remotely sensed debris thickness mapping of bara shigri glacier indian himalaya despite the important role of supraglacial debris in ablation knowledge of debris thickness on himalayan glaciers is sparse a recently developed method based on reanalysis data and thermal band satellite imagery has proved to be potentially suitable for debris thickness estimation without the need for detailed field data in this study we further develop the method and discuss possibilities and limitations arising from its application to a glacier in the himalaya with scarce in situ data surface temperature patterns are consistent for 13 scenes of advanced spaceborne thermal emission and reflection radiometer aster and landsat 7 imagery and correlate well with incoming shortwave radiation and air temperature we use an energy balance approach to subtract these radiation or air temperature effects in order to estimate debris thickness patterns as a function of surface temperature both incoming shortwave and longwave radiation are estimated with reasonable accuracy when applying parameterizations and reanalysis data however the model likely underestimates debris thickness probably due to incorrect representation of vertical debris temperature profiles the rate of heat storage and turbulent sensible heat flux moreover the uncertainty of the result was found to increase significantly with thicker debris a promising result since ablation is enhanced by thin debris of 1 2 cm

the silica heat flow interpretation technique application to continental australia abstract the silica heat flow interpretation technique swanberg c a and morgan p j geophys res 117 1979 227 241 j geophys res 85 1980 7206 7214 has been applied and tested in mainland australia using a database of approximately 41 000 australian groundwater analyses the silica geotemperature of the groundwaters was obtained by substituting the dissolved silica content of a groundwater into the quartz geothermometer equation of truesdell 1976 proceedings of the second united nations symposium on the development and use of geothermal resources san francisco ca usa 20 29 may 1975 vol 1 the average silica geotemperature value for 1 1 latitude longitude grid cells has been calculated and the results plotted against published traditional heat flow values for those grid cells cull j p 1982 bmr j aust geol geophys 7 11 21 to form silica heat flow estimation models data exclusion criteria based upon data quantity and statistical spread have been applied to both the groundwater data and the traditional heat flow data this was done in order to exclude areas that were poorly categorized in terms of data quality and quantity for the remaining data a significant linear relationship between the groundwater geotemperature estimates and traditional heat flow measurements has been identified for four of the models with a t test on the correlation coefficient estimates of regional heat flow were then made by applying the calibration models in areas with no traditional heat flow measurements but adequate groundwater data a silica heat flow map has been constructed using one of the models and the differences between it and the traditional heat flow map evaluated good correlations exist between the silica heat flow map and the traditional heat flow map except for the northwest yilgarn of wa australia where silica heat flow data give significantly higher values than traditional data the silica heat flow map identifies areas of high heat flow associated with some large scale geological features such as the granite belt in southeastern australia the calibration models result in some silica heat flow estimates exceeding natural likely values this suggests that the calibrations need further refining for australian conditions

magnetic mineralogy of the lower continental crust rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths as parts of high grade metamorphic and plutonic terrains and as obducted lower crustal cross sections a review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks thermodynamic considerations indicate that tipoor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust regardless of the primary magmatic composition of the spinel phase furthermore it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle thus magnetite in the deep crust is the most likely candidate for the source of the long wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys if so the entire crust should be magnetic in most continental areas including shields in areas of high heat flow the bottom of the magnetic crust may lie considerably above the moho

a record of astronomically forced climate change in a late ordovician sandbian deep marine sequence ordos basin north china abstract the late ordovician pingliang formation on the southwestern margin of the ordos basin north china consists of rhythmic alternations of shale limestone and siliceous beds to explore the possible astronomical forcing preserved in this lithological record continuous lithological rank and magnetic susceptibility ms stratigraphic series were obtained from a 34 m thick section of the pingliang formation at guanzhuang power spectral analysis of the ms and rank series reveal 85 5 cm to 124 cm 23 cm to 38 cm and 15 cm to 27 cm thick sedimentary cycles that in ratio match that of late ordovician short eccentricity obliquity and precession astronomical cycles the power spectrum of the ms time series calibrated to interpreted short orbital eccentricity cycles aligns with spectral peaks to astronomical parameters including 95 kyr short orbital eccentricity 35 3 kyr and 30 6 kyr obliquity and 19 6 kyr and 16 3 kyr precession cycles the 15 cm to 27 cm thick limestone shale couplets mainly represent precession cycles and siliceous bed deposition may be related to both precession and obliquity forcing we propose that precession forced sea level fluctuations mainly controlled production of lime mud in a shallow marine environment and transport to the basin precession and obliquity controlled biogenic silica productivity and temperature dependent preservation of silica may have been influenced by obliquity forcing

the ivrea zone as a model for the distribution of magnetization in the continental crust magnetic properties of the principal lithologies of the ivrea zone of northern italy have been measured maficultramafic granulite facies rocks and amphibolites from the granulite amphibolite facies transition zone are highly magnetic with susceptibilities greater than 0 5 10 3 cgs saturation magnetization values greater than 90 10 3 emu cm and curie points between 570 and 580 c the magnetic phase is magnetite all other lithologies are moderately to weakly magnetic the magnetic mafic granulites are thick and laterally continuous and thus would make an appropriate deep crustal source for long wavelength magnetic anomalies in regions typified by low to average geothermal gradients

crustal structure of the churchill superior boundary zone between 80 and 98 w longitude from magsat anomaly maps and stacked passes a modeling technique using spherical shell elements and equivalent dipole sources has been applied to magsat signatures at the churchill superior boundary in manitoba ontario and ungava a large satellite magnetic anomaly 12 nt amplitude on pogo and magsat maps near the churchill superior boundary was found to be related to the richmond gulf aulacogen the averaged crustal magnetization in the source region is 5 2 a m 1 stacking of the magnetic traces from magsat passes reveals a magnetic signature 10 nt amplitude at the churchill superior boundary in an area studied between 80 w and 98 w modeling suggests a steplike thickening of the crust on the churchill side of the boundary in a layer with a magnetization of 5 a m 1 signatures on aeromagnetic maps are also found in the source areas for both of these satellite anomalies

magnetization of lower crust and interpretation of regional magnetic anomalies example from lofoten and vesteralen norway the interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near surface magnetic data has remained enigmatic in its requirement for laterally extensive thick highly magnetic and deep seated sources presented here are the results of a detailed field analytical and experimental study of the magnetic mineralogy and magnetic properties of rocks from lofoten and vesteralen norway a province of deep seated origin with associated regional magnetic and gravity anomalies these rocks have a bulk magnetic character of surprising uniformity with susceptibilities hysteresis and curie temperatures defined solely by coarse grained magnetite stable remanence is observed in samples from sw lofoten but is not of significant magnitude the thermal enhancement of magnetic susceptibility the hopkinson effect is not important rocks from the deepest part of the section have the largest average susceptibilities 6 10 2 si and magnitudes of natural remanent magnetization nrm 5 10 4 a m2 kg these values of susceptibility together with the oft observed viscous character of the nrm suggest that these granulite facies lithologies particularly those of sw lofoten are perfect examples of lower crustal sources of typical regional magnetic anomalies furthermore these intermediate pressure granulites define a unique magnetic petrology especially for mafic compositions at higher grades of metamorphism high pressure granulites and eclogites or lower grades greenschist and amphibolite facies such magnetite bearing assemblages will be unstable and their high or low grade equivalents will be only weakly magnetic this evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons it portrays the lower crust as a highly magnetic unit defined by metamorphic grade with a curie isotherm of 575 c magnetite and magnetization parallel to the ambient geomagnetic field induced and viscous components

the moho as a magnetic boundary revisited we have re examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non magnetic mantle whereas the original conclusion was based on limited data the present study is based on about 400 globally distributed xenolith samples while it is now clear that the moho represents a maximum velocity gradient in a complex transition from crust to mantle and while crustal magnetization is highly variable the original concept of a magnetic crust overlying a non magnetic mantle is strongly supported by the new data a large upper mantle xenolith suite of ultramafic composition is characterized by non magnetic chrome spinels and magnesian ilmenites such rocks are magnetic only where demonstrably altered the most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature prograded rocks have characteristic magnetite curie points and are strongly magnetic especially the more mafic compositions while the mafic rocks formed by crystallization at depth have curie points between 400 570 c and are strongly magnetic induced magnetizations are commonly several a m and can readily account for long wavelength magnetic anomalies measured by satellite and aircraft thus there is no need to search for exotic sources for missing crustal magnetization no metal or other peculiar magnetic species have been observed by us in any of our unaltered samples mineral assemblages including magnetic components are consistent with oxidizing conditions close to the fmq buffer

orbital scale changes in redox condition and biogenic silica detrital fluxes of the middle jurassic radiolarite in tethys sogno lombardy n italy possible link with glaciation abstract orbital forcing has been shown to be a fundamental driver of climate change through both icehouse and greenhouse periods to reveal the impact of orbital forcing on the oceanic environment through a greenhouse icehouse transition we established 4 myr long cyclostratigraphy of the bajocian callovian middle jurassic 160 ma basal radiolarites at the torre de busi and corre di sogno sections in the lombardian basin n italy stratigraphic changes in chert abundance chert shale thickness ratio and color darkness of radiolarites show hierarchal periodicities of 8 cm 16 cm 40 cm 160 cm and 4 m corresponding to 20 kyr 40 kyr 100 kyr 400 kyr and 1 myr cycles based on the biostratigraphic age model black cherts in intervals with high chert abundance might reflect oxygen depleted conditions due to orbital scale high productivity on the other hand black cherts in intervals with low chert abundance high detrital input might reflect oxygen depleted conditions probably due to orbital scale sea level drop and stratification on 40 kyr and 100 kyr cycles the anoxic condition occurred in low chert abundance intervals across 8 m above 2 myr after the base of the radiolarites these results imply that the formation of the restricted basin resulted from tectonic and or eustatic sea level drop which is consistent with increased black chert deposition and redox sensitive elements abundances mo toc mo u their out of phase relationships on the 405 kyr cycle throughout the sequence 4 myr long with increasing amplitude above 8 m level would be caused possibly by tectonic activity or more likely by glacio eustatic sea level changes reported from sequence stratigraphy similar to those of the oligocene to pliocene glacial cycles but probably with less amplitude

magsat and pogo magnetic anomalies over the lord howe rise evidence against a simple continental crustal structure magsat average scalar and pogo reduced to pole magnetic anomaly data both show prominent positive signatures over the lord howe rise although generally assumed to be continental in nature the lord howe rise cannot simply be submerged ordinary continental crust which would produce a negative anomaly contrast with respect to the surrounding higher susceptibility oceanic crust three dimensional modeling of the plateau using the known crustal structure and assuming an induced origin for the satellite elevation anomaly leads to a model in which the lowest crustal layer in an otherwise continental crustal structure has an unusually high susceptibility in the range cgs 0 008 0 010 si 0 10 0 13 replacement or alteration of the lowest layer by a high susceptibility rock type may be related to the subsidence of the plateau

permian palaeogeographic evolution of the jiangnan basin south china abstract the most striking geographic feature of the permian of south china is the jiangnan basin which intimately reflects the interplay between the yangtze and cathaysia blocks geographic and sedimentological evolution suggests that the jiangnan basin superposed on a precambrian collisonial megasuture and a folded early paleozoic deep basin was an extensional intracratonic basin it opened in the longlinian and became more passive and restricted in the latest maokouan the remarkably similar geographic patterns of the jiangnan basin in the early and late palaeozoic suggest that the yangtze and cathaysia blocks were coherent during the paleozoic

what is magnetic in the lower crust long wavelength and satellite magnetic anomalies require that some regions of the earth s crust contain minerals that are magnetic at lower crustal conditions studies of high grade metamorphic rocks emphased multi domain magnetite as a likely source members of the hematite ilmenite series were not considered because hematite commonly has a relatively weak magnetization and though ilmenite compositions in the range ilm 50 70 can be strong ferrimagnets their low curie t 260 c makes them an unlikely source however recent investigations have described hematite ilmenite magnetism and coercivity in terms of a new ferrimagnetic substructure related to interfaces between ilmenite and hematite in exsolution lamellae down to unit cell thickness in this lamellar magnetism ferrimagnetism is coupled to principal af moments of hematite and retains many properties of titanohematite including high coercivity and thermal stability if exsolution lamellae are the source of the magnetism a temperature above that predicted for lamellar resorption should destroy it to look at behaviour of lamellae at mid to lower crustal conditions we selected a hemo ilmenite sample ilm 84 on which detailed studies of microstructures mineral chemistry and magnetic properties had been made we designed experiments at 10 kbar as an analog for lower crustal conditions after initial 5 day runs at 540 685 and 950 c a temperature of 580 c was chosen for 8 and 21 day runs at one atmosphere this t for ilm 84 is above that predicted for complete resorption of all lamellae to produce a homogeneous paramagnetic r3 ilmenite in these experiments exsolution lamellae coarsened if these features were reproduced in the lower crust hemo ilmenite or ilmeno hematite would retain a magnetic moment and higher than expected coercivity due to the microstructures emp and tem analyses were used to understand the compositional differences between the natural sample products of the experiments and one atmosphere phase diagrams if the experimental changes are interpreted as moving the compositions closer to equilibrium at 580 c 10 kbar then a considerable widening of miscibility gaps as a function of pressure and the 20 geikielite component in the ilmenite as well as an increase in neel temperature of hematite is predicted an alternate is to consider the response as ostwald steps short of equilibrium experimental results indicate that exsolution microstructures play a key role in mineral magnetism and that such microstructures as well as coarse hematite may be preserved at pressures and temperatures of the lower crust

viscous remanent magnetization model for the broken ridge satellite magnetic anomaly a range of crustal models for the interpretation of the broken ridge satellite magnetic anomaly have been constructed from bathymetric data the source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth s field at the center of the source region the model calculations were made at the observation points of a selected set of magsat investigator b passes with a variable magnetic field direction calculated from the mgst 4 81 field model the horizontal perimeter of all the models was defined by the departure of the broken ridge bathymetry from an average oceanic depth of 4 5 km a topographic model with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth requires a magnetization of 42 a m to account for the observed anomaly amplitude an airy type isostatic model with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry requires a magnetization of 6 a m to account for the observed anomaly amplitude a midcrust model with an upper surface at the average oceanic depth and a lower surface at the average oceanic moho requires a magnetization of 13 a m to account for the observed anomaly amplitude a comparison of the locations of the model and observed anomaly peaks shows that the broken ridge anomaly can be modeled assuming present field directions there is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location the model magnetizations vary from 6 a m where the whole crust is uniformly magnetized to 42 a m where the topographic expression of the ridge is the source of the anomaly the strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values but the higher magnetizations are associated with very young rocks and dredge samples it is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth s field

an equivalent layer magnetization model for the united states derived from satellite altitude magnetic anomalies long wavelength 500 km magnetic anomalies measured by the pogo series satellites at altitudes 400 700 km over the united states and adjacent areas are inverted to an equivalent layer magnetization model based on an equal area dipole source array at the earth s surface minimum source spacing giving a stable solution and a physically meaningful magnetization distribution is 300 km but a scheme is presented for effectively sampling the distribution on a grid twice as fine the model in effect expresses lateral variation in the vertical integral of magnetization and is a starting point for models of lateral variation in the form of the magnetization depth curve in the magnetic crust the magnetization model contours correlate with large scale tectonic features and in the western part of the country probably reflect curie isotherm undulations

cyclostratigraphy and astrochronology case studies from china abstract a high precision geologic time scale is the essential key for understanding the earth s evolutionary history and geologic processes astronomical tuning of orbitally forced stratigraphic records to construct high resolution astronomical time scales ats has led to a progressive refinement of the geologic time scale over the past two decades in turn these studies provide new insights regarding the durations and rates of major earth events evolutionary processes and climate changes all of which provide a scientific basis for contextualizing and predicting future global change trends south china hosts some of the best exposed and well dated neoproterozoic through mesozoic stratigraphic sections in the world many of which are suitable for cyclostratigraphy and calibrating the geologic time scale in north china several cenozoic oil bearing basins have deep boreholes with continuous sampling and or well logging that enable derivation of astronomically tuned time scales for an improved understanding of basin evolution and hydrocarbon generation this special issue focuses on case studies of astrochronology and applied cyclostratigraphy research using reference sections within china in this introductory overview we 1 summarize all existing astrochronology studies of the neoproterozoic through cenozoic sections within china that have been used to enhance the international geologic time scale 2 examine briefly the astronomically forced paleoclimate information recorded in various depositional systems and the modern techniques employed to analyze the periodicity of these signals encoded within the sedimentary record and 3 summarize the 20 contributions to this special issue of palaeogeography palaeoclimatology palaeoecology on cyclostratigraphy and astrochronology case studies from china

sedimentary environments of the middle permian phosphorite chert complex from the northeastern yangtze platform china the gufeng formation a continental shelf radiolarian chert abstract radiolarian chert deposited on a continental shelf occurs in the middle permian gufeng formation on the northeastern yangtze platform china the sedimentary environments of radiolarian cherts from accretionary complexes have been well studied however there are few studies about radiolarian chert deposited on continental shelves therefore we have completed a sedimentological and geochemical study of the gufeng formation the gufeng formation is subdivided into the phosphate nodule bearing mudstone member pnmm and the siliceous rock member srm in ascending order the basal pnmm consists of glauconite bearing mudstone which indicates deposition under aerobic shallow marine conditions whereas the upper pnmm is composed of mudstone including abundant phosphate nodules suggesting deposition near the outer shelf in suboxic conditions the srm consists mainly of alternating beds of black chert mudstone and siliceous mudstone with minor tuffaceous mudstone and porous chert beds the black chert contains abundant radiolarians sponge spicules and organic matter framboidal pyrite occurs in the black chert porous chert in the srm includes abundant rhombohedral cavities which are dolomite moulds surrounded by quartz suggesting dolomitization before silicification during early diagenesis the gufeng chert is characterized by high si mo ni cu and zn contents and extremely low mn content the gufeng chert has high normal paraffin concentrations with petroleum like markers these geochemical features suggest that the gufeng chert was deposited under sulfate reducing conditions and is not hydrothermal but rather biogenic in origin the sedimentological and geochemical data suggest that the gufeng formation was deposited mainly on the outer shelf in suboxic anoxic conditions caused by organic matter produced during upwelling the upwelling probably led to high radiolarian productivity abundant silica and organic matter were deposited in the gufeng basin compared with chert from other sites the gufeng chert shows many similarities with chert from platformal basins but not with chert from accretionary complexes especially with respect to age rock type stratigraphy and geochemical features the chert of the gufeng formation is similar to that of the phosphoria formation in the western u s a which is a world class phosphorite giant these similarities suggest that upwelling occurred around the eastern margins of panthalassa and the paleotethys concurrently during the middle permian in general shelf type radiolarian chert represented by the gufeng chert usually shows some of the following features dark color lack of rhythmical bedding association with phosphorite platformal limestone felsic tuff abundant organic matter and sulfides shelf type radiolarian chert is deposited in a poorly aerated restricted basin or in an oxygen minimum zone

magnetic properties of exposed deep crustal rocks from the superior province of manitoba abstract a traverse across precambrian rocks of the superior province in central manitoba provided 56 samples for magnetic study from the pikwitonei granulite domain and the adjacent cross lake subprovince these rocks represent a cross section of the continental crust presently exposed at the surface thirty one samples are dominantly paramagnetic and three contain pyrrhotite twenty two samples from silicic plutonic rocks which make up about 80 of the section by volume contain nearly pure magnetite and have susceptibilities that are almost constant between room temperature and 540 c averaging about 1 10 2 si units the mafic supracrustal rocks with much higher iron content are about an order of magnitude less magnetic because they contain little or no magnetite susceptibility is generally independent of metamorphic grade or apparent depth in the crust the discrepancy between induced magnetizations of measured lower crustal rocks and the source requirements of long wavelength magnetic anomalies may be due to a high temperature viscous magnetization in the deep crust

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

magnetization models for the source of the kentucky anomaly observed by magsat abstract both the aeromagnetic data and magnetic anomaly data obtained by magsat indicate the presence of a very magnetic source region within the crust beneath kentucky and tennessee a source model was previously developed to fit surface gravity and long wavelength aeromagnetic data using limited seismic constraint for the present study the model was further developed and it is demonstrated that the source region for the satellite anomaly is considerably more extensive than the kentucky body sensu stricto the extended source region is modeled using both prismatic model sources and dipole array sources magnetization directions for the source region found by inversion of various combinations of scalar and vector data are found to be close to the main field direction implying the lack of a strong remanent component it is shown by simulation that in a case such as this where the geometry of the source is known if a strong remanent component is present its direction is determinable but by scalar data as readily as vector data magnetization magnitude for the extended source region is about 4 a m if the vertical extent of the source includes the whole of the crust

elevated temperature viscous remanent magnetization of natural and synthetic multidomain magnetite abstract the time temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists rock magnetists and magnetic anomaly modellers we have investigated this relationship by studying the viscous remanent magnetization vrm of coarse grained multidomain md magnetite our experiments covered the temperature range from 22 to 400 c times from minutes to months and included both australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method vrm acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories thermal cycling between room temperature at which the measurements were made and the vrm acquisition temperature sharply decreased the temperature dependence of the vrm room temperature vrm acquisition accelerates with time when plotted on a semilog plot whereas at elevated temperature the curves are quasilinear against log time for both the natural and synthetic samples this change in behavior may suggest a variation in the vrm acquisition mechanism as a function of temperature for md magnetite the granulites have a nearly linear increase in vrm acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 c but increase by nearly a factor of 3 by 400 c the increase in vrm of the glass ceramics between 200 and 400 c is in general qualitative agreement with thermal fluctuation theory there was no systematic change in the rate of vrm acquisition with grain size for the multidomain magnetites used in this study elevated temperature e g 400 c vrm acquisition by the deep crustal granulites if extrapolated over the brunhes chron would produce a magnetization of several a m which if true is of the order required by models for the source of long wavelength magnetic anomalies

the granulite eclogite facies transition a comparison of experimental work and a natural occurrence in the bergen arcs western norway a unique segment of the deeper parts of the caledonian continent continent collision zone is exposed in one of the thrust sheets in the bergen arcs of western norway it demonstrates that precambrian granulite facies rocks underwent fluid controlled eclogitization on a regional scale the transition from granulite to eclogite facies is sharp it occurs over distances of centimetres to metres and follows shear zones and infiltration fronts major anastomosing shear zones typically 50 to 100 m thick with well banded eclogite facies rocks crosscut the lithological boundaries they can be followed continuously for several kilometres breccias where metre size blocks of granulite float in an eclogite matrix outcrop over areas of 0 5 km2 areas comprising more than 80 eclogite occupy several square kilometres this mixture of metamorphic facies indicates the metastable occurrence of granulite facies rocks in the lower crust of modern and ancient orogenic belts in contrast to earlier experimental work such field relations imply that the granulite eclogite facies boundary in nature can be sharp and independent of lithological boundaries a partly eclogitized lower crust of the kind exposed in the bergen arcs can explain the 7 2 7 8 km s vp layer present in many crust mantle transition zones anastomosing shear zones and areas of almost complete eclogitization represent potential lower crustal reflectors

astronomical cycle origin of bedded chert a middle triassic bedded chert sequence inuyama japan abstract astronomical forcing is one of the main drivers of climate change and astronomical cyclicity recorded in sediments provides a clue to understand the dynamics of the global climate system bedded cherts consist of rhythmic alternations of chert and shale beds although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles e g fischer 1976 hori et al 1993 conclusive proof remains elusive to explore this possibility we established a continuous high resolution lithostratigraphy of middle triassic bedded chert in central japan the average duration of each chert shale couplet is 20 kyr similar to that of the precession cycle spectral analysis of a bed number series of thickness variations in chert beds was performed assuming that each chert shale couplet represents a 20 kyr precession cycle the results reveal cycles involving approximately 200 20 5 and 2 3 beds corresponding to periodicities of approximately 4000 400 100 and 40 60 kyr respectively by further assuming that the 20 bed cycle represents a 405 kyr eccentricity cycle of constant and stable periodicity we converted the bed number series to a time series spectral analysis of the time series revealed distinct periodicities of 3600 117 97 and 38 kyr in addition to 405 kyr besides 3600 kyr these periodicities agree well with the 120 95 and 37 kyr periodicities for eccentricity cycles and the obliquity cycle during the triassic moreover we detected amplitude modulation of the approximately 100 kyr cycle of thickness variations in chert beds with a 405 kyr periodicity which may correspond to amplitude modulation of 100 kyr climatic precession cycle with the 405 kyr periodicity the approximately 3600 kyr periodicity described above and 1800 kyr periodicity manifested as the amplitude modulation of the 405 kyr cycle are correlated to present day long term eccentricity cycles of 2400 and 4800 kyr evolved by chaotic behavior of solar planets collectively these similarities in the periodicities of dominant cycles their hierarchy and the nature of amplitude modulation of cycles in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles thereby indicating the potential of bedded chert as a template for a mesozoic cyclostratigraphy

diagenetic formation of bedded chert implications from a rock magnetic study of siliceous precursor sediments abstract bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz rich chert beds and clay rich interbeds the origin of the lithologic variations is not well understood a rock magnetic study of a sequence of alternating bands of radiolarian rich and clay rich bands recovered by odp leg 129 which is considered to be the best known analog for the bedded chert precursor sediments shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types with only a minor 1 5 times on average increase in bulk concentration in clay rich bands comparison with magnetic properties of the diagenetically mature chert shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification a simple silica transfer model with dissolution of silica in proto shale donor beds and deposition in proto chert receiver beds indicate that the known features of the bedded chert sequences such as the difference in thickness cosmic dust content between chert and shale can be reproduced in case of donor beds having initially lower clay higher biogenic silica content compared to receiver beds while contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays an association of proto shale beds with high productivity episodes is supported by an independent paleoproductivity proxy their higher al ti values and is in full agreement with the known silica diagenesis trends particularly with the retarding influence of detrital minerals on silica transformations the impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession silica dissolution starts in clay poor intervals when diffusing silica encounters clay rich beds adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz early deposition of quartz cement prevents significant compaction of chert layers while the loss of silica in donor beds leads to their compaction and concomitant increase in non reactive clay and cosmic dust particles severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies

a 70 million year astronomical time scale for the deep sea bedded chert sequence inuyama japan implications for triassic jurassic geochronology abstract the astronomical time scale ats has provided high resolution geochronology however the early mesozoic ats is still under construction partly due to the lack of continuous pelagic sequences of the early mesozoic here we present ca 70 myr long ats constructed from the early mesozoic deep sea bedded chert sequence exposed in the inuyama area central japan the sedimentary rhythms of bedded chert display a full range of climatic precession related cycles 20 kyr cycle as a chert shale couplet and 100 405 2000 to 4000 and 10 000 kyr cycles as chert bed thickness variation the newly established ats inuyama ats is tuned by 405 kyr eccentricity cycle and is anchored at the end triassic radiolarian extinction level as 201 4 0 2 ma this inuyama ats gives ages consistent with the radiometric ages projected to the inuyama deep sea sequence using biostratigraphy and carbon isotope stratigraphy the inuyama ats provides the age constraints for the triassic and jurassic stage boundaries which support the long norian option of muttoni et al 2004 because the deep sea bedded chert sequence covers a long time interval before the cretaceous the ats for the bedded chert will serve as a template for the astrochronology of mesozoic and older ages

astronomical forcing of a middle permian chert sequence in chaohu south china abstract astronomical forcing has been shown to be a fundamental driver of climate change through geological time pelagic bedded cherts deposited in mesozoic ocean basins with chert mudstone cycles have been shown to contain the imprint of milankovitch astronomical climate forcing in the chaohu region south china we studied a middle permian radiolarian chert sequence gufeng formation with chert mudstone couplets reminiscent of the mesozoic cherts but deposited on a continental shelf spectral analysis of lithologic bed thickness data from two sections of this chert sequence reveals that 13 cm to 20 cm chert mudstone cycles in the stratigraphic domain match theoretical 32 kyr middle permian obliquity cycling together with a hierarchy of other cycles with 12 cm 9 cm 7 cm 6 6 cm and 5 4 cm wavelengths tuning the 13 cm to 20 cm stratigraphic cycles to earth s obliquity cycle periodicity indicates that the cm scale cycles are precession scale variations with a strong 400 kyr amplitude modulation tuning to theoretical precession terms provides further support for the astronomical forcing of the chert sequence we propose that monsoon controlled upwelling contributed to the development of the chert mudstone cycles a seasonal monsoon controlled by astronomical forcing i e insolation influenced the intensity of upwelling stronger upwelling increased radiolarian productivity in the surface ocean increasing silica deposition glacio eustatic oscillations from ice sheet dynamics in southern gondwana modulated terrigenous mud flux to the basin the two processes jointly contributed to the astronomical rhythms of these tropical chert mudstone sequences which are characterized by comparably strong obliquity and precession responses subsequent diagenesis distorted the chert and mudstone layering but not enough to destroy the original stratigraphic patterns the resulting astronomical time scale ats assumes a roadian wordian boundary age of 268 8 ma for the onset of the first chert layer at the base of the sequence and ends at 264 1 ma for a total duration of 4 7 myr

advances in characterizing the cyclostratigraphy of binary chert mudstone lithologic successions permian roadian lower capitanian chaohu lower yangtze south china abstract successions of mesozoic and paleozoic radiolarian bedded cherts display a binary cyclic hierarchy of cm scale chert and mudstone couplets some of these couplet successions contain an apparent imprint of milankovitch cycles in which the couplets may reflect changes in silica deposition related to milankovitch forcing for example high insolation for chert deposition and low insolation for mudstone the challenge has been how to represent such binary lithologic series in a format that is suitable for signal processing to search for milankovitch cycles here we present an analysis of chert mudstone binary series by considering in lieu of a boxcar linear interpolation a midpoint triangle linear interpolation the procedure is demonstrated first on synthetic data then on the binary chert mudstone stratigraphic series of the permian guadalupian roadian lower capitanian gufeng formation chaohu lower yangtze south china we assign 1 to the midpoint of each mudstone bed and 1 to the midpoint of each chert bed then linearly interpolate from midpoint to midpoint along the succession time frequency analysis of the midpoint triangle series highlights a previously unnoticed stratigraphic alternation of obliquity and precession band power finally we present a revised astrochronology for the gufeng succession that is anchored at 270 ma at the base of the jinogondolella nankingensis conodont zone base of roadian stage and ending at 264 6 ma in the undifferentiated follicucullus scholasticus ruzhencivisponus uralicas radiolarian zone mid capitanian stage for a duration of 5 4 myr

astronomical forcing of middle permian terrestrial climate recorded in a large paleolake in northwestern china abstract geological records of milankovitch cycles provide a temporal framework for reconstructing earth climate evolution and mapping out the history of the ancient solar system understanding the fundamental celestial mechanics driving astronomical rhythms during the permian period from stratigraphic record is challenging because of the inherent limitations of astronomical solutions the chaotic nature of solar system motion and the paucity of cyclostratigraphic studies of geological records the junggar and adjacent basins of northwestern china constituted a large tectonic lake in the middle permian which deposited one of the thickest and richest petroleum source rock intervals in the world constructing an accurate timescale for the permian junggar basin is critical to understand the processes that controlled organic carbon burial as well as paleoclimatic and paleoenvironmental background conditions in this study detailed spectral analyses of natural gamma ray ngr data were performed on the lacustrine muddy dominated lucaogou formation from five exploitation wells the results reveal significant meter scale sedimentary cycles of 37 8 m 10 7 10 m 3 85 3 2 m and 2 0 1 58 m reflecting oscillations in lithologies variations in changing clay content within the mudstone sandstone and dolomite these cycle wavelength ratios match well with those of the milankovitch cycles predicted for middle permian period the depositional duration of the lucaogou formation was estimated at 3 myr correlation of the 1 2 myr obliquity modulation cycles among the ngr logs sedimentation rates and lake levels of the junggar basin as well as with the middle permian global sea level suggests that long term astronomical grand cycles tightly forced climatic and sedimentary processes throughout the lucaogou formation according to two independent approaches we reconstruct middle permian astronomical parameters earth precession constant k and fundamental frequencies terms gi i 1 2 3 4 5 and calculate astronomical periods eccentricity obliquity and precession these results document astronomical forcing of paleozoic lake systems constrain earth moon orbital evolution and refine the fundamental astronomical frequencies of the solar system during the middle permian

an equivalent layer magnetization model for australia based on magsat data abstract an equivalent layer magnetization model for australia and adjacent oceanic areas is presented the model is obtained by linear inversion of magsat anomaly data measured in the altitude range 325 550 km the anomaly data set has been isolated from the raw data set by use of models of the core field and very long wavelength external fields and is internally consistent certain major structural features of the australian continent are geographically associated with magnetization anomalies a first order difference is seen between the tasman zone and the precambrian cratonic areas magnetization anomalies are much more subdued in the former possibly reflecting a shallowing of the curie isotherm within the crust a profile of the vertical integral of magnetization is presented for a crustal section extending from the gawler block to the southeast coast it is shown that the magnetization variations are probably due partly but not wholly to depth to curie isotherm variations gross magnetization variations among at least three distinct crustal units must be involved

crustal magnetization and temperature at depth beneath the yilgarn block western australia inferred from magsat data variations in crustal magnetization along a seismic section across the archean yilgarn block of western australia inferred from magsat data are interpreted as a subtle thermal effect arising from variations in depth to the curie isotherm the isotherm lies deep within the mantle of the eastern part of the province but transects the crust mantle transition and rises well into the crust on the western side the model is consistent with heat flow variations along the section line the mean crustal magnetization implied by the model is approximately 2 a m the temperature variation implied by the model is consistent with the hypothesis that the crust mantle transition seen seismically corresponds to the mafic granulite eclogite phase transition within a zone of igneous crustal underplating

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

calibrating the guadalupian series middle permian of south china abstract the guadalupian epoch represents one of the most critical intervals during the earth history which is characterized by a series of global geological and biological events however it is poorly constrained by radioisotopic ages in comparison with the high precision dates from the lopingian and lowest triassic in south china and the cisuralian in southern urals russia in this study we report combined ca id tims and sims u pb zircon geochronology for three ash beds from the base and lower part of the kuhfeng formation in the chaohu area southeastern china zircons from ash beds a c and d were initially analyzed by sims and yielded weighted mean 206pb 238u dates of 269 0 1 4 ma mswd 1 03 n 31 266 3 2 2 ma mswd 2 2 n 29 and 262 2 1 7 ma mswd 1 0 n 20 in ascending order however high precion u pb geochronology of the same ash beds by the ca id tims method point to important inaccuracies in the sims results due to pb loss in zircons ash beds a and c yielded two tims weighted mean 206pb 238u dates of 272 95 0 11 ma mswd 1 9 n 7 and 271 038 0 097 ma mswd 2 2 n 8 respectively the depositional age of 272 95 0 11 ma for ash bed a is postulated as the traditional cisuralian guadalupian boundary cgb age in south china our new geochronology suggests that the guadalupian series in south china had a total duration of 13 85 0 52 myr given a guadalupian lopingian boundary glb age of 259 1 0 5 ma the new age calibration of the base of the guadalupian series is ca 0 65 0 51 myr older than and improves upon the previous estimates for the cgb age and the duration of the guadalupian epoch based on the 2016 international chronostratigraphic chart icc and the 2012 geologic time scale gts