in the case of a complex exposure this information is necessary for an evaluation of erosion rates and to define geological processes such as uplift and variations in the antarctic ice cover

storm induced precipitation variability control of long term erosion abstract erosion is often treated as a continuous process yet it occurs through discrete events such as floods and landslides of variable magnitude and periodicity it has also long been expected to be strongly dependent on precipitation however the influence of temporal rainfall variability upon long term evolution of landscapes remains unclear here we report high erosion rates 0 8 to 10 mm yr 1 over 70 ka estimated from paleovolcanic reconstructions across a steep rainfall gradient on reunion island which show that long term erosion rates are influenced by the cyclone induced variability of precipitation geostatistical analysis of 30 years of daily rainfall records reveals that erosion rates are high where the local climate is the driest and where the difference in intensity between extreme rainfall events and prevailing precipitation is the strongest this implies that the intrinsic variability of precipitation impacts landscape evolution not only through extreme meteorological events but also through background rainfall induced parameters such as humidity and dryness which modulate the erosion threshold of the earth s critical zone

cosmic ray produced neon and helium in the summit lavas of maui a 3he component attributed to cosmic ray induced spallation reactions 3hec in silicate minerals has recently been identified in lava flows on the summit of haleakala volcano on maui1 3 this3hec is observed in the helium extracted by fusion of olivine and pyroxene phenocrysts after first removing almost all of the mantle derived helium by vacuum crushing a procedure which releases the volatiles trapped in fluid inclusions within the phenocrysts cosmic ray produced 3he was found in the high altitude 3 000 m summit lavas which are 500 000 years old but was not observed in sub surface samples which were overlain by younger basalts2 or in a 200 yr old lava flow at sea level on maui1 2 we report here the identification of cosmic ray produced 21nec in addition to 3hec in gases extracted by fusion of olivines and clinopyroxenes after vacuum crushing the observed 3he 21ne c ratios and the ratio of 21nec in olivine to that in clinopyroxene are consistent with an in situ origin of 3hec and 21nec by cosmic ray spallation reactions this component could be important for interpreting helium isotopic data in terrestrial reservoirs geophysical applications could include determinations of erosion rates and exposure histories of terrestrial rocks

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

average properties of ellipsoidal fabrics implications for two and three dimensional methods of strain analysis abstract many rocks contain ellipsoidal objects such as pebbles or reduction zones which display a variety of shapes and orientations in deformed rocks such objects may be used for strain analysis by using the concept of an average ellipsoid here called the fabric ellipsoid two fabric ellipsoids are defined which are the results of two different algebraic averaging processes during deformation of ellipsoidal distributions the fabric ellipsoids change as if they were themselves material ellipsoids and are therefore of fundamental importance in strain analysis in most studies to date such 3 d fabric ellipsoids have been obtained from 2 d average ellipses determined on section planes cut through the rock sample previous work has assumed that the average ellipses will approximate to section through a single fabric ellipsoid i show here that this is not the case as sectioning introduces a systematic bias into the section ellipse data this bias is distinct from the statistical errors due to finite sample size and measurement errors discussed in other work and must be considered in any method of strain analysis using section planes

ages of the second caldera of piton de la fournaise volcano reunion determined by cosmic ray produced3he and21ne abstract cosmic ray produced 3 he and 21 ne in olivine separates from the plaine des rempart close to the second caldera of piton de la fournaise reunion indicate identical exposure ages of 62 000 yrs this age is only slightly less than the formation age of the basalt which is 65 000 2000 yrs and represents the final stage of eruption phase 2 the lower part of the wall of the second caldera just above the plaine des sables shows a cosmic ray exposure age of 23 800 2000 yrs this is probably the age for the collapse of the second caldera and represents the first direct dating of a fault in addition this age agrees with the range of 41 000 19 000 yrs given by k ar ages of different lava flows further six rocks that represent the final filling of the second caldera show low exposure ages of between 3300 and 5100 yrs these ages date the final stage of eruption phase 3 at 3400 1000 yrs and set an upper limit on the formation of the third caldera the enclos fouque this work shows that 21 ne and 3 he exposure age dating are powerful techniques for determining young ages in mafic samples and for obtaining ages for geological phenomena such as faulting because neon has three isotopes cosmogenic initial and morb type neon can be effectively resolved and combined use of 21 ne c and 3 he c represents a significant improvement in exposure age dating

seismology tectonic strain in plate interiors arising from r smalley jr m a ellis j paul r b van arsdale nature 435 1088 1090 2005 r smalley et al reply it is not fully understood how or why the inner areas of tectonic plates deform leading to large although infrequent earthquakes smalley et al 1 offer a potential breakthrough by suggesting that surface deformation in the central united states accumulates at rates comparable to those across plate boundaries however we find no statistically significant deformation in three independent analyses of the data set used by smalley et al and conclude therefore that only the upper bounds of magnitude and repeat time for large earthquakes can be inferred at present

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

mineral virginia earthquake illustrates seismicity of a passive aggressive margin 1 the august 2011 m 5 8 mineral virginia earthquake that shook much of the northeastern u s dramatically demonstrated that passive continental margins sometimes have large earthquakes we illustrate some general aspects of such earthquakes and outline some of the many unresolved questions about them they occur both offshore and onshore reaching magnitude 7 and are thought to reflect reactivation of favorably oriented generally margin parallel faults created during one or more wilson cycles by the modern stress field they pose both tsunami and shaking hazards however their specific geologic setting and causes are unclear because large magnitude events occur infrequently microseismicity is not well recorded and there is little if any surface expression of repeated ruptures thus presently active seismic zones may be areas associated with higher seismicity over the long term the present loci of activity that migrates or aftershock zones of large prehistoric earthquakes the stresses causing the earthquakes may result from platewide driving forces glacial isostatic adjustment localized margin stresses and or dynamic topography the resulting uncertainties make developing cost effective mitigation strategies a major challenge progress on these issues requires integrating seismic geodetic and geological techniques

the development of iron crust lateritic systems in burkina faso west africa examined with in situ produced cosmogenic nuclides we have investigated the development of iron crust laterites on the stable west african craton in northern burkina faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles lateritic soils develop in tectonically stable slowly eroding tropical environments and are a major component of the earth s surface to examine processes affecting laterite formation we determined 10be and 26al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3 8 m myr 1 in addition data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years quartz cobbles presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial colluvial material have10be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode presumably associated with a wetter climate roughly 300 kyr bp these results illustrate the practicality and the potential of the use of in situ produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in situ chemical weathering and mechanical transport

cobble imbrication as a sensitive indicator of subtle local changes in river flow direction imbrication of cobbles in pleistocene terrace gravels along the shoshone river near powell wyoming shows significant changes in orientation from a lower set to an upper set within a gravel layer the lower gravels show a mean orientation of 126 with spreads from s 38 to s 56 the mean orientation of the upper gravels is 070 with spreads from s 19 to s 26 change from the lower to the upper deposit is abrupt with no noticeable variability near the contact in fact the contact can easily be overlooked without close scrutiny of the cobble fabric this suggests that cobble imbrication may be a sensitive indicator of subtle changes in the flow direction of a braided stream because the earlier fabric was not disturbed by deposition of the younger unit there is every reason to believe that such fabrics can and will be preserved even in the consolidated rock record and can be used for paleoenvironmental interpretations

chronology of quaternary glaciations in east africa abstract a new glacial chronology for equatorial east africa is developed using in situ cosmogenic 36 cl measured in 122 boulders from moraines on mount kenya and kilimanjaro the oldest deposits sampled on kilimanjaro yield a limiting 36 cl age of 360 calendar kyr all 36 cl ages are in calendar years cal kyr or cal yr on mount kenya the oldest moraines give ages of 355 420 kyr liki i and 255 285 kyr teleki given the uncertainty in our 36 cl ages the liki i moraine may correspond to either marine isotope stage 10 or 12 whereas the teleki moraine correlates with stage 8 there is no evidence for stage 6 on either mountain the liki ii moraines on mt kenya and moraines of the fourth glaciation on kilimanjaro give ages of 28 3 kyr and 20 1 kyr respectively they represent the last glacial maximum lgm and correlate with stage 2 of the marine isotope record a series of smaller moraines above the lgm deposits record several readvances that occurred during the late glacial on mt kenya these deposits date to 14 6 1 2 kyr liki iia 10 2 0 5 kyr liki iii 8 6 0 2 kyr liki iiia and 200 yr lewis the corresponding deposits on kilimanjaro have mean ages of 17 3 2 9 kyr fourth glaciation saddle 15 8 2 5 kyr little glaciation saddle and 13 8 2 3 kyr fourth glaciation kibo these data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle

cosmogenic neon and helium at reunion measurement of erosion rate abstract we report analyses of both helium and neon for olivine separates from a drill core at an altitude of 2330 m in oceanite at piton de la fournaise volcano we show for the first time that cosmogenic 21 ne like cosmogenic 3 he decreases exponentially with core depth we obtain an attenuation length of 165 6 g cm 2 1 for cosmogenic 21 ne identical to the value found previously for cosmogenic 3 he alone 1 for each depth in the core the measured amount of cosmogenic 21 ne is about 13 lower than predicted based on the high precision k ar age of 65 200 2000 yrs 1 2 and this result is attributed to erosion assuming a constant erosion rate we derive a value of 3 5 1 7 mm yr 1 from the 21 ne data the same order of magnitude as the value obtained from cosmogenic 3 he at haleakala 1 3 this work highlights the important potential of both cosmogenic helium and neon for measuring in situ exposure ages and erosion rates

rates of continental erosion and mountain building the first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny the dominant part is played by mechanical erosion which acts most strongly in regions of large topographic expression chemical erosion depends strongly on precipitation or runoff in individual river drainage basins but because most continents have very similar average rainfall chemical erosion is fairly uniform for continental sized areas and will succeed in planing down all continents to a level peneplain if given enough time the exception to this rule is australia which has a very low chemical erosion rate because of its dryness the time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 my depending on the continent and the assumptions made mountain building occurs throughout the geological time scale but at a non uniform rate although there will not be a balance between erosion and mountain building over a short time scale due to the non uniform rate of mountain building the long term situation must be that the two phenomena should balance out it is shown that the freeboard of continents will respond to the long term balance between mountain building and erosion an expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth it is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the proterozoic as mountain building is predicted to decrease on average with time because of the reduction in internal heat generation and as erosion is dependent on the average elevation this average elevation will decrease slowly through time the opposite of what some workers have predicted a more complicated model of mountain building is then investigated in which one component of mountain building has a sinusoidal signal the oscillations in average elevation depend on the period of the sinusoid being smaller for shorter periods finally an average continental elevation is derived using a list of real orogenic events although this list of orogenies is incomplete there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper

australia s large earthquakes and recent fault scarps abstract nineteen large earthquakes up to magnitude ms 6 8 have occurred within continental australia this century and at least seven recent prehistoric fault scarps have been discovered the magnitude of the earthquake causing each of these prehistoric fault scarps has been estimated from the observed fault length and maximum scarp height and none exeeds magnitude 7 0 their occurrence demonstrates that the australian plate is undergoing significant internal deformation but as yet there is no model to explain their cause nor the likely site of the next large earthquake the absence of earthquake epicentres on the current tectonic map of australia is a regrettable omission

cosmic ray exposure cre dating in a wet tropical domain late quaternary fan emplacements in central sulawesi indonesia accurate chronological records are critical for studying the alluvial fan sequences emplaced during local global climatic changes but in general there are no satisfactory quantitative dating methods available in situ produced be 10 concentrations have been measured in quartz boulders exposed on sulawesian late quaternary fan surfaces these concentrations suggest that the two fan units evidenced by geomorphic study were emplaced during two distinct major climatic events the calculated minimum exposure ages imply abandonment ages of 11 kyr and 120 kyr for the younger and older units respectively this study demonstrates that using both the neutron and muon components implicated in the in situ be 10 production surficial erosion rates can be estimated and alluvial fans dated ln particular it shows that in situ be 10 may be used to date fan emplacement during the last 120 kyr under humid tropical conditions significantly helping to constrain continental palaeoclimatology

cosmogenic and nucleogenic 3he in apatite titanite and zircon cosmogenic 3he was measured in apatite titanite and zircon and cosmogenic 21 ne in quartz at 13 depth intervals in a 2 7 m long drill core in a miocene ignimbrite from the altiplano of bolivia all three 3he depth profiles as well as the 21 ne profile attenuate exponentially with depth indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources the attenuation lengthscale for 3he production of 180 11 g cm 2 is consistent with expectations for neutron spallation and is identical to that found for the cosmogenic 21 ne in quartz by normalizing the measured 3he concentrations to 21 ne and using the independently known cosmogenic 21 ne production rate the apparent cosmogenic 3he production rates in apatite titanite and zircon were respectively found to be 112 97 and 87 atoms g yr at sea level and high latitude the formal uncertainty on these estimates is 20 2 and arises in equal parts from uncertainties in the measured 3he 21 ne ratios and the uncertainty in the 21 ne production rate however an additional factor affecting the apparent 3he production rate in these phases arises from the long stopping range of spalled 3he and tritium which decays to 3he because all three accessory phases have higher mean atomic number than major rock forming minerals they will have lower 3he production rates than their surroundings as a consequence the long stopping ranges will cause a net implantation of 3he and therefore higher apparent production rates than would apply for purely in situ production thus these apparent production rates apply only to the specific grain sizes analyzed analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size from 50 to 100 m cross section yields a 10 decrease in apparent production rate while this effect warrants further study the grain sizes analyzed here are typical of the accessory phases commonly encountered so the apparent rates provide an appropriate starting place for surface exposure dating using 3he in these minerals