the ktb apatite fission track profiles building on a firm foundation abstract deep boreholes serve as natural laboratories for testing thermochronometers under geological conditions the kontinentale tiefbohrung ktb is an interesting candidate because the geological evidence suggests that approximate isothermal holding since the last documented exhumation in the late cretaceous to palaeocene is a reasonable assumption for the thermal histories of the ktb samples we report 30 new apatite fission track ages and 50 new mean confined track lengths determined on cores from the 4 km deep pilot hole the and external detector ages are consistent with the population ages from earlier studies and together define a clear age profile the mean track lengths from this and earlier studies reveal the effects of experimental factors the measured age and length profiles are compared with the predictions of 24 annealing models for isothermal holding there are clear discrepancies between the measured and calculated profiles down to 1 5 km depth the measured mean track lengths are shorter than the predicted the balance of methodological evidence indicates that this is due to seasoning i e a shortening of the fossil confined tracks without attendant age reduction from 2 5 to 4 0 km depth the mean track lengths are longer than the predictions this suggests that the bias model that weights the probabilities of observing tracks of different length and which is based on experiments relating surface track densities to mean track lengths is not appropriate for confined tracks experimental and methodological factors are sometimes difficult to disentangle but present a sufficient margin for there to be no need to go against the independent geological evidence unknown geological events cannot be ruled out but their existence cannot be inferred from the fission track data alone much less can the nature or magnitude of such events be specified

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the post variscan thermal and denudational history of ireland the thermal and denudational history of ireland is evaluated using an extensive new apatite fission track aft dataset derived from surface samples modelled thermal histories are used to construct maps of denudation for a number of time slices from triassic time to 10 ma using a time dependent palaeogeotherm the maps illustrate the spatial variability of denudation and subsidence within each time slice the patterns of denudation are complex showing considerable variability at the length scale of 101 102 km with especially high denudation rates found over known igneous centres such as the mournes of county down based on the onshore aft data alone there is no definitive signature of an irish sea dome extending significantly across ireland in early tertiary time the cumulative amount of denudation during tertiary time varies depending on the aft annealing model used but is generally in the region between 1 and 2 km and without clear spatial trends high amounts of denudation have been mapped over the tertiary intrusions in county down reflecting their unroofing since emplacement in paleocene time the cumulative denudation from triassic time to 10 ma shows relatively low amounts of denudation 2 km in the irish midlands and the extreme ne of the island consistent with the observation that mesozoic tertiary sediments and igneous products are preserved in the ulster basin the western flank of ireland and the region between dublin and county down show high cumulative amounts of denudation 4 km the latter being consistent with the high amounts of denudation interpreted for the irish sea region this denudation pattern explains in part the outcrop of precambrian and lower palaeozoic rocks in these areas the spatial integration of the denudation over the entire landmass gives the average denudation rate and the sediment discharge from ireland as a function of time average denudation rates are moderately high in triassic time falling to low values in cretaceous time and increasing substantially in tertiary time however the total volumetric discharge of sediment in tertiary time is an order of magnitude smaller than the preserved solid volume of tertiary sediment in the basins offshore western ireland

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

comment on experimental evidence for the pressure dependence of fission track annealing in apatite by a s wendt et al earth planet sci lett 201 2002 593 607 abstract the pressure experiments reported by wendt et al are cast in such a way as to question the fundamental basis underpinning apatite fission track thermochronology implying the possibility of a large systematic error in previous interpretations we find however that the study is severely compromised by fundamental errors in both experimental design and execution as well as lacking in consideration of a substantial body of previous work in the area of fission track annealing studies wendt et al have not attempted to extrapolate the results of their experiments to geologically relevant heating times and temperatures this has been the fundamental test that all previous annealing models have had to pass their study clearly fails this test moreover if significant pressure dependence does exist then it is implicit in existing deep borehole consistent annealing models whilst such models are interpolated in terms of temperature the borehole test means that they also accommodate any other factors which correlate with temperature including pressure the implications of the results reported by wendt et al have been overstated far from being intimidating we find that they have little relevance either to previous studies of apatite fission track annealing and extrapolations based on them or to the routine application of fission track analysis to elucidate thermal history information under geological conditions

fast rise times and the physical mechanism of deep earthquakes earthquakes at depths of 300 km are similar to shallower events in that they are dominantly of double couple character1 implying that shearing motion has taken place at depth but because increased friction at these high pressures inhibits brittle fracture2 3 various other mechanisms related to phase transformations have been invoked to explain the occurrence of deep earthquakes4 10 as yet however no consistent differences have been found between the source characteristics of deep 300 km and intermediate depth 300km earthquakes2 11 13 here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes the rise time is defined as the time from rupture initiation to peak moment release rate when the rise times are scaled to the seismic moment release of the events their average is nearly twice as fast for events deeper than 450 km as for shallower events this difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity

aftershock sequences of intermediate depth earthquakes in the pamir hindu kush seismic zone we examine the space time seismicity patterns associated with the 40 largest m 5 6 intermediate depth earthquakes that occurred in the pamir hindu kush zone between 1962 and 1984 to determine if any of these events were followed by aftershock sequences to address this issue we examined data from the soviet central asia regional earthquake catalog with two complementary techniques 1 a subjective technique based on inspection of epicentral maps and 2 a quantitative method which assumed that the background seismicity was a poisson process and which detected aftershocks as significant departures as quantified by the poisson model from background rates of the 40 events we examined only three were identified by both methods as having clearly defined aftershock sequences these three events are also remarkably similar in several other ways 1 they occurred at almost the same point in space 2 they have indistinguishable focal mechanisms 3 they have similar magnitudes and 4 they define two nearly equal time intervals 9 37 and 9 41 years no other events in the pamir hindu kush zone exhibit comparable aftershock behavior six other events show rate anomalies but the subjective method rated them negative or at least ambiguous four of these six we argue are caused by coincidental or man made rate increases at the time of the mainshock the remaining two are noteworthy because they are the two largest events to have occurred beneath the central pamir each of these events shows a small short burst of seismicity around the time period of the largest event in the sequence we suggest two alternative hypotheses to explain our observations 1 more events may have aftershock sequences but the aftershocks are too small to be detected or 2 the three events that had clear aftershocks are unique and trigger a burst of aftershocks only because they have a special relationship to the the most active nest of activity in the hindu kush zone

double seismic zone for deep earthquakes in the izu bonin subduction zone a double seismic zone for deep earthquakes was found in the izu bonin region an analysis of sp converted phases confirms that the deep seismic zone consists of two layers separated by 20 kilometers numerical modeling of the thermal structure implies that the hypocenters are located along isotherms of 500 to 550 c which is consistent with the hypothesis that deep earthquakes result from the phase transition of metastable olivine to a high pressure phase in the subducting slab

a deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes a distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1 2 here we report the first extensive deep earthquake aftershock sequence to be observed it was recorded by an array of eight broadband seismographs following the 9 march 1994 deep tonga earthquake the aftershocks show a power law decay with time following the main shock as is typical of shallow events most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main shock mechanism and the mechanisms of three large aftershocks assuming these aftershocks denote the main shock rupture area they define a 50 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region the width of the aftershock zone is wider than the expected width of the metastable olivine wedge demonstrating that either the width of the metastable olivine material exceeds previous estimates or the aftershocks are not confined in such a wedge

localized polymorphic phase transformations in high pressure faults and applications to the physical mechanism of deep earthquakes earthquake mechanisms based on frictional instabilities are widely accepted for relatively shallow earthquakes such mechanisms in unmodified form are highly unlikely for deep earthquakes because of the expected high frictional shear stresses at depths of 300 700 km several alternative mechanisms have been proposed here i discuss a new type of shear instability observed in experiments on ordinary polycrystalline ice and polycrystalline tremolite by my colleagues and me these shear instabilities lead to rapid stress drops and sound emissions and have the following characteristics 1 a single sharp shear zone forms in the maximum shear stress orientation 2 confining pressure does not suppress the instability and under some conditions raising confining pressure pc lowers the differential stress at which it occurs 3 stress drops can occur repeatedly after reloading at about the same differential stress 4 the mean stress vs temperature conditions at which the instabilities occur in ice and tremolite place them in the fields of thermodynamic stability of ice ii and diopside talc respectively although i have not as yet confirmed the existence of these high pressure phases in the shear zones transformation in the shear zone is implicated because such shear instabilities do not occur in similar materials in which polymorphic phase changes do not occur reconstructive polymorphic phase changes localized along shear zones may be a general phenomenon in crystalline materials stressed non hydrostatically near transition boundaries moreover by analogy the reconstructive phase transformations that are thought to occur in deeply subducted lithosphere may lead to localized shear instabilities manifested by deep earthquakes and produce the observed double couple first motion distribution finally if this is the earthquake source mechanism in deeply subducted lithosphere then earthquakes should shut off when all of the polymorphic phase changes have occurred this may explain the maximum depth of earthquakes at about 680 km

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

experimental evidence for the pressure dependence of fission track annealing in apatite abstract fission track analysis has seen a major expansion in application to general geological problems reflecting its advances in understanding the temperature dependence of track annealing and track length distributions however considerable uncertainties still persist in particular concerning the stability of fission tracks subjected to the interaction of environmental physical parameters e g pressure temperature stress and in extrapolation of laboratory data to geological time scales in this work we studied the fading behavior of spontaneous fission tracks in basic apatite hexagonal ca5 po4 3 oh f cl when exposed simultaneously to laboratory pressures temperatures and stress over varying time spans the experiments showed that track fading is a complex recovery mechanism which is extremely sensitive to the coupling of these three parameters in particular a strong decrease in the fission track fading rate was observed as a function of increasing pressure and a nearly temperature independent dramatic increase in fission track recovery was observed as a function of stress consequently 1 the stability field of fission tracks in apatite increases towards temperatures higher than 110 c depending on the absolute pressure 2 closure ages in apatite are underestimated 100 for an ideal geothermobarometric gradient 3 related exhumation and erosion rates are overestimated above the closure temperature and underestimated below the closure temperature and 4 since the widely applied statistical description of thermally induced fading kinetics does not account for the influences of either pressure or stress and is based on fission track annealing data produced at ambient pressure the accuracy in extrapolating fission track data to geological time scales and in their application to dynamical systems must be cast into doubt

variation in apatite fission track length measurement implications for thermal history modelling abstract predictive thermal history modelling using apatite fission track ft data is dependent on an algorithm to describe the time and temperature dependency of ft annealing which in turn relies on the empirical determination of ft length as a measure of the annealing process assessment of variation in ft length measurement is poorly described with few comparisons between analysts and little interlaboratory standardisation using apatites of various compositions containing induced tracks annealed to differing degrees this study has assessed variation in horizontal confined track length measurement for a variety of procedural conditions replicate analysis by a single observer is typically within 3 but increases inversely with track length comparison between observers on the same samples shows significant generally nonsystematic variation between observers for a complex length distribution variation is 12 sources of variation are identified as a variation from track revelation including etching track in track tint vs track in cleavage tincle measurement and use of 252 cf irradiation to produce additional etching channels b bias in measurement including equipment analytical procedures and sample size and c observer bias principally differences in and consistency of personal technique 5 m hno 3 is preferred to weaker etchants although more anisotropic tracks are better defined permitting more rigorous measurement while c axis parallel sections where 2 4 geometry is better defined are more easily identified for all but the longest length distributions tincles are significantly longer than tints with few short tincles at high angles measurement of tincles effectively masks the anisotropy of annealing 252 cf irradiation is effective in increasing the number of tints sampled and measured variation between values measured for unirradiated and cf irradiated aliquots does not exceed that found for a single analyst although a slight systematic shift to longer lengths for cf irradiated samples is seen as reported by other workers track length distributions are anisotropic anisotropy increasing with annealing level track angle exerts a major influence on measured length summing affects from annealing and etching anisotropies with observer bias track angle should be accommodated within the annealing algorithm it is recommended that similar track revelation observation and measurement conditions are used for the analysis of field samples as are used in annealing experiments and subsequently employed in numerical models to predict thermal history a parallel argument can be advanced for using samples of similar composition further we recommend that the ft community should seek as a matter of some urgency a programme of interlaboratory comparison of track length measurement using standard apatite samples containing artificial length distributions typifying various levels of complexity such comparisons would provide a more rigorous baseline for thermal history prediction in geological case studies

the effect of long term low temperature exposure on apatite fission track stability a natural annealing experiment in the deep ocean abstract since studies on deep sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing due to sluggish annealing kinetics this effect cannot be quantified by laboratory annealing experiments the unknown amount of low temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data particularly for samples which experienced slow cooling in shallow crustal levels to further elucidate these uncertainties we studied volcanogenic sediments from five deep sea drill cores that were exposed to maximum temperatures between 10 and 70 c over geological time scales of 15 120 ma mean track lengths mtl and etch pit diameters dpar of all samples were measured and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis thermal histories of the sampled sites were independently reconstructed based on vitrinite reflectance measurements and or 1d numerical modelling these reconstructions were used to test the most widely used annealing models for their ability to predict low temperature annealing our results show that long term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11 both chlorine content and dpar values explain the downhole annealing patterns equally well low chlorine apatite from one drill core revealed a systematic relation between si content and dpar value the question whether si substitution in apatite has direct and systematic effects on annealing properties however cannot be addressed by our data for samples which remained at temperatures 30 c the predictions of the laslett et al 1987 model agree with the measured mtl data within error levels with few exceptions predictions by the ketcham et al ketcham r donelick r and carlson w 1999 variability of apatite fission track annealing kinetics iii extrapolation to geological time scales am mineral 84 9 1235 1255 annealing model are consistent with the measured data for samples which remained at temperatures below 30 c for samples which experienced maximum temperatures between 30 and 70 c and which are rich in chlorine the ketcham et al 1999 model overestimates track annealing

a bayesian approach to calibrating apatite fission track annealing models for laboratory and geological timescales abstract we present a new approach for modelling annealing of fission tracks in apatite aiming to address various problems with existing models we cast the model in a fully bayesian context which allows us explicitly to deal with data and parameter uncertainties and correlations and also to deal with the predictive uncertainties we focus on a well known annealing algorithm laslett g m green p f duddy i r gleadow a j w 1987 thermal annealing of fission tracks in apatite 2 a quantitative analysis chem geol 65 1 1 13 and build a hierachical bayesian model to incorporate both laboratory and geological timescale data as direct constraints relative to the original model calibration we find a better in terms of likelihood model conditioned just on the reported laboratory data we then include the uncertainty on the temperatures recorded during the laboratory annealing experiments we again find a better model but the predictive uncertainty when extrapolated to geological timescales is increased due to the uncertainty on the laboratory temperatures finally we explictly include a data set vrolijk p donelick r a quenq j cloos m 1992 testing models of fission track annealing in apatite in a simple thermal setting site 800 leg 129 in larson r lancelet y eds proceedings of the ocean drilling program scientific results vol 129 pp 169 176 which provides low temperature geological timescale constraints for the model calibration when combined with the laboratory data we find a model which satisfies both the low temperature and high temperature geological timescale benchmarks although the fit to the original laboratory data is degraded however when extrapolated to geological timescales this combined model significantly reduces the well known rapid recent cooling artifact found in many published thermal models for geological samples

spatial patterns of aftershocks of deep focus earthquakes this paper presents precise relative relocations for aftershocks of 59 intermediate and deep earthquakes using p pp and pkp arrival times read by us or reported in the literature these aftershocks included 36 rupture subevents occurring within 1 min of the initial event and 71 true aftershocks occurring later and identified statistically as being related to the initial event we compare the relative locations with the orientations of the wadati benioff zones and with the available focal mechanisms using statistical methods not previously applied to earthquake data surprisingly neither true aftershocks nor rupture subevents cluster along the nodal planes of intermediate or deep earthquake focal mechanisms aftershocks more than 20 km from the initial events occur preferentially in the plane of the wadati benioff zone while those lying closer have isotropically distributed directions rupture subevents occur after the travel time of the s wave from the initial event larger magnitude earthquakes generally possess rupture subevents lying farther from the initial event whereas true aftershocks can occur 50 km or more from initial events having magnitudes as small as 5 3 except following a few unusually large earthquakes with focal depths of about 100 km we find no rupture subevents or true aftershocks more than about 80 km from the initial hypocenter the existence of many aftershocks far from nodal planes does not favor models in which deep earthquake failure is simply slip along a planar fault rather the aftershocks may occur in response to a general redistribution of stress caused either by the occurrence of the initial event or possibly by nearby continuing phase transitions alternatively the failure zones of deep earthquakes may be surfaces which are frequently curved by 40 or more from the orientation of the initial nodal plane perhaps due to inhomogeneous stress distributions

exhumation and relief development in the pelvoux and dora maira massifs western alps assessed by spectral analysis and inversion of thermochronological age transects 1 we have used dedicated sampling and analysis of apatite fission track aft and apatite u th he ahe thermochronological data sets in an attempt to quantify relief evolution and exhumation rates in the pelvoux and dora maira massifs western european alps a dual approach comparing spectral analysis and thermal kinematic model inversion was applied we sampled age elevation relationships at a range of topographic wavelengths along two north south transects crossing these massifs for the 40 km long pelvoux transect we report 35 new aft ages that range between 3 0 0 4 ma and 12 6 1 0 ma and 8 new ahe ages between 3 5 1 5 and 4 7 0 7 ma the dora maira transect spans a distance of 60 km and includes 28 23 new and 5 previously published significantly older aft ages which vary between 13 1 0 2 and 27 3 0 3 ma inferred exhumation rates of 0 7 km m y 1 since 8 ma in the pelvoux and only 0 1 km m y 1since 20 ma in dora maira are consistent between methods as well as with previous estimates although they are associated with large uncertainties due to imperfect sampling and analytical errors neither massif displays clear evidence for relief change during the intervals constrained by the data data from the dora maria massif suggest moderate recent relief increase whereas the results from the pelvoux are inconclusive but could imply relief decrease our study highlights the difficulties of applying thermochronology techniques to constrain relief changes we show that an unreasonable number of samples is needed to perform reliable spectral analysis of age elevation transects and discuss the overall limitations of both techniques to address relief evolution we suggest that a more efficient approach would be to apply thermal kinematic model inversion to relatively small study areas using spatially distributed sampling and multiple thermochronometer analyses per sample

assessing quaternary reactivation of the main central thrust zone central nepal himalaya new thermochronologic data and numerical modeling we study the recent dynamics of the central nepal himalaya focusing on possible reactivation of the footwall of the main central thrust which is marked by an abrupt topographic transition different tectonic mechanisms such as overthrusting of a major crustal ramp underplating or out of sequence thrusting have been suggested to explain the morphology and exhumation patterns in this area we present 25 new apatite fission track ages collected along a north south transect in central nepal as well as two age elevation profiles ages are consistently younger than 3 ma old in the main central thrust zone and increase continuously to 4 6 ma old in the south no jump in apatite fission track ages is observed across the topographic transition apparent exhumation rates from age elevation relationships vary from 0 46 0 13 0 09 km ma in the palung granite south of kathmandu to 4 4 4 8 1 5 km ma in the main central thrust zone the latter rate is probably overestimated by a factor of two due to topographic effects as shown by a new numerical model these strongly varying exhumation rates can be explained by overthrusting of a crustal ramp which exerts a primary control on age patterns and do not require out of sequence reactivation of thrusts in the main central thrust zone

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

on apatite fission track dating and the tertiary evolution of west greenland topography abstract combined with geological information apatite fission track aft data can impose valid thermal and temporal constraints however their uncritical interpretation may also lead to unsupported conclusions because fission tracks in apatite undergo measurable shortening even at surface temperatures model artefacts can potentially be mistaken for geological cooling events this problem can be especially acute when older fanning kinetic models are utilized instead of the newest curvilinear ones in any circumstances aft model results must be interpreted with considerable caution and should rarely be considered to supersede conventional geological wisdom the possibility that young cooling events are simply artefacts ought always to be entertained as an example this study considers aft data and model results that have been cited to suggest that today9s high mountains of west greenland are erosional remnants of a landmass uplifted during the neogene however in the best of circumstances the aft data in question are heavily contaminated and in the worst case they carry virtually no geological meaning although geological constraints require the faulted westward facing escarpment of west greenland9s disko region to have been erected after commencement of sea floor spreading in baffin bay the aft model based hypothesis that it was constructed in purely neogene time remains an unproven speculation

inversion of thermochronological age elevation profiles to extract independent estimates of denudation and relief history ii application to the french western alps abstract thermochronologic data collected along age elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates however thermochronologic age elevation relationships are known to be perturbed by topographic effects and potential changes in relief which are neglected in the inherently 1 d interpretation commonly applied such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging in a companion paper we develop a methodology for rigorously interpreting thermochronologic age elevation profiles in terms of exhumation rates and relief development through time and to independently quantify the resolution of these constraints here we test this approach using a thermochronological dataset consisting of apatite and zircon fission track and u th he data collected at la meije peak in the pelvoux ecrins massif french western alps our data and models suggest a three phase exhumation history in the pelvoux ecrins massif including a pulse of rapid exhumation at 6 5 5 ma preceded and followed by more moderate rates of denudation in the order of 0 3 0 4 km myr 1 this rapid exhumation event appears to occur coevally in other external crystalline massifs in the alps but is not detected by qualitative inspection of the age elevation relationships both our synthetic results and inversion of the la meije data strongly suggest that apatite fission track age elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required combining apatite fission track and u th he ages and particularly including fission track length data greatly improves the resolution of the inferred exhumation histories although denudation rates through time and the timing of rate changes are generally well resolved our data have insufficient resolution to satisfactorily constrain the relief history synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data

seismological evidence for metastable olivine inside a subducting slab the nature and location of the olivine spinel phase transition inside subducting slabs differ greatly from the situation in the surrounding mantle due to the different temperature distribution inside the slabs two models have been proposed for this phase transition one in which the location of the phase boundary between olivine and modified phase spinel is determined by equilibrium thermodynamics1 and the other including a metastable olivine phase which persists to a depth of 550 km ref 2 the location of the olivine spinel transition in the slab may be relevant to the generation of deep earthquakes3 5 and to the buoyancy forces driving subduction6 here we use travel time residuals from deep earthquakes recorded by the dense seismograph network in japan to investigate the configuration of the olivine spinel phase boundary inside the subducting pacific plate theoretical travel time residuals for the equilibrium model do not fit the observed residuals whereas those for the metastable model do implying the presence of metastable olivine inside the subducting slab

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

single link cluster analysis of earthquake aftershocks decay laws and regional variations using single link cluster analysis the authors investigate how various properties of aftershock sequences depend on their tectonic regime and focal depth for international seismological centre earthquakes of m sub b ge 4 8 they find that earthquakes deeper than 70 km have the fewest and smallest aftershock sequences even after accounting for differences in detectability and maximum magnitude they find that ridge transform earthquakes have smaller aftershock sequences that shallow subduction zone earthquakes among different subduction zones they find that zones with high moment release rates possess larger aftershock sequences comparing ridge transform zones they find those with slower spreading rates possess larger aftershock sequences by transposing origin times of several different aftershock sequences as if all had main shocks occurring at time zero they are able to study the properties of aftershock sequences which individually have too few aftershocks to study by other means secondary events determined by single link cluster analysis follow a modified omori s power law decay for time separations of 0 1 day to 20 days from the parent event with p values ranging from 0 539 plus minus 0 022 intermediate and deep focus earthquakes to 0 928 plus minus 0 024 ridge transform earthquakes they find that earthquake foreshocks and multiplets also follow a modified omori s law atmore greater times from the main shock the decay is steeper than a power law decay more like an exponential decay aftershocks in the adak catalog m sub b ge 2 0 show a marked decrease in activity between 40 and 50 km depth they speculate that the observed differences in number of aftershocks and p values may be caused by variations in fault heterogeneity or in fluid pressures less

denudation history of the continental margin of western peninsular india since the early mesozoic reconciling apatite fission track data with geomorphology a comprehensive apatite fission track aft study of the passive margin of western peninsular india between 12 and 16 n is used to reconstruct the denudation chronology of the continental hinterland in common with other rifted margins the morphology is characterised by a low lying coastal plateau separated from an elevated inland plateau by an erosionally controlled escarpment western ghats we modelled the fission track data using the commonly adopted annealing algorithm of laslett et al chem geol 65 1987 1 13 using the default parametrisation i e an initial track length of 16 3 m it was found that during the mesozoic denudation rates remained extremely low increasing sporadically when erosion peaked at 130 ma rifting with antarctica and 80 ma rifting with madagascar denudation rates rose considerably during the cenozoic reaching maxima of ca 120 m myr such values are however considered as major overestimates and the effects of the seychelles rifting at 65 ma remain suspiciously unrecorded we explored the consequences of changing the initial track length in this model to a value of 14 5 m in practice this reduces the rapid cenozoic denudation artefact model peak rates during the mesozoic are much more variable and during the cenozoic reach values an order of magnitude lower than with the original initial track length the response to the seychelles rifting event is almost immediate just as previous model calibrations in aft analysis have been relatively empirical this revised approach does not provide insights into the physical mechanisms of low temperature annealing however it is shown to agree much better with independently established geomorphic cosmogenic stratigraphic and tectono magmatic evidence in this and other stable shield regions in terms of both the timing and the magnitude of geological events and the geomorphic response of the landscape to them

contrasting lipid biomarker composition of terrestrial organic matter exported from across the eurasian arctic by the five great russian arctic rivers contrasting lipid biomarker composition of terrestrial organic matter exported from across the eurasian arctic by the five great russian arctic rivers

inversion of thermochronological age elevation profiles to extract independent estimates of denudation and relief history i theory and conceptual model abstract we determine to what extent low temperature thermochronology data in particular from age elevation profiles provide independent and quantitative estimates on denudation rates and relief development thermochronological age elevation profiles have been widely used to infer exhumation histories however their interpretation has remained inherently one dimensional neglecting potential effects of lateral offsets between samples furthermore the potential effects of transient topography on crustal isotherms and consequently on thermochronological data have not yet been addressed in detail we investigate this problem with the aim of deriving independent estimates of both denudation rates and relief history from low temperature thermochronometers measuring the relative uncertainties on these parameters and finally constraining the timing of potential variations in denudation rate and or relief development we adopt a non linear inversion method combining the three dimensional thermal kinematic model pecube which predicts thermal histories and thermochronological ages from an input denudation and relief history with an inversion scheme based on the neighbourhood algorithm we use synthetic data predicted from imposed denudation and relief histories and quantitatively assess the resolution of thermochronological data collected along an age elevation profile our results show that apatite fission track aft ages alone do not provide sufficient quantitative information to independently constrain denudation and relief histories multiple thermochronometers apatite u th he ahe ages and or track length measurements combined with aft ages are generally successful in constraining denudation rates and timing of rate changes the optimum combination of thermochronometers varying with the input scenario relief change or varying denudation rates however relief changes can only be quantified and precisely constrained from thermochronological age elevation profiles if the rate of relief growth is at least 2 3 times higher than the background denudation rate this limited resolution is due to the depth of the closure isotherm between 70 and 110 c for the aft and ahe systems which only partly record topographic change new thermochronometers such as 4 he 3 he or osl that are sensitive to lower temperatures may be the key for resolving this issue

seismotectonics of intermediate depth earthquakes from properties of aftershock sequences abstract aftershock sequences following two intermediate depth earthquakes have been analyzed to determine their spatial and temporal distributions the kanaga pass earthquake m b 6 0 occurred at 105 km depth in the central aleutian islands data were provided by the central aleutians seismic network the iwaizumi town earthquake m b 6 4 occurred at 75 km depth under northern honshu japan data were provided by the network of tohoku university for both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others with down dip compression for both both events were followed by numerous aftershocks that decayed with time initially according to the modified omori relation with p value close to 1 in both cases activity at a higher rate than that expected from the omori relation began after several months without a strong aftershock to trigger it the kanaga pass activity is modeled by two omori sequences the second beginning 64 days after the mainshock the p values are 0 92 and 1 04 respectively the corresponding b values are 1 01 and 1 03 the iwaizumi town sequence is modeled by a single omori sequence p 0 96 plus a constant rate of activity 0 25 events day for 63 days beginning 120 5 days after the mainshock the b value of the main sequence was 0 91 0 14 up to the start of the linear surge for which the b value was 1 23 0 47 no strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity in either case the cause of the secondary activity is not known the two sequences occurred in subduction zones with similar properties and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes a strong creep event or silent earthquake in a weakly coupled interplate contact zone is a plausible explanation for the secondary sequences the unusual setting of the iwaizumi event at the bottom of the upper plane of the double planed wadati benioff zone under honshu may play a role in that case

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

evaluating balanced section restoration with thermochronology data a case study from the central pyrenees we present a new method that can be used to quantitatively evaluate the consistency between balanced section restorations and thermochronological data sets from orogenic belts we have applied our method to a crustal scale area balanced cross section restoration along a profile in the central pyrenees this restoration is well constrained and supported by a wide variety of geological and geophysical data moreover an extensive thermochronological data set has been collected independently in the area we use the structural kinematic software 2d move to constrain a set of velocity fields that describes the kinematics of the central pyrenees using these velocity fields as input for the thermokinematic code pecube we derive predictions of the thermal history and a range of thermochronometric ages for the modeled area we find that the kinematic history of the belt as inferred from section balancing is in good agreement with the published thermochronological data high temperature zircon fission track and k feldspar ar ar data constrain the thermal structure of the belt as well as the timing of underplating low temperature apatite fission track and u th he data require late synorogenic sedimentary burial of the southern flank of the pyrenees between late eocene 40 ma to late miocene 9 ma times consistent with previous studies and imply that no such burial occurred on the northern flank

a fission track transect across nares strait canada greenland further evidence that the wegener fault is a myth many workers continue to model nares strait between greenland and ellesmere island canada as a plate boundary and locus of major cenozoic compression and strike slip marked by the postulated wegener transform fault this despite continuity of precambrian paleozoic provinces across the seaway including an undeformed neoproterozoic mafic dyke swarm that crosses at a high angle as a test of the speculative plate kinematic modelling apatite fission track aft dating was undertaken along a transect across the paleoproterozoic shield of smith sound at the south end of nares strait running from sea level to ca 600 m elevation and as much as 70 km inland on both sides the shield is overlain by three sedimentary basins separated by major hiatuses the aft ages fall into two groups 282 34 and 650 51 ma they indicate maximum depth was 4 km in the lower paleozoic from both coasts to 50 km inland and at 50 km inland temperatures were below the annealing interval throughout phanerozoic time durin

anticrack associated faulting at very high pressure in natural olivine shallow earthquakes are produced by brittle shear fracture of rock and or fictional sliding on pre existing fault surfaces1 at very high pressures however brittle fracture and frictional sliding are impossible because frictional resistance to movement on any potential fault surface is so great that ductile stress relief processes accomodate strain at lower shear stresses than those necessary to activate faulting nevertheless more than 20 of earthquakes with magnitudes greater than five occur at depths greater than 300km where the pressure exceeds l0 gpa ref 2 possible explanations for this paradox have been offered by several recent experimental studies of shearing instabilities associated with phase transformations at high 3 8 one of these mechanisms is associated with anticrack development during the olivine spinel phase transformation in mg2geo4 at pressures of 1 2 gpa refs 4 6 it is particularly attractive because it operates between mineral structures known to occur in the mantle we show here that this mechanism also can operate in natural silicate olivine mg fe 2sio4 during onset of the transformation at the much higher pressures at which deep earthquakes occur these results lend strong support to the hypothesis that the anticrack mechanism is responsible for such earthquakes4

variable late neogene exhumation of the central european alps low temperature thermochronology from the aar massif switzerland and the lepontine dome italy 1 several recent studies proposed an important increase in exhumation rate in the western european alps since circa 5 4 ma in order to assess potential spatial differences in exhumation histories we present new apatite fission track aft and apatite u th he ahe ages from the central aar massif guttannen area switzerland and the western lepontine dome formazza area italy internal u th zoning in apatites explains alpha ejection corrected ahe ages that are older than the corresponding aft ages in this study a qualitative interpretation of aft and ahe age elevation relationships suggests a two phase 9 7 and 5 3 ma exhumation scenario affecting the central alps with a stronger expression of the pliocene signal in the formazza area however a quantitative evaluation of exhumation scenarios using the 3 d heat equation solver pecube highlights the existence of several other likely scenarios casting doubt on the validity of a qualitative interpretation of the age elevation relationships in formazza scenarios suggested by quantitative modeling include continuous denudation at a rate of 750 m ma and a one step exhumation rate change from 300 to 1000 m ma at 5 ma in guttannen they include continuous denudation at a rate of 400 m ma with valley deepening and two periods of higher exhumation rate increasing from 300 to 700 m ma repeatedly at 9 7 and at 5 3 ma contingent upon further flexural isostatic modeling the magnitude of exhumation recorded in the axial region of the alps since circa 5 ma does not appear sufficient to solely explain the denudation recorded in the north alpine foreland basin

preservation of contrasting geothermal gradients across the caribbean north america plate boundary motagua fault guatemala 1 strike slip plate boundaries juxtapose crustal blocks that may have different geodynamic origins and therefore different thermal structures thermo kinematic modeling of this type of strike slip plate boundary predicts an asymmetric signature in the low temperature thermochronologic record across the fault age elevation profiles of zircon u th he ages across the motagua fault a 500 km long segment of the transform boundary between the north american and caribbean plates document a sharp cooling age discontinuity across the fault this discontinuity could be interpreted as a difference in denudation history on each side of the fault however a low relief miocene erosional surface extends across the fault this surface has been uplifted and incised and provides a geomorphic argument against differential denudation across the fault by integrating magmatic volcanic and heat flow data age elevation profiles and thermo kinematic modeling we propose that large horizontal displacement along the motagua fault has offset a persistent geothermal asymmetry across the fault and explains both the age discontinuities and the age elevation patterns this study illustrates how thermochronology can be used to detect large strike slip displacements and more generally opens new perspectives in understanding the impact of nonuniform thermal structures on thermochronologic results

interpreting data dispersion and inverted dates in apatite u th he and fission track datasets an example from the us midcontinent abstract new apatite u th he ahe and apatite fission track aft data were acquired for cratonic basement samples from an 80 m span of drillcore in northeastern kansas the short depth interval over which the samples were collected indicates that they should have undergone thermal histories that would be indistinguishable using low temperature thermochronometry techniques individual ahe dates from four samples range from 99 to 464 ma three samples yield dates 300 ma higher eu 39 113 ppm and substantial data dispersion uncorrelated with eu and grain size these apatites have strong and variable eu zonation aft dates for five samples range from 242 to 291 ma the sample with the highest eu apatites and oldest ahe dates yields the youngest aft results these results are inverted with ahe dates distinctly older than the corresponding aft date we explore both the causes of data dispersion and the overall compatibility of this cratonic dataset we find that geologically reasonable thermal histories can 1 explain the distribution of the moderate eu ahe data when accounting for the influence of radiation damage grain size and eu zonation on apatite he diffusivity 2 reproduce the observed dispersion in the high eu ahe data when using a viable range of eu zonation and grain size and 3 explain the aft data for the same samples the ahe and aft data are mutually consistent and viable thermal histories successfully predict the observed pattern of older ahe than aft dates for the high eu apatites together these results suggest that appropriately accounting for the known controls on apatite he diffusivity can explain the observed dispersion and inverted ahe and aft results in some thermochronometry datasets a range of ahe dates should be especially common in cratonic data because small differences in apatite he diffusivity are amplified by the thermal histories that typify cratonic settings we use these results to develop some guidelines for interpreting dispersed ahe datasets first date eu and date grain size correlations should be evaluated and if these patterns occur they can be used to better resolve the thermal history second for samples that yield inexplicably large dispersion of ahe dates uncorrelated with eu and crystal size the appropriate strategy is either to reject these samples from the suite used for thermal history interpretation or to acquire additional data to help decipher the significance of the age distribution

low temperature thermochronology in the peruvian central andes implications for long term continental denudation timing of plateau uplift canyon incision and lithosphere dynamics abstract in peru the western edge of the 4 5 km high western cordillera is cut by a 3 km deep canyon to understand incision by the cotahuasi ocona river and the regional uplift history of this orogenic plateau capped by volcanic rocks 26 crystalline rock samples were collected for low temperature thermochronology from vertical profiles parallel and perpendicular to the canyon rock cooling histories confirm that most plateau denudation had occurred prior to 24 ma but plateau incision peaked after c 14 9 ma in response to rapid surface uplift the abrupt occurrence of a rock heating event is also detected during middle miocene time this was either a response to the emplacement of low conductivity regionally extensive ignimbritic caprock or a response to crustal scale fluid circulation caused by wet melting of the overriding plate when magmatism resumed c 24 ma the potential for thermochronology to provide information on past geothermal gradients is discussed showing how it can be used as a proxy for understanding change in subducting slab dynamics with oscillations in subduction angle having perhaps been the main on off switch for magmatism in this cordilleran setting supplementary material fission track and apatite u th he results and apatite chemistry indicators are available at http www geolsoc org uk sup18405