tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin an example from the cretaceous early tertiary mugi melange the shimanto belt japan abstract a tectonic melange exposed on land is examined to reveal relationships between melange formation underplating and deformation mechanisms focusing on the deformation of basaltic rocks the studied mugi melange of the shimanto belt is composed of a shale matrix surrounding various blocks of sandstone pelagic sediments and basalts the melange was formed during late cretaceous to early tertiary times in a subduction zone under p t conditions of 150 200 c and 6 7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions the melange represents a range of deformation mechanisms pressure solution with micro scale cataclasis in the shale matrix brittle tension cracking in the blocks and ubiquitous strong cataclasis in the basal portion of basaltic layers the cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth

change in fabric of melange in the shimanto belt japan change in relative convergence systematic changes in fabrics of melanges have been detected in rocks of the shimanto belt japan several shear indicators including s c structures riedel shears and folds in the melanges indicate consistent sense of shear both in outcrop and microscopic scales the deformation mechanisms independent particulate flow and pressure solution suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening after lithification cataclastic deformation occurred locally but most of the earlier deformation features are preserved this process appears to have taken place along the decollement beneath the off scraped accretionary prism sinistral reverse sense of shear is observed in the campanian melange whereas a dextral reverse sense of shear is preserved in the early eocene melange in the shimanto belt this change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates three different plate models have been proposed for the late cretaceous to early tertiary western pacific margin 1 subduction of the kula pacific ridge along the continental margin 2 subduction of the pacific plate since 85 ma and 3 subduction of the aborted kula pacific ridge after 43 ma the observed change in fabric of melange is consistent only with the third model which predicts a change in relative convergence from sinistral to dextral at the appropriate time

thermochronological investigation of fault zones abstract the timing of faulting episodes can be constrained by radiometric dating of fault zone rocks fault zone material suitable for dating is produced by tectonic processes such as 1 fragmentation of host rocks followed by grain size reduction and recrystallization to form mica and clay minerals 2 secondary heating melting of host rocks by frictional fault motions and 3 mineral vein formation as a result of fluid advection associated with the fault motions the thermal regime of fault zones consists primarily of the following three factors a regional geothermal structure across the fault zone and background thermal history of studied province bounded by fault systems b frictional heating of wall rocks by fault motions and c heating of host rocks by hot fluid advection in and around the fault zone thermochronological methods widely applied in fault zones are k ar 40ar 39ar fission track and u th methods for which methodological principles as well as analytical procedures are briefly described the thermal sensitivities of individual thermochronological systems are then reviewed which critically control the response of each method against the thermal processes based on the knowledge above representative examples as well as key issues are highlighted to date fault gouges pseudotachylytes mylonites and carbonate veins placing new constraints upon geological geomorphological and seismological frames finally the nojima fault is presented as an example for multiple applications of thermochronological methods in a complex fault zone

sediment deformation and hydrogeology of the nankai trough accretionary prism synthesis of shipboard results of odp leg 131 the main objective of leg 131 was to provide data on the deformational processes and associated hydrogeology of the nankai prism toe drilling succeeded for the first time in the history of ocean drilling in penetrating the complete sedimentary sequence to basaltic basement reaching 1327 mbsf metres below seafloor with good core recovery 55 excellent correlation of the lithology and structure including the frontal thrust and the decollement with seismic reflection images was also determined bedding dips faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to nw se shortening as expected from the plate tectonic convergence vector below the decollement no significant deformation features were observed indicating that the decollement is a sharp discontinuity in stress transmission physical properties data show major discontinuities at the decollement notably an increase in porosity below the later this may indicate excess pore pressure in the subducted section and decollement zone a less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature attempts to make downhole measurements were severely hampered by unstable hole conditions but useful constraints have been placed on the thermal regime and some calibration of laboratory physical properties toin situ conditions has been provided andin situ stress and pore pressure were measured in the uppermost sediments evidence of channelized fluid flows is inconclusive no sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found thermal measurements are not significantly different from those predicted by a purely conductive heat flow model a signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events we conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site

complete 40ar resetting in an ultracataclasite by reactivation of a fossil seismogenic fault along the subducting plate interface in the mugi melange of the shimanto accretionary complex southwest japan abstract we used the k ar ages of clay sized mineral grains to investigate the timing of activity on the minami awa fault which is a fossil seismogenic fault along a subducting plate interface separating the coherent strata of the shimanto accretionary complex to the north from the tectonic melange to the south the k ar ages from the matrix shale of the melange range from 85 to 48 ma and decrease with decreasing amount of detrital mica indicating that they record a mixture of authigenic illite related to burial diagenesis and detrital mica in contrast the k ar ages of an ultracataclasite within the fault core are significantly younger ranging from 29 to 23 ma and are unrelated to grain size and amount of detrital mica this indicates that s ar diffused completely from the ultracataclasite between 29 and 23 ma which postdates the formation of authigenic illite by at least several million years the diffusion of 40 ar in the ultracataclasite was probably caused by frictional heating or high temperature fluid migration that occurred when the fault was reactivated the results indicate that seismogenic faults that separate tectonic melange from coherent strata in accretionary complexes may slip not only during accretion but also long after accretion

links between orogenic wedge deformation and erosional exhumation evidence from illite age analysis of fault rock and detrital thermochronology of syn tectonic conglomerates in the spanish pyrenees abstract we present new geochronologic data illuminating the tectonic and erosional history of the orogenic wedge exposed in the south central pyrenees spain we interpret illite age analyses from four fault gouges that record thrust belt development and document the importance of out of sequence thrusting fault activity occurs in pulses with slip occurring contemporaneously on multiple faults throughout the wedge new apatite fission track data from syn orogenic sediments of the sis conglomerate body reveal ages of 48 to 42 ma with no consistent variation upsection in strata deposited from 41 to 30 ma we interpret these data as well as thermal modeling of track length distributions to imply rapid cooling in the interior of the pyrenean wedge during the middle eocene the record of fault activity and erosion suggests that orogenic wedges may not evolve in a steady fashion but generally exhibit significant changes in rates of deformation and exhumation the observed correlation in the timing of tectonism and erosional exhumation provides evidence for links between tectonic and surface processes

k ar illite dating to constrain multiple events in shallow crustal rocks implications for the late phanerozoic evolution of ne asia abstract a recently developed illite age analysis iaa approach was applied to determine the multiple events for the chugaryeong fault belt korea each event was determined by a combined approach of the optimized illite polytype quantification and the k ar age dating of clay fractions separated from the fault clays the late cretaceous to paleogene events 76 5 0 8 69 1 0 6 59 3 0 7 and 48 2 0 7 ma were recognized by calculating the authigenic 1m 1m d illite ages on the iaa plots of the fault clays the early cretaceous ages 121 7 124 7 and 112 4 1 5 ma were also obtained from the convergent intercepts of 100 2m 1 illite on the iaa plots the absence of the 2m 1 illites in the host rock indicates that the early cretaceous ages represent the timings of high t hydrothermal events of 280 c the 2m 1 illites in the fault clays should be pre formed by a fluid rock interaction under a relatively high t subsurface condition and be mechanically reworked into the near surface along the fault by post tectonic events this is the first report determining the absolute age constraints of multi activated tectonic events from a fault these geochronological determinations of the multiple events recorded in the chugaryeong fault belt are crucial to establish the tectonic evolution of the korean peninsula since the late cretaceous

geological record of thermal pressurization and earthquake instability of subduction thrusts abstract during an earthquake fault strength decreases with slip over the slip weakening distance dc to a residual strength the estimation of dc is crucial for the evaluation of fault instability during earthquakes however it has been difficult to determine dc from natural faults we found geological evidence of thermal pressurization from the on land analog of a subduction thrust exhumed from seismogenic depths thermal pressurization was indicated by the fluidization of comminuted material and increase in the volume of fluid inclusions by frictional heating numerical analysis of thermal pressurization with the constraints on the thickness of the seismic slip zone the temperature range of frictional heating and ambient conditions of subduction thrusts indicates that the dc of subduction thrusts ranges from 0 03 to 0 22 m which is independent of initial pore fluid pressure on subduction thrusts the short dc associated with the effect of thermal pressurization on subduction thrusts indicates the occurrence of rapid stress relief and high radiated energy during subduction earthquakes

a geochemical estimation of fluid flux and permeability for a fault zone in mugi melange the cretaceous shimanto belt sw japan abstract the fluid flux and permeability along seismogenic subduction interfaces were estimated roughly based on geological evidences a decrease in silica from the host rocks to the deformed rocks due to rock fluid interaction within the fault fluid system was qualitatively examined the area under study was the northernmost part of the mugi melange in the cretaceous shimanto belt shikoku sw japan a fault zone with pseudotachylytes is located at the area as the roof thrust of the mugi melange the pressure temperature conditions of the fault zone correspond to the depth around the onset of the seismogenic zone along the subduction interface on the basis of deformation texture rocks are classified into three types melange fault breccia and ultracataclasite the bulk chemical analysis of the abovementioned rocks was conducted by employing the x ray fluorescence xrf analysis method the average mass loss from the melange host rocks to the fault rocks was estimated by the isocon method in particular a decrease in silica was observed clearly based on the decrease in silica and certain other geological constraints such as the pressure temperature conditions of fluid length of the fault and the duration between deposition and exhumation the amount of fluid reacting with rocks and the fluid flux were examined roughly the estimated fluid volume was approximately thousand times the rock volume and the fluid flux was about 10 8 10 6 m s based on the fluid flux and some constraints in temperature pressure conditions of fluid and pressure gradient the permeabilities of fault zone rocks were estimated which was approximately of the order of about 10 17 10 16 m 2 and about 10 16 10 14 m 2 in the fault breccia and ultracataclasite respectively while the estimated value of the permeability of the fault breccia is similar to that obtained in laboratory experiments the estimated value of the permeability of the ultracataclasite is considerably larger than that obtained from laboratory experiments this probably indicates that the permeability of the ultracataclasite is not constant as observed in the laboratory experiments but can evolve during seismic cycle

origin of illite in the lower paleozoic of the illinois basin evidence for brine migrations in the lower paleozoic of the illinois basin three illite polytypes are found 2m 1 of detrital origin and 1m d and 1m of diagenetic origin illite polytype quantification of detrital 2m 1 illite and diagenetic 1m d and 1m illite combined with k ar age dating allows extrapolation to apparent detrital and diagenetic illite ages kinetic modeling of smectite illitization combined with the calculated age of illitization can evaluate different origins of illite the diagenetic illite in the lower paleozoic of the illinois basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events the upper ordovician maquoketa group contains diagenetic illite dominantly 1m d with minor 1m with an extrapolated age of 360 m y 356 377 m y and formed from smectite at temperatures of 50 100 c this age falls within the span of dates for illite smectite i s in k bentonites from the upper mississippi valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a k rich brine from the michigan basin upper mississippi valley area or forest city basin in ordovician and cambrian shale partings and sandstone older than the maquoketa group the diagenetic illite 1m d in shale and 1m in sandstone has an age of 300 m y and formed at temperatures 140 c this late paleozoic age falls within the range of illites from sandstone in the upper mississippi valley and k bentonites of the appalachian basin it coincides with the alleghany orogeny and is interpreted as having formed by gravity driven flow from the uplifted alleghanian ouachita orogenic belt that drove hot 140 c fluids through the illinois basin

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

mechanisms of argon retention in clays revealed by laser 40ar 39ar dating a method for dating clays is important for studies of weathering diagenesis hydrocarbon migration and the formation of major metalliferous deposits however many attempts have produced imprecise or inaccurate results data from shales show that contrary to expectations the 40ar 39ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40ar in nature rather than by direct recoil as is generally assumed

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

timing of alpine fault gouges k ar ages from clay rich fault gouges in the european alps are consistent internally with established field constraints and with fission track ages demonstrating the applicability of this method for direct dating of brittle deformation illite grown by retrograde hydration of granitic and or high grade metamorphic protoliths is the major k bearing mineral in the fractions that have been separated 0 1 to 6 10 m the ages obtained are bracketed by apatite and zircon fission ages from adjacent localities this together with the interpreted cooling curve for one well constrained sample from the simplon fault indicates that illite grew or the k ar isotopic system in illite closed at temperatures in the range 120 150 c consistent with the stability of illite alone rather than illite smectite average ages obtained for fractions finer than 2 m were 1 5 2 2 6 ma 2 for 11 analyses from the detachment and footwall of the simplon fault 2 8 8 0 6 ma for four analyses from the centovalli fault at trontano 3 19 9 2 6 ma for two analyses from a major riedel fault related to the periadriatic fault offsetting the tertiary bergell tonalite at gesero 4 21 5 5 5 ma for two analyses from a zone parallel to the periadriatic fault in the adjacent southern alps in val morobbia and 5 14 5 3 9 ma for two analyses from a gouge zone on the northern rim of the tertiary mauls tonalite in the eastern alps the ages from the simplon centovalli and periadriatic faults are not identical and do not lend support to models linking these three faults as a single coeval structure

depth of oceanic crust underplating in a subduction zone inferences from fluid inclusion analyses of crack seal veins fluid inclusions in crack seal veins are analyzed in accreted melange now on land the vein i inclusions are observed in the necked parts of sandstone blocks in melange and the vein ii inclusions developed in shale below the thrust fault which cuts the melanges the pressure and temperature estimations from the inclusions within vein i show that the cracks were sealed at 125 195 8c under 92 144 mpa of fluid pressure vein ii formed in cracks that might have opened in a damage zone caused by the thrust fault as it broke through oceanic crust and into the melange these veins contain fluid inclusions trapped at 135 245 8c under 107 149 mpa of fluid pressure the depth 4 6 km below the seafloor and temperature estimates are consistent with the conditions where an aseismic decollement within sedimentary rocks steps down into the oceanic basement so that a thin section of oceanic crust underplates in the hanging wall such a step down site is the updip limit of the seismogenic zone in the modern nankai trough

comparative 40ar 39ar and k ar dating of illite type clay minerals a tentative explanation for age identities and differences abstract the 40 k 40 ar k ar and 40 ar 39 ar dating methods are applied here to the same very small micrometric illite type particles that crystallized under low temperature 5 9 ma the same size fractions dated by the 40 ar 39 ar method give total gas ages ranging from 173 3 2 0 to 228 8 1 6 ma nearly all 40 ar 39 ar total gas ages are slightly older which cannot be explained by the recoil effect only the impact of which being amplified by the inhomogeneous shape of the clay minerals and their crystallographic characteristics with varied crystallinity indices and a particle width about 10 times large than thickness the 40 ar 39 ar data outline some advantages such as the plateaus obtained by incremental step heating of the various size fractions even if not translatable straight as ages of the illite populations they allow identification of two generations of authigenic illite that formed at about 200 and 175 ma and one detrital generation however 40 ar 39 ar dating of clay minerals remains challenging as technical factors such as the non standardized encapsulation may have potential unexpected effects both dating methods have their limitations 1 k ar dating requires relatively large samples ca 10 20 mg incurring potential sample homogeneity problems with two aliquots required for k and ar analysis for an age determination also inducing a higher analytical uncertainty 2 an identified drawback of 40 ar 39 ar dating is ar recoil and therefore potential loss that occurs during neutronic creation of 39 ar from 39 k mostly in the finer mineral particles if all the recoiled 39 ar is redistributed into adjacent grains minerals the final 40 ar 39 ar age of the analyzed size fraction remains theoretically identical but it is not systematic in clay type material the finest grain sizes e g 39 ar recoil and are therefore only datable by the 40 ar 39 ar method using an encapsulation technique that still needs to be technically evaluated in this study 39 ar recoil during irradiation was quantified by encapsulation it ranged from about 20 of the total 39 ar for the 39 ar recoil confirms that grain size increasing minimizes the recoil effect but how it proceeds has still to be explained despite recent developments in 40 ar 39 ar dating the conventional k ar method is still a valuable tool for clay dating due to a convenient and straightforward use supported by a standardized and well controlled technical approach the present comparison of the two ar dating methods as applied to clay material shows that neither method is presently outdated and that they are even of reciprocal use both methods have distinct application fields in clay geochronology and complementary application fields in clay crystallography

exhumation of an accretionary prism results from fission track thermochronology of the shimanto belt southwest japan abstract the along arc variation of the thermal history of the shimanto accretionary prism was studied with fission track thermochronology to investigate tectonic factors that control the exhumation of an accretionary prism twelve apatite and 35 zircon ages were analyzed on sandstone and granite samples from the kii and kyushu regions southwest japan and the samples are interpreted in combination with the previously reported data from the shikoku region zircon samples show various degrees of age resetting after deposition that are dependent on their position within the prism some of the zircon samples constrain the time of maximum heating within or cooling through the zircon partial annealing zone two lines of observation i e simultaneous exhumation revealed along across arc transect with relatively high maximum burial at the rear part of the prism and along arc coeval rebound from burial to exhumation indicate that the exhumation of the entire wedge was probably caused by a progressive accretionary process such as underplating at depth because the driving force for this type of exhumation is the growth of the wedge this exhumation process is probably a general exhumation process that occurs in all accretionary complexes apatite samples record cooling at 10 ma except for samples from sites in the north of the kii region whose ages are 35 ma suggesting the effect of the southwest japan block obduction onto the subducting young philippine sea plate this implies along with the distribution of locally reset zircon ages that the exhumation of the prism was also forced by local episodic events for example collision of blocks during differential rotational movement change of the subduction parameters and possible accelerated underplating beneath a narrow region probably caused by the increased material influx from the igneous activities

effect of atmospheric water vapor on modification of stable isotopes in near surface snow on ice sheets south pole are used to simulate the d 18 o of near surface snow as it evolves in time the sensitivity of d 18 o of near surface snow to wind speed surface temperature and accumulation rate is tested through model experiments a steady wind of 5 10 m s 1 results in annual mean d 18 o enrichment of 3 7 enrichment of the heavy isotope occurs predominantly in buried winter snow layers during summer however in many simulations there is a slight depletion of d 18 o summer layers that occurs as atmospheric water vapor is deposited within the snow during winter postdepositional modification of water stable isotopes in near surface snow also depends on the snow accumulation rate high accumulation rates quickly advect snow layers away from the influence of the atmosphere preventing significant modification low accumulation rates e g at vostok and dome c allow significant postdepositional modification because the near surface snow is exposed to forced ventilation for several annual cycles postdepositional modification during the last glacial maximum lgm at summit greenland is estimated to be greater than the present day postdepositional modification therefore the temperature difference for lgm may be larger than that previously inferred from the d 18 o record however because of the compensating effects of lower temperatures and smaller accumulation during the lgm in antarctica the postdepositional enrichment in east antarctica may be approximately the same for lgm as for the modern climate

timing of the cataclastic deformation along the akaishi tectonic line central japan this paper describes k ar ages of cataclasites and fault gouges from the akaishi tectonic line atl central japan petrological and mineralogical features of these rocks are also examined based on the results we test the hypothesis that the k ar ages of these rocks represent the age of hydrothermal alteration associated with the fault movement intensity of deformation and alteration increase passing from host rock into cataclasite and finally into fault gouge this increase corresponds to an increase of the value of crystallinity index kubler index of the micaceous minerals contained in these rocks furthermore the degree of rejuvention of k ar ages of the micaceous minerals increases in the same sense a correlation of k ar ages plotted on the y axis versus kubler index plotted on the x axis yields a concave curve asymptotically parallel with the x axis at approximately 15 ma this curve is interpreted to represent the mode of decrease of the relative amounts of inherited argon in the k ar system of the micaceous minerals corresponding to an increase in the intensity of deformation and alteration inclination of the curve becomes zero if all the inherited argon is lost from the k ar system of the micaceous minerals thus it is concluded that the hydrothermal alteration occurred at approximately 15 ma the strike slip basin along the atl formed the middle miocene the k ar dating of atl gouges indicates that the atl was active simultaneously with formation of the strike slip basin along its trace during the middle miocene

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

dating of shallow faults new constraints from the alptransit tunnel site switzerland methods for dating shallow faults in the earth9s crust are still evolving and their reliability remains controversial based on results from well preserved fault gouge samples from the alptransit deep tunnel site in switzerland we propose a simplified and readily applicable approach to investigate and date clay rich gouge zones we collected 10 samples covering a north south section of 15 km all sampled faults are developed in orthogneiss host rock have a rather consistent approximately northwest southeast strike are mainly normal and are related to northeast southwest orogen parallel extension during alpine exhumation fine grained clay separates from the fault gouges consist of illite 1m 1m d and 2m 1 polytypes smectite and chlorite with minor amounts of feldspar in some sample fractions the k ar ages for

illite k ar dating and crystal growth processes in diagenetic environments a critical review k ar dating of illitic minerals is commonly used in studies of diagenetic series applied to oil prospecting in spite of a great number of specialized papers some problems remain unresolved these are mostly due to a misunderstanding of the argon accumulation process during illitization criteria for identifying detrital authigenic mineral mixtures crystal ripening fast precipitation or continuous nucleation growth processes are discussed using k ar data available in the literature using different parameters such as age agek ar agestrati cryst diagenetic agek ar agestrati or frac agek arfraction agek arfinest it is shown that the k ar age significance depends on the illite nucleation growth processes a diagenetic age is obtained when these processes are rapid the k2o accumulation period is shorter than 2 if lower than this value the k ar ratio depends on the proportions of new and old particles respectively which are controlled by the relative rates of nucleation crystal growth and ripening

fault dating in the canadian rocky mountains evidence for late cretaceous and early eocene orogenic pulses fault rocks from the classic rocky mountain foreland fold and thrust belt in southwestern canada were dated by ar analysis of clay grain size fractions using x ray diffraction quantification of the detrital and authigenic component of each fraction these determinations give ages for individual faults in the area illite age analysis the resulting ages cluster around 72 and 52 ma here called the rundle and mcconnell pulses respectively challenging the traditional view of gradual forward progression of faulting and thrust belt history of the area the recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges which result in geologically reasonable strain rates for the area in addition to regional considerations this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper crustal kinematics and regional tectonic analysis of ancient orogens

pseudotachylytes in an ancient accretionary complex and implications for melt lubrication during subduction zone earthquakes abstract pseudotachylyte bearing fault zones found in the shimanto accretionary complex southwest japan developed during subduction or underplating at seismogenic depths the pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange the microstructures of pseudotachylytes are represented by a fragment laden glass supported texture resulting from the rapid cooling of the frictional melt transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed the compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite rich slip zone with a minimum melting temperature of 1100 c the viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains the viscosities at 1100 c range from 85 to 290 pa s and the corresponding shear resistance along a 1 mm thick melt layer at a slip rate of 1 m s was 0 1 0 3 mpa the formation of a melt layer in an illite rich slip zone can possibly induce large stress drops increase the slip rate and enhance rupture propagation which together could affect the earthquake magnitude in a subduction zone

transition of accretionary wedge structures around the up dip limit of the seismogenic subduction zone the nankai accretionary prism is divided into three segments outer and inner wedges and their transition zone these wedges reflect different aspects of wedge taper internal deformation and basal plate boundary fault the outer wedge is characterized by narrow critical taper internal deformation by in sequence fold and thrust and aseismic decollement the inner wedge represents a stable narrow taper weakly deformed internal structure and seismogenic plate boundary fault along its base the transition zone between the two wedges shows large critical taper with steep surface slope internal structure of out of sequence thrust and step down of decollement onto the sediment oceanic basement interface the trench slope break and oceanward margin of forearc basin is located around the landward edge of this transition zone these common aspects might be related to the lithification of both accreted and underthrust sediments and the resultant switch of the plate boundary fault deformation and lithification process recorded in exhumed on land melange of accretionary complexes suggest that the step down of the plate boundary decollement occurs around the up dip limit of seismogenic subduction zone

spatiotemporal characterization of smectite to illite diagenesis in the nankai trough accretionary prism revealed by samples from 3 km below seafloor

crustal architecture and tectonic evolution of the cauvery suture zone southern india abstract the cauvery suture zone csz in southern india has witnessed multiple deformations associated with multiple subduction collision history with incorporation of the related accretionary belts sequentially into the southern continental margin of the archaean dharwar craton since neoarchean to neoproterozoic the accreted tectonic elements include suprasubduction complexes of arc magmatic sequences high grade supracrustals thrust duplexes ophiolites and younger intrusions that are dispersed along the suture the intra oceanic neoarchean neoproterozoic arc assemblages are well exposed in the form of tectonic melanges dominantly towards the eastern sector of the csz and are typically subjected to complex and multiple deformation events multi scale analysis of structural elements with detailed geological mapping of the sub regions and their structural cross sections geochemical and geochronological data and integrated geophysical observations suggest that the csz is an important zone that preserves the imprints of multiple cycles of precambrian plate tectonic regimes

smectite to chlorite conversion by frictional heating along a subduction thrust abstract an ancient subduction thrust developed at the top of oceanic basaltic crust is exposed in the shimanto accretionary complex of eastern shikoku southwest japan the fault zone structure indicates that slip was localized along a few centimeters thick layer in which fluidization of comminuted material is recorded to identify possible clay mineral reactions during frictional heating along subduction thrusts we examined the clay mineralogy of the basalt derived fault zone rocks including ultracataclasite altered basalt foliated cataclasite and massive basalt all samples include chlorite mixed layer chlorite smectite c s and corrensite among the samples the highest chlorite content in the c s is found in the ultracataclasite the significant chloritization in the ultracataclasite is unlikely to reflect hydrothermal alteration and regional very low grade metamorphism during subduction down to seismogenic depths instead it is interpreted to have resulted from frictional heating associated with localized slip based on chlorite geothermometry and the presence of small quantities of corrensite we estimate the temperatures during this frictional heating to have been in the range of 185 260 c which is 35 130 c higher than the ambient temperature this difference of 35 130 c is consistent with a temperature rise of 50 150 c indicated by the stretching of fluid inclusions in calcite therefore the crystallographic and chemical properties of c s are potentially useful in detecting small temperature rises associated with frictional heating and the method could be widely applicable to c s bearing faults

direct dating of eocene reverse faulting in northeastern tibet using ar dating of fault clays and low temperature thermochronometry abstract paired together fault gouge dating and low temperature thermochronometry overcome the limitations and assumptions inherent in each independent technique here we establish timing of brittle faulting along the west qinling fault of northeastern tibet by dating several size fractions of fault gouge clay that represent variable populations of illite polytypes results show that the authigenic or fault generated component of illite formed at 50 8 ma and that the detrital component formed at 236 7 ma indicating a middle eocene age of faulting and a middle triassic age of the wall rocks comparing this dataset with published thermochronology from hanging wall rocks supports the interpretation that the west qinling fault initiated at 50 ma and continued until at least middle miocene time and that authigenic clay growth occurred at ambient temperatures of 110 c lack of overprinting of younger clay ages at this site may indicate that rocks were out of the thermal window for authigenic clay formation during later faulting episodes the potential for temperature to control illite growth has implications for interpretation of authigenic illite ages and their relationship to deformation episodes within fault zones

dating fault gouges geological faults are fractures in the earth s crust along which there has been some displacement and are thus zones of weakness which are of importance in planning civil engineering projects constructing underground caverns and conducting earthquake research to assess the potential for future displacements and fluid circulation along faults it is helpful to establish the age of the latest episode of mineral growth in fault zones however there have been few attempts to date recrystallized and newly grown minerals developed in fault zones at low temperature1 4 here we report k ar and rb sr ages for clay fractions from two fault zones in the eastern alps and compare them with ages of the same fine fractions from the undeformed host rock the results suggest that the bulk of the clays along these faults were formed during early cretaceous and tertiary times respectively no major displacements seem to have taken place in recent times

distributed deformation along the subduction plate interface the role of tectonic melanges recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically however the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified tectonic melange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate in this work we examine several tectonic melange zones hyuga okitsu mugi from the shimanto belt an accretionary prism in southwest japan connecting to the active nankai subduction zone these tectonic melange zones have a block in matrix structure with lenses of sandstones and basalts within a metapelitic matrix and their deformation is distributed over zones of hundreds of meters in thickness in addition the examples of melange considered here are bounded by sharp faults some of them bearing pseudotachylyte layers so that distributed deformation within the melange and localized deformation on its boundary are juxtaposed distributed deformation involves the development of a foliation as well as of a pervasive network of macroscopic and microscopic shear zones along with slip on this network strain proceeds by fracturing and precipitation of quartz in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins the analysis of shear band kinematics shows in all three examples a dominant top to the trench sense of shear consistent with deformation along the plate boundary during subduction moreover most shear zones when foliation is restored back to syn subduction position are extensional structures finally the geometry and kinematics of the melange bounding faults as well as radiometric constraints show that in most cases the faults localized structures were formed during a later stage than melange internal deformation these findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths first there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation second rather than proposed models of underplating where all deformation is localized into the thrusts bounding the tectonic sheets we suggest that underplating was to a large extent accommodated by distributed deformation within the melange sheets this underplating model accounts for i the large amount of strain within the melange ii the absence of contractional structures during underplating iii thinning of the melange required by the network of extensional shear bands and stretched boudins third melanges appear as likely candidates for portions of the plate interface deforming by aseismic slip the seismic vs aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain which in turn depends on the efficiency of pressure solution to operate

short paper accretion and uplift in the shimanto belt sw japan k ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the shimanto belt a cretaceous neogene accretionary complex cleavage ages record two major phases of accretion during the late cretaceous and late eocene early oligocene the minimum time periods between sedimentation and deformation are constrained to 9 5 9 and 0ma intervals within four tectonic units containing youngest sediments of maastrichtian late eocene late eocene and early oligocene ages respectively one phase of middle to late miocene uplift is indicated for the whole study area by the fission track cooling ages frontal underplating combined with out of sequence thrusting are suggested as mechanisms for this regional uplift the cretaceous neogene shimanto belt fig la forms the outermost onshore exposure of accreted material in sw japan the boundaries of constituent accreted units of the shimanto belt are defined by a detailed stratigraphy taira et al 1980taira et al 1988 providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution the butsuzo tectonic line fig la juxtaposes the shimanto belt to the jurassic sanbosan belt in the north younger accreted material extends offshore to the south as far as the present day boundary of the eurasian plate the nankai trough fig la continued accretion seaward of the shimanto belt has back rotated older accreted units to their present day steeply dipping orientations the type section of the shimanto belt taira et al 1980 lies in the eastern hata peninsula fig 1b which contains the study

stretching of fluid inclusions in calcite as an indicator of frictional heating on faults fluid inclusions trapped in calcite tend to increase in volume i e stretching when heated to temperatures higher than those of the initial entrapment at the entrapment pressure and the result is an increase in the homogenization temperature we examined whether the stretching of fluid inclusions in calcite occurred by frictional heating on the subduction thrust exhumed from seismogenic depths 4 6 km at which the slip is localized along the few centimeters thick ultracataclasite layer the results indicate that the histograms of the homogenization temperatures of the fluid inclusions in the ultracata clasite are marked by the pronounced skewness with the frequency mode near the low temperature end of the range and a tapering tail toward higher temperatures these skewed frequency distributions are absent in the surrounding rocks and are best explained by the stretching of the fluid inclusions by the frictional heating associated with the localized slip experimental stretching of fluid inclusions in calcite demonstrated that the tapering tail toward high temperatures in the histograms of the homogenization temperatures from the ultra cataclasite can be reproduced by a 50 150 c temperature rise along the ultracataclasite with respect to the ambient temperature stretching of fluid inclusions in calcite could record a small temperature rise during frictional heating on faults and is potentially useful in identifying the restraint of the temperature rise by fluid pressurization during an earthquake

variation of illite muscovite 40 ar 39 ar age spectra during progressive low grade metamorphism an example from the us cordillera 40ar 39ar step heating data were collected from micron to submicron grain sizes of correlative illite and muscovite rich cambrian pelitic rocks from the western united states that range in metamorphic grade from the shallow diagenetic zone zeolite facies to the epizone greenschist facies with increasing metamorphic grade maximum ages from 40ar 39ar release spectra decrease as do total gas ages and retention ages previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation recrystallization of neo formed illite while recognizing the importance of these processes in evaluating our results we suggest that the inverse correlation between apparent age and metamorphic grade is controlled primarily by thermally activated volume diffusion analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments our results suggest that complete resetting of the illite muscovite ar thermochronometer occurs between the high anchizone and epizone or at roughly 300 c this empirical result is in agreement with previous calculations based on muscovite diffusion parameters which indicate that muscovite grains with radii of 0 05 2 m should have closure temperatures between 250 and 350 c at high anchizone conditions we observe a reversal in the age grain size relationship the finest grain size produces the oldest apparent age which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo formed illite muscovite crystallites typically surpass the size of detrital muscovite grains it is also approximately the stage at which neo formed illite muscovite crystallites develop sufficient ar retentivity to produce geologically meaningful 40ar 39ar ages results from our sampling transect of cambrian strata establish a framework for interpreting illite muscovite 40ar 39ar age spectra at different stages of low grade metamorphism and also illuminate the transformation of illite to muscovite at frenchman mtn nv where the cambrian bright angel formation is at zeolite facies conditions illite muscovite 40ar 39ar data suggest a detrital muscovite component with an apparent age 967 ma the correlative carrara fm is at anchizone conditions in the panamint and resting spring ranges of eastern california and in these locations illite muscovite 40ar 39ar data suggest an early permian episode of subgreenschist facies metamorphism the same type of data from equivalent strata at epizone conditions greenschist facies in the footwall of the bullfrog fluorspar canyon detachment in southern nevada reveals a period of slow to moderate late cretaceous cooling