inclination variation in the late jurassic to eocene red beds from southeast asia lithological to locality scale approach summary shallowpalaeomagneticinclinationshavebeenfrequentlyreportedfromtheredbedsofcentral and southeast asia to trace the origin of this phenomenon inclination variation in southeast asia is examined on lithological to locality scale lithological aspect of this variation has been studied using the early cretaceous bailong and cangxi formations of the bazhong area 32 1 n 106 7 e the northern sichuan basin samples from 36 sites consisting 18 pairs of successive sandstones and mudstones layers were collected for this purpose from a synclinal structure stepwise thermal demagnetization of most samples revealed the presence of stable characteristic remanent magnetization which is generally unblocked by 680 c positive fold and reversal tests suggest a primary origin for this component yielding the early cretaceous palaeomagnetic direction of declination inclination 20 9 26 5 ks 37 2 95 4 4 n 30 10 pairs of sandstone and mudstone layers show almost identical inclinations isandstones 23 3 3 7 and imudstones 24 7 2 4 but 27 shallower than that expected from the eurasian apparent polar wander paths apwps indicating that no lithological variation in inclination has occurred location wise variation in inclination shallowing is examined through palaeomagnetic data from late jurassic to eocene red beds distributed around southeast asia based on these investigations no inclination shallowing is observed in the eastern part of the south china block scb whereas large degree of shallowing is observed in the sichuan and xining ninhe basins variation in inclination shallowing from one sedimentary basin to another could probably be caused by changes in the depositional environment the eastern part of the scb where no inclination shallowing have been observed is characterized by basin and range type tectono geological setting in contrast the foreland basins are bounded to the north by east west striking high altitude orogenic belt swift developmentofthesebasinsasaresultofflexuralsubsidenceisthemostlikelyreasonforinclination shallowing

area balanced model of the late cenozoic tectonic evolution of the central andean arc and back arc lat 20 22 s age data and estimates on upper crustal shortening together with observations on the present day deep seismic structure magmatic activity and geomorphology indicate that the central andean arc and back arc lat 20 22 s reflect two major thickening mechanisms active during crustal convergence the first process from ca 27 to 5 ma involved crustal underthrusting and pervasive lower crustal shortening and thickening this caused thickening and uplift in the altiplano and shortening in the eastern cordillera and interandean zone the second process from ca 5 ma to the present involved crustal stacking and caused thickening and uplift in the eastern cordillera and shortening in the subandes this concept of two thickening mechanisms is synthesized in an area balanced model describing the crustal evolution from the late oligocene to the present along a profile from the western cordillera to the brazilian shield

laboratory depositional and compaction caused inclination errors carried by haematite and their implications in identifying inclination error of natural remanence in red beds summary undetected depositional and or compaction caused inclination errors may result in an overestimation of tectonically caused latitudinal offset hodych buchan used a single component isothermal remanent magnetization irm acquired in a dc field at a 45 angle to the bedding to test for inclination errors in silurian red beds this approach was criticized by stamatakos et al we produced synthetic depositional and compaction caused inclination errors to test hodych and buchan s approach red bed samples collected from the eocene suweiyi formation tarim basin northwest china were disaggregated using an ultrasonic cleaner and the sediments were mixed with distilled water to make a sediment slurry the sediment slurry spontaneously separated into silt dominated and clay sized parts so deposition and compaction experiments were conducted with three categories of slurries coarse grained the fine grained and a 1 1 volume ratio mixture of the coarse and fine grained slurries during compaction clay sized sediments experienced 17 19 inclination shallowing at 58 magnetic field inclination while coarser sediments showed little laboratory compaction caused inclination error acquisition of irm coercivity spectra and unblocking temperature spectra reveal that the magnetic carrier for the fine grained sample is dominated by pigmentary haematite a deposition experiment was also conducted with the coarse grained sediments which showed a range of depositional inclination error 0 30 carried by larger high unblocking temperature coercivity particles the intermediate unblocking temperature or coercivity component carried presumably by pigmentary haematite is an accurate record of the ambient magnetic field direction our data indicate that the single component irm method of hodych buchan can be used to identify and correct for the compaction caused inclination error of the fine grained samples while it failed to detect any depositional inclination error in the coarse grained samples therefore to better quantify the inclination error in red beds it is suggested that multiple irms be measured in various directions

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

identifying goethite and hematite from rock magnetic measurements of soils and sediments we describe a sequence of experiments involving the acquisition of isothermal remanent magnetization irm in high 2 t fields followed by orthogonal demagnetization and the cooling and heating of remanent magnetization mr over the temperature range 196 c to 680 c the main aim of these experiments is to improve the basis for identifying the imperfect antiferromagnetic minerals contributing to the magnetic properties of natural soil and sediment samples which often contain complex mineral mixtures the procedures developed are first applied to a series of ore and synthetic samples of hematite and goethite each characterized by x ray diffraction xrd spectra and second to controlled mixtures of the two minerals the results confirm the mineral diagnostic nature of the experiments used they are subsequently used to identify goethite and hematite contributions to the rock magnetic properties of a variety of soils and recent sediments goethite can be unambiguously identified often accompanied by hematite in the majority of the samples studied despite indications from the synthetic mixtures that the ratio of goethite to hematite may have to exceed an order of magnitude before the goethite signatures are readily recognizable the rock magnetic tests are a much more sensitive basis for recognizing the presence of goethite in natural samples than are xrd analyses the results suggest that goethite s contribution to the rock magnetic properties of soil and sediment samples may often have been underestimated

magnetic properties of oolitic iron ore on bell island newfoundland abstract magnetic properties of samples from bell island sedimentary rocks have been studied x ray analysis indicates that the main magnetic mineral is hematite in all samples the other iron bearing minerals identified are siderite and chamosite microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite the intensity of natural remanent magnetization nrm varies in the range of 0 03 0 4 a m 1 depending on the percentage of hematite the thermal demagnetization curves of nrm show in some cases a sharp increase in magnetization at temperatures in the range 500 600 c the peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating x ray analysis confirmed that the newly formed material is magnetite since the original nrm has been masked by the new intergrown material this would result in a serious error in the determination of paleomagnetic pole positions the samples showing this behaviour were not considered for paleomagnetic study the samples containing oolitic hematite in a calcite matrix exhibit very high stability of nrm including directional stability until almost 670 c for these samples a virtual pole position based on n 6 samples 32 specimens demagnetized to 665 c is 34 n 114 e not far from published ordovician poles for the north american craton

high unblocking temperature haematite magnetizations of late palaeozoic red beds from the okcheon zone southern part of the korean peninsula summary the late carboniferous and permo triassic red beds of the pyeongan supergroup in the okcheon zone southern part of the korean peninsula were found to contain three components of remanence these components which are resolved by thermal demagnetization are characterized by high hhc for late carboniferous and hhpc for permotriassic intermediate hm and low hi unblocking temperatures as the maximum unblocking temperature spectra of the hhc hhpt component often overlapped the hm spectra above 680 c 3 o to 5 c steps were needed above 670 c to resolve the hhc hhpt component magnetization the tilt corrected mean direction of the hhc component red beds is d 255 5 i 2 4 a 20 8 and that of the hhpt component is d 271 9 i 33 4 c 16 5 the precisions of mean directions of hhc and hhpt components improve after tilt correction and allow the directions to be distinguished from each another this indicates that both hhc and hhpt component magnetizations were acquired during or shortly after the deposition of the red beds according to micropetrographic observations the haematite in the red beds can be divided into three types detrital specularite a non detrital specularite b and pigmental haematite c type a grains are tens of micrometres in size and many of them are martite the remanent magnetization of type a is possibly depositional remanent magnetization drm post drm or chemical remanent magnetization crm acquired close to the deposition time whereas that of types b and c is of crm origin acquired during and after folding respectively the time of folding is well established as a triassic to jurassic event we believe that the remanent magnetization carried by type a grains is primary and is correlated to hhc or hhpt component magnetization

a comparative study of the arm and isr of some natural magnetites of md and psd grain size summary arm and isr stability with respect to alternating field and low temperature treatment were measured for a set of artificial rock specimens each of the specimens contained a known amount of a natural magnetic mineral of a known grain size the following minerals were used a homogeneous natural magnetite a natural titanomagnetite with submicroscopic fe rich inclusions and a natural titanomagnetite and a natural titano maghemite that both show exsolution phenomena the grain size of the fractions varied from 5 to 250 m it is found that the stability of arm and irs of multi domain grains with respect to alternating fields and low temperature treatment is similar whereas for the smallest grains studied 5 m arm is more stable than isr multi domain arm data from this study are in agreement with current multi domain arm theory low temperature behaviour of arm and isr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains

some new aspects for the modelling of isothermal remanent magnetization acquisition curves by cumulative log gaussian functions rock samples containing ferri magnetic minerals acquire an isothermal remanent magnetization irm after exposure to a direct magnetic field measuring irm b allows to produce irm acquisition curves which were modelled by cumulative log gaussian clg functions robertson and france 1994 to discriminate the different magnetic phases an approach of how to find the best model has been added to this method from the investigation of a magnetite sample it is inferred that with the method the remanence acquisition coercive force for each magnetic component in a sample can be deduced the method allows to detect minute concentrations of magnetically hard material in a predominant magnetically soft sample and vice versa it furthermore allows to extrapolate the irm behaviour of samples that do not reach saturation irm in the experiment the study of a natural hematite renders it possible that even well defined hematite may be contaminated by ferri magnetic impurities biasing the remanence acquisition coercive force of the sample towards lower values

dependence of coda attenuation on frequency and lapse time in central greece the dependence of coda attenuationq c on frequency and lapse time was studied data from small local earthquakes recorded at three stations vmr vsi and vfi of the volnet network operating in central greece were used q c was estimated by applying the single scattering model to bandpass filtered seismograms over a frequency range of 1 to 12 hz analysis was performed every 10 s until the end on overlapping time windows q c is found to depend on frequencyf in hz according to a power law q c q0f n observedq0 ranges from 30 to 100 and the powern ranges from 0 90 to 0 70 q0 increases andn decreases with lapse time increasing a strong dependence ofq c on lapse time was also found in the frequency range of 1 to 8 hz and at a short lapse time q c values were found to be similar for all three stations on the other hand at the longest analyzed time window 50 s the estimatedq c values show a discrepancy which is more obvious at a higher frequency band the scattering coefficient around the central station vsi is found to range from 0 029 to 0 0041 km 1 q c from the single scattering model andq s from the amplitude ratio of directs to coda waves for the vsi station are similar we believe dependence of coda attenuationq c on frequency and lapse time is caused by a combination of geotectonic features and depth variation asq s

magnetic intensity variations in red beds of the lodeve basin southern france and their bearing on the magnetization acquisition process summary an investigation of small scale variations of magnetization of a red bed sequence in the permian lodbve basin southern france has been carried out by means of 512 disc shaped samples each 25 mm in diameter and about 6 mm thick it is found that magnetic intensity variations reaching an order of magnitude can occur over stratigraphic distances of only a few centimetres these fluctuations show a clear lithostratigraphic control with intensity peaks systematically located at bed tops rock magnetic experiments and petrographic observations indicate that intensity variations are not due to changes in the amount or properties of the magnetic material external causes such as geomagnetic field fluctuations or superposition of reversed and normal components can be ruled out petrographical evidence suggests that the magnetization is carried by detrital haematite grains and it is proposed that the controlling factor of intensity variations is the efficiency of alignment of these grains along the earth s magnetic field the characteristic intensity patterns with peaks near bed tops can be explained by post depositional alignment of the grains with the highest efficiency near the exposed surface grain mobility can be enhanced by mechanical perturbations bioturbations hydraulic phenomena increased water content and reduced load all these factors tend to see their effect diminish away from the surface and could therefore produce the observed magnetization pattern the proposed model points to an early acquisition of the remanence in the lodbve red beds

relationship between median destructive field and remanent coercive forces for dispersed natural magnetite titanomagnetite and hematite summary remanent acquisition curves remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well defined grain size fractions between 5 and 250 m of natural magnetite titanomagnetite and hematite from these curves the remanent acquisition coercive force h cr the remanent coercive force hcr and the median destructive field of irm h i were determined theoretically these parameters should be the same for an assembly of non interacting homogeneously distributed randomly oriented single domain grains for a given hematite specimen h cr hcr and h i have about the same value in spite of the strong grain size dependence of these parameters for each specimen of magnetite and titanomagnetite the value of h cr is larger than hcr which again is larger than h i however the ratios h cr hcr and h i hcr appear to have a different constant value an interesting relationship which appears to hold for dispersed magnetite titanomagnetite or hematite grains between 5 and 250 m independently of grain size quantity and packing density of the magnetic material is

rockmagnetic properties of fine grained natural low temperature haematite with reference to remanence acquisition mechanisms in red beds summary grain size dependence of some remanent magnetic properties of a natural low temperature haematite is reported the haematite was obtained from a gossan in montarnu france and upgraded by magnetic separation seven grain size fractions were prepared ranging from 10 down to 0 25 m by means of microprecision sieving and centrifuging the haematite has an average crystallite size of 60 nm and contains minor contents of si al and h2o its unit cell dimensions are very close to those of ideal haematite the magnetic properties of the haematite are strongly dependent on its thermal history for the original unheated haematite fractions the isothermal saturation remanence jrs decreases with grain size the remanent coercive force hcr remanent acquisition coercive force hcr and median destructive field of the saturation remanence h1 2i are maximal in the 1 0 0 5 m fraction this grain size range can be understood as the sd threshold size in analogy with ferrimagnetic minerals jrs hcr x xin is a useful grain size indicator lowtemperature cycling of jrs shows identical behaviour for all fractions the remanence decreases slightly and shows no sharp morin transition after annealing the haematite fractions at 700 c jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment both hcr and hcr have increased and their maximal values are shifted to the 2 1 1 0 m fraction all fractions show a much sharper morin transition during low temperature cycling of jrs due to crystallite enlargement trm decreases with grain size the different rock magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite drm with a high temperature origin and a haematite crm which has a low temperature origin two natural red beds one with a crm and one with a drm associated with high temperature haematite broadly show the expected differences in coercivity and in annealing behaviour although a full account of all aspects of the observed behaviour can not yet be given

unique thermoremanent magnetization of multidomain sized hematite implications for magnetic anomalies intense magnetic remanence 100 1000 a m associated with md hematite and or titanohematite and associated with high koenigsberger ratios 40 1000 indicate that magnetic remanence may dominate the total magnetization if these minerals are volumetrically significant titanohematite behaves similarly to hematite and thus the grain size dependence of trm acquisition in hematite is considered as a generalization the transition between truly md behavior and tendency towards sd behavior in hematite has been established to be between grain sizes of 0 1 and 0 05 mm in contrast to magnetite and titanomagnetite hematite exhibits inverse grain size dependence with md hematite acquiring a relatively intense trm in the geomagnetic field comparable to sub micrometer sized magnetite and only an order of magnitude less than sd magnetite consequently md hematite and by analogy titanohematite remanence may be of significance as a source of magnetic anomalies at all scales md hematite exhibits trm weak field acquisition behavior that is different from all other magnetic minerals being the only magnetic mineral having an rem trm sirm value e0 1 for trm acquisition in the geomagnetic field the very different trm behavior of md hematite in contrast to magnetite is due to two factors the first is the lesser influence of demagnetizing energy with respect to wall pinning energy at temperatures almost up to the curie temperature for hematite the second is the greater importance of the magnetostatic energy in the applied field which for hematite dominates the total energy at high temperatures fl 2000 published by elsevier science b v all rights reserved

near surface characterization using a roadside distributed acoustic sensing array abstract due to the broadband nature of distributed acoustic sensing das measurement a roadside section of the stanford das 2 array can record seismic signals from various sources for example

crossover plots a useful method for plotting sirm data in paleomagnetism the effective grain size of magnetic minerals and therefore domain type controls the saturation isothermal remanent magnetization acquisition and subsequent demagnetization response and can be used to create crossover plots for paleomagnetic specimens where sirm is plotted as a function of the field applied using a logarithmic scale this paper presents templates of such plots for magnetite pyrrhotite hematite and goethite onto which sample data can be plotted for conventional paleomagnetic specimens to characterize their magnetic mineralogy and effective grain size

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

estimating the concentration of aluminum substituted hematite and goethite using diffuse reflectance spectrometry and rock magnetism feasibility and limitations this study was supported by the national natural science foundation of china 41374073 and 41430962 the national program on global changes and air sea interaction gasi 04 01 02 and the chinese continental shelf deep drilling program gzh201100202 pengxiang hu was further supported by the china scholarship council 2013 3009 david heslop and andrew roberts were supported by australian research council discovery project dp110105419

high temperature characteristics of a natural titanomagnetite summary continuous recordings were made of thermal demagnetizations of isr arm and trm induced in artificial rock specimens the artificial rock specimens contained well defined grain size fractions of a natural titanomagnetite which under an optical microscope was homogeneous using a transmission electron microscope it was shown that submicroscopic inclusions exist within the titanomagnetite screening of the remanence of these inclusions by the surrounding titanomagnetite is thought to be responsible for the occurrence of peaks in the continuous thermal decay curves of isr and arm higher peaks observed in the case of trm are thought to be caused by the combined effects of screening and an anti parallel remanence in the titanomagnetite further submicroscopic exsolution processes which occur in the titanomagnetite as a result of repeated heating can be deduced from the development of new peaks from changes in the temperature at which the peak s occur their shape and their relative height the use of stepwise demagnetization techniques on the titanomagnetite yielded erroneous directional results due to the screening in the titanomagnetite the biasing effects due to screening could be avoided by using continuous thermal demagnetization techniques

integrated stratigraphy of the smirra core umbria marche basin apennines italy a new early paleogene reference section and implications for the geologic time scale pelagic sections of the umbria marche basin in the northern apennines italy have provided key geological archives for studying critical intervals of early paleogene time in addition to classical sections the smirra coring project provides a new record of relatively undisturbed sediments 120 m from 4 overlapping holes of the upper scaglia formation paleocene middle eocene here we present a new high resolution integrated magneto bio stratigraphy of the 93 m composite section drilled in smirra holes 1 and 2 the correlation of the magnetostratigraphy to published geomagnetic polarity timescales gptss ck95 gpts2004 gpts2012 as constrained by calcareous nannofossil biostratigraphy reveals that the cores range from chron c26r to c21n thanetian lutetian 60 to 46 ma sedimentation rates range from 0 6 to 1 cm kyr and are comparable with those from coeval sections in the umbria marche basin they point to significant discrepancies between existing time scales which mainly involve the duration of chron c23 and subchrons within c23n and c24n application of the average spectral misfit method on the magnetic susceptibility record reveals a strong signature of orbital forcing and allows an independent estimate of the sedimentation rates this analysis offers additional insights into solving time scale issues which mainly concern the duration of chron c23n our results imply a significantly longer duration for c23n 2n which would accommodate the presence of an extra third 405 kyr eccentricity related cycle in c23n the smirra record thus provides new constraints on the early paleogene time scale a key record for further cyclostratigraphic and astrochronological research and a reference framework for paleo climatic and oceanographic studies