ductile fabrics in the zone of active oblique convergence near the alpine fault new zealand identifying the neotectonic overprint the mid crustal alpine schist in central southern alps new zealand has been exhumed during the past similar to3 m y on the hanging wall of the oblique slip alpine fault these rocks underwent ductile deformation during their passage through the similar to 150 km wide pacific australia plate boundary zone likely to be cretaceous in age peak metamorphism predates the largely pliocene and younger oblique convergence that continues to uplift the southern alps today late cenozoic ductile deformation constructively reinforced a pre existing fabric that was well oriented to accommodate a dextral transpressive overprint quartz microstructures below a recently exhumed brittle ductile transition zone reflect a late cenozoic increment of ductile strain that was distributed across deeper levels of the pacific plate deformation was transpressive including a dextral normal shear component that bends and rotates a delaminated panel of pacific plate crust onto the oblique footwall ramp of the alpine fault progressive ductile shear in mylonites at the base of the pacific plate overprints earlier fabrics in a dextral reverse sense a deformation that accompanies translation of the schists up the alpine fault ductile shear along that structure affects not only the 12 km thick section of alpine mylonites but is distributed across several kilometres of overlying nonmylonitic rocks c 2001 elsevier science ltd all rights reserved

a comparison of equations commonly used to calculate organic carbon content and marine palaeoproductivity from sediment data abstract a comparison is made of different methods for calculating total organic carbon toc in deposits and marine primary productivity using sediment data the calculation methods include both equations derived by pure regression analysis and equations that include descriptions of processes such as carbon flux and burial efficiency the results are compared with present day measured values of total organic carbon and marine primary productivity using measured sedimentation rate dry bulk density and water depth these measurements cover a large range of marine depositional environments i e shallow to deep water oxic to anoxic conditions etc both toc and productivity calculations show some consistent over or under prediction of computed values compared with the measured values while some equations lead to unrealistic results toc calculations using process descriptions are influenced more by the choice of burial efficiency equation than by the choice of carbon flux equation productivity calculations are influenced by the choice of carbon flux equation calculations using measured values from a core show that productivity estimates vary by a factor of two

synmetamorphic veining origin of andalusite bearing veins in the vedrette di ries contact aureole eastern alps italy discordant andalusite biotite quartz bearing veins occur in the contact aureole of the vedrette di ries pluton italian eastern alps never outside the area of contact metamorphic andalusite development andalusite veins are found only within andalusite bearing hornfelses and vein biotite occurs wherever host rock garnet is partially replaced by biotite veins formed during contact metamorphism synchronously with the crystallization of andalusite and biotite within host rocks their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid filled cavities that opened by hydraulic fracturing a mechanism of synmetamorphic veining is proposed to explain rock failure and subsequent mineral deposition within veins during hydrofracturing induced by dehydration reactions in response to heating in the aureole fissures were immediately filled with locally derived fluids the lack of large scale flux together with high fluid pressures required by hydrofracturing suggest fluid in the cavities was a virtually stagnant passive medium and that mass transport toward fractures was driven by intergranular diffusion because temperature and pf values within veins are similar to those in the host rock vein assemblages are interpreted as the stable high t side of reactions taking place within pelitic schists at the time when fractures opened once nucleation of product phases occurred chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients since nucleation was favoured at the strained grains of vein walls andalusite and biotite simultaneously grew in vein and host rock the proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of al2sio5 bearing veins in not requiring large fluid rock ratios or a highly aggressive fluid composition the mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium and deep crustal rocks which have undergone extensive devolatilization

brittle failure mode plots for compressional and extensional tectonic regimes abstract equations governing different macroscopic modes of brittle rock failure extensional fracturing extensional shear fracturing compressional shear failure and reshear of existing faults can be represented on plots of differential stress vs effective vertical stress for a set of material properties such plots can be constructed for different tectonic regimes and correlated to depth for particular fluid pressure conditions allowing easy evaluation of the physical controls on brittle rock failure and ready comparison of the fields occupied by the three failure modes in different tectonic settings they emphasize the relative ease in terms of differential stress and fluid pressure levels of deforming a rock mass by brittle fracturing and faulting in extensional regimes compared with compressional aside from their relevance to general structural mechanics these generic failure plots have wide ranging application to understanding the initial development and progressive evolution of fault fracture systems both in sedimentary basins and as hosting structures for hydrothermal mineralization in different tectonic settings

metamorphic and post metamorphic fluid flow in the low grade rocks of the harlech dome north wales a combination of fluid inclusion stable isotope and geochemical techniques has been used to study the nature of fluids present and their behaviour during caledonian low grade metamorphism of the harlech dome north wales fluid inclusion studies show that in most of the metasedimentary sequence the peak metamorphic fluid was an aqueous na k cl brine but in the graphitic clogau formation and in parts of the overlying maentwrog formation immiscible h2o rich and ch4 rich fluids coexisted late stage inclusions are of calcium rich brine and a dilute aqueous fluid the chemical composition of chlorite in metamorphic veins and rocks varies between different formations and quartz oxygen isotopic compositions show considerable variation between different units both of these features are taken to indicate that there was little or no pervasive movement of fluid between different units at the peak of metamorphism after the metamorphic peak there was focused flow of fluid upward through the sequence along fractures in response to end caledonian uplift and unloading where the migrating fluid crossed the graphitic shales interaction between the fluid and the shales gave rise to the formation of the auriferous veins of the dolgellau gold belt subsequent to this mineralizing event there was widespread development of 18o enriched calcites and micas in the case of vein minerals it is possible that these crystallized directly from late stage fluids at lower temperature than the quartz in the same veins alternatively the original vein minerals may have re equilibrated with later 18o enriched or cooler fluid in the case of muscovites in the rock matrix it is proposed that the isotopically heavy compositions are the result of re equilibration of initially light grains with an introduced fluid requiring considerable influx of fluid this event may relate to either of two late stage fluids observed as inclusions

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

esr dating of the wangsan fault south korea abstract the wangsan fault is exposed at kyeongju korea the andesite is unconformably covered by quaternary alternating conglomerate and sandstone deposits the unconformity is cut by the thrust fault which displaces a hanging wall block of about 30 m exposed at the surface of this fault is a light gray and brown fault gouge about 40 120 cm thick in order to test the consistency of esr ages from a single fault gouge zone we collected six gouge samples systematically along and across the wangsan fault we found that six samples collected from the same gouge zone show consistent esr date estimates average 550 ka because fault rock is rare along faults in unconsolidated sediments due to low confining stress near the surface we consider that this fault gouge has been moved up with the hanging wall block along the fault the estimated average uplift rate of the hanging wall block is about 0 04 cm year based on the age of the displaced quaternary deposits vertical separation about 20 m dated by osl dating methods the depth of the fault gouge at the time of reactivation which was estimated from uplift rate and the esr ages is about 220 m therefore we conclude that the results of esr age estimates represent the time of reactivation of the fault gouge at a depth of 220 m in the past during fault movement because later movements which occurred during uplift near the surface may not have zeroed esr signals significantly for esr dating of fault movements

deformation partitioning and porphyroblast rotation in meta morphic rocks a radical reinterpretation abstract most porphyroblasts never rotate during ductile deformation provided they do not intern ally deform during subsequent events with the exception of relatively uncommon but spectacular examples of spiralling garnets instead the sur rounding foliation rotates and reactivates due to partitioning of the deformation around the porphyroblast consequently porphyroblasts commonly preserve the orientation of early foliations and stretching lineations within strain shadows or inclusion trails even where these structures have been rotated or obliterated in the matrix due to subsequent deformation these relationships can be readily used to help develop an understanding of the processes of foliation development and they demonstrate the prominent role of reactivation of old foliations during subse quent deformation they can also be used to determine the deformation history as porphyro blasts only rotate when the deformation cannot partition and involves progressive shearing with no combined bulk shortening component

40ar 39ar muscovite dates from the nappe region of southwestern norway dating extensional deformation in the scandinavian caledonides abstract the caledonian thrust nappes and basal decollement zone above the baltic shield record penetrative caledonian deformation related to ese directed nappe transport and subsequent wnw directed ductile shearing during extensional collapse of the orogen muscovite within proterozoic quartzite conglomerates in the lower bergsdalen nappe that record only ese vergent caledonian deformation display internally concordant 40ar 39ar age spectra which yield plateau ages of 403 ma and 398 ma similar plateau ages 402 and 399 ma were obtained from muscovite from mica schists which record additional penetrative wnw directed shearing suggesting that muscovite intracrystalline argon systems did not cool below appropriate closure temperatures until during or after extensional wnw vergent movements quartz microfabrics indicate that temperatures were higher than those required for argon retention in muscovite until the end of the wnw directed nappe translation extension and that lower temperatures were maintained during subsequent movements along w and nw dipping extensional ductile shear zones the 40ar 39ar muscovite ages are therefore interpreted to post date the extensional wnw directed shearing along the decollement zone and closely date the initiation of w and nw dipping extensional shear zones when compared to the age of contractional deformation the results confirm that extension followed orogenic contraction very shortly after the main scandian orogenic phase

a new hypothesis for organic preservation of burgess shale taxa in the middle cambrian wheeler formation house range utah cambrian konservat lagerstatten are the most significant fossil deposits for our understanding of the initiation of phanerozoic life although many modes of preservation may occur these deposits most frequently contain nonmineralized fossils preserved in the form of kerogenized carbon films a rare yet important taphonomic pathway that has not previously been explained for any unit by a comprehensive model the middle cambrian wheeler formation of utah one of these lagerstatten contains abundant kerogenized preservation of nonmineralized tissues which occurs within a distinctive taphofacies that accumulated under the following conditions 1 domination of the siliciclastic fraction by clay sized particles 2 close proximity to a carbonate platform which resulted in mixed carbonate clay sediments 3 a well developed oxygen minimum precluding benthic colonization and burrowing and 4 relative proximity to oxic bottom waters facilitating transport of organisms from a habitable environment to one that favored their preservation we propose that preservation of nonmineralized tissues in the wheeler formation may have resulted from a combination of influences that reduced permeability and thus lowered oxidant flux which in turn may have restricted microbial decomposition of some nonmineralized tissues those influences include near bottom anoxia preventing sediment irrigation by restriction of bioturbation reducing conditions near the sediment water interface that may have acted to deflocculate aggregations of clay minerals resulting in low permeability face to face contacts early diagenetic pore occluding carbonate cements and an absence of coarse grains such as silt skeletonized microfossils fecal pellets or bioclasts this model may be applicable to kerogenized preservation of macrofossils in other fossil lagerstatten

porphyroblast nucleation growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development in regional metamorphic rocks the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain rate gradients across the boundaries between the partitioned zones these generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain because of their uniquely layered crystal structure however most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients and hence are susceptible to dissolution and solution transfer as a consequence zones of progressive shearing become zones of dissolution of most minerals and of concentration of phyllosilicates and graphite exceptions are mylonites where strain rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing as they would be dissolved by the effects of shearing strain on their boundaries however they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation this includes a build up of stored strain energy relative to zones of progressive shearing in which dissolution is occuring porphyroblast growth is generally syndeformational and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account porphyroblasts in a contact aureole do not grow statically either as microfracturing associated with emplacement allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening the criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning reactivation of deforming foliations and a general lack of rotation of porphyroblasts with the spectacular exception of genuinely spiralling garnet porphyroblasts

significance of high c n ratios in organic carbon rich neogene sediments under the benguela current upwelling system corganic ntotal values change from 5 in middle miocene sediments deposited under the benguela current upwelling system to 15 in latest miocene and pliocene sediments although the change appears to record a larger proportion of land derived material in younger sediments bulk 13corg values indicate a predominance of marine origin for organic matter throughout the sequence marine paleoproductivity as represented by mass accumulation rates of caco3 and organic carbon increased over this time interval we postulate that the larger corganic ntotal values record a shift towards greater recycling rates of nitrogen rich relative to carbon rich organic matter components as the flux of total organic matter to the seafloor increased furthermore we propose that high corganic ntotal values may record enhanced paleoproductivity in other sedimentary sequences dominated by marine organic matter

application of electron spin resonance esr dating to ductile shearing examples from the qinling orogenic belt china abstract shear zones are common structures in orogenic belts and elucidation of the tectonic evolution of these orogenic belts to a large degree depends on understanding the kinematics and timing of shear deformation however there is a lack of an accurate fast and convenient way to determine the timing of deformation in this paper we apply the esr electron spin resonance spectroscopy dating method to three syntectonic quartz veins from the funiushan tectonic belt in the qinling orogenic belt in central china the results agree well with the available ages of deformation in the area obtained through other dating methods this demonstrates the accuracy and feasibility of using the esr method to date quartz crystals formed during deformation the method is fast and convenient and satisfies the accuracy requirement it is an effective means for determining the timing of deformation especially in areas with intensive fluid activity during deformation

electron spin resonance esr dating abstract esr dating has been systematically applied in earth sciences and archaeology since 1975 when ikeya dated a stalactite from the akiyoshi cave in japan in the meantime this new technique has been successfully applied to the dating of materials such as speleothems spring deposited travertines mollusc shells corals and tooth enamel the present research focuses on the dating of quartz which allows the age assessment of heated ceramics volcanic rocks intrafault material or windblown sediments esr dating is based on the measurement of the number of paramagnetic centers in a mineral these centers are generated by alpha beta and gamma radiation of the matural radioelements such as u th and k and have accumulated over time two parameters have to be determined the accumulated dose ad and the dose rate d to which the sample was exposed this review comprises three major parts a introduction into esr dating b application of the method and c appendices giving detailed description of dose rate calculation physical basis of electron spin resonance the esr spectrometer and esr lines the major emphasis lies on the outline of the application of esr dating this includes the discussion of the esr signals of the various materials sampling sample preparation and measurement techniques

yellow river terrace sequences of the gonghe guide section in the northeastern qinghai tibet implications for plateau uplift abstract the uplift of the tibetan plateau was a significant event in terms of global landforms and climate but the exact nature of the uplift process remains contested among geologists in the uplifted inland area of the plateau river terrace formation has been controlled mainly by tectonic uplift which means that analyses of river terraces are an excellent means of investigating the uplift process in this paper we establish the terrace sequence of the yellow river in the gonghe guide section of the northeastern tibetan plateau making use of field investigations gravel analysis and electron spin resonance esr dating terraces t3 t9 of the numbered sequence formed at 0 13 0 18 0 23 0 41 0 85 0 93 and 1 32 ma moving backwards in time and upwards from the valley floor whereas terraces t11 t20 formed at 1 71 1 75 1 88 1 94 2 01 2 12 2 23 2 31 2 36 and 2 47 ma this suggests that the yellow river existed in gonghe and guide basins for at least the last 2 47 ma the incision rates of the yellow river in the gonghe guide section indicate that there were three distinct phases of uplift of the northeastern tibetan plateau occurring at different rates with an average uplift rate of 0 26 mm yr during the quaternary these results support the multi stage uplift model which states that the tibetan plateau has experienced continuous uplift since 8 ma but contradicts both the early uplift theory which holds that the uplift of the tibetan plateau occurred mainly before the pliocene and the notion of a late and rapid uplift of the plateau that began at 3 6 ma

neoproterozoic chuar group 800 742 ma grand canyon a record of cyclic marine deposition during global cooling and supercontinent rifting abstract chuar group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western north america and australia the top of the chuar group is 742 ma thus this succession offers a potentially important record of global climatic and tectonic changes during the mid neoproterozoic a critical time leading to possible global glaciation during supercontinent rifting chuar group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the sturtian ice age 750 700 ma as well as extension related to the dispersal of rodinia the mid neoproterozoic chuar group 1600 m thick is predominantly composed of mudrock with subordinate meter scale beds of dolomite and sandstone facies analysis leads to the interpretation of a wave and tidal influenced marine depositional system diagnostic marine features include 1 marine fossils and high local pyrite content in the mudrock facies 2 mudcracked mud draped symmetric ripples and reverse flow indicators in the sandstone facies 3 facies associations between all facies and 4 no unequivocal terrestrial deposits chuar facies stack into 320 dolomite and sandstone capped meter scale cycles 1 20 m thick and non cyclic intervals of uniform mudrock facies 20 150 m thick nearly all cycles have mudrock bases indicating subtidal water depths dolomite capped cycles shallow to peritidal environments peritidal cycles some indicating subaerial exposure of varying degrees exposure cycles sandstone capped cycles shallow to peritidal environments with no evidence of prolonged exposure peritidal cycles non cyclic intervals represent the deepest water depths 10s of meters peritidal cycles and non cyclic intervals dominate the lower and middle chuar group in the upper chuar group the percentage of exposure cycles increases as does the thickness of individual cycles and non cyclic intervals and the degree of subaerial exposure in the cycle caps correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness some thickness changes are coincident with local and regional extensional structures these cycle trends reflect a resolvable mixture of tectonic and climatic controls cycles are interpreted to be high frequency and glacioeustatically controlled based on comparison of cycle character and thickness to phanerozoic examples and lateral continuity of many cycles respectively cycles in the lower and middle chuar group show characteristics suggestive of low amplitude sea level changes similar to meter scale cycles from global greenhouse climates in the phanerozoic upper chuar group cycles show characteristics suggestive of moderate amplitude sea level changes similar to phanerozoic greenhouse icehouse transition climates indicating an increase in continental ice volume variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence evidenced by thickening trends coincident with active rift related structures and also by variability in sediment supply the presence of dolomite caps in this otherwise siliciclastic succession is interpreted to be due largely to rapid changes in climate related to short term glacioeustatically controlled sea level changes four crude lithostratigraphic sequences 150 775 m thick are defined based on dolomite poor to dolomite rich stratigraphic intervals long term changes in cycle cap lithology may be driven by long term changes in climate similar to short term glacioeustatic controls on meter scale cycle cap lithology large scale thickness variability in these sequences is due to local differential tectonic subsidence

fluid evolution during metamorphism of the otago schist new zealand i evidence from fluid inclusions fluid inclusion salinities from quartz veins in the otago schist new zealand range from 1 0 to 7 3 wt nacl eq in the torlesse terrane and from 0 4 to 3 1 wt nacl eq in the caples terrane homogenization temperatures from these inclusions range from 124 to 350 c with modal values for individual samples ranging from 163 to 229 c but coexisting low salinity inclusions exhibiting metastable ice melting show a narrower range of t h from 86 to 170 c with modes from 116 to 141 c these data have been used in conjunction with chlorite chemistry to suggest trapping conditions of 350 400 c and 4 1 6 0 kbar for inclusions showing metastable melting from lower greenschist facies rocks with the densities of many other inclusions reset at lower pressures during exhumation of the schist the fluid inclusion salinities and br cl ratios from veins from the torlesse terrane are comparable to those of modern sea water and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid some modification of the fluid may have taken place as a result of interaction with halogen bearing minerals and dehydration and hydration reactions the salinity of fluids in the caples terrane is uniformly lower than that of modern sea water and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites the contrast between the two terranes may be a result of the original sedimentary provenance as the torlesse terrane consists mainly of quartzofeldspathic sediments whilst the caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis and hence may provide more fluid via dehydration at higher grades

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

electron spin resonance esr dating of pre quaternary faults in the sichuan basin sw china abstract the electron spin resonance esr dating method is thought to be limited to the quaternary period in this study we attempt to expand the esr dating range to the pre quaternary using three fault zone samples collected from faults in the sichuan basin and its surroundings we adopt a ray radiation dose of up to 2 104 gy which is much greater than that typically employed and find that the esr intensity of the e1 center a type of signal for esr dating in quartz continuously increases with the ray dose and is slightly influenced by the radiation dose rate this indicates the potential of esr to date pre quaternary faults by simulating environmental radiation under geological burial conditions we use esr to date the latest extension event in the western hubei and hunan thrust belt to 9 17 0 76 ma and the thrusting of the longmenshan fault zone toward the sichuan basin to approximately 6 14 0 73 ma conversely the saturated esr intensity of fault zone samples from the daba fault zone suggests a lack of strong tectonic activity in the southern daba fault zone in the last 10 ma our results are consistent with fission track ages and the regional tectonic evolution history therefore esr is a practical method for dating pre quaternary faults in structural and petroleum geology applications

magnitude frequency relations for northeastern japan magnitude frequency relations are investigated for the on land area covered by the tohoku university seismic network northeastern japan we combine the japan meteorological agency jma catalog with the tohoku university catalog for the crustal events the suite for the upper plane events of the double planed wadati benioff zone from 60 km through 100 km deep is based on the tohoku university catalog only completeness analyses of the catalog reveal that the crustal events and the upper plane events suite is complete down to magnitude 2 0 and 2 1 respectively including events only with magnitude above the completeness threshold the magnitude frequency relations are found to be nonlinear both for the crustal events slightly and the upper plane events strongly consistency between total seismic wave duration magnitudes and velocity amplitude magnitudes suggests that the magnitude scale in the catalog is nonbiased the completeness analyses and the non biased magnitude scale suggest that the nonlinear magnitude frequency relations are not artificial but substantial features of the earthquake occurrence