first early permian paleomagnetic pole for the yili block and its implications for late paleozoic postorogenic kinematic evolution of the sw central asian orogenic belt we conducted a paleomagnetic study on the early permian volcanic and sedimentary rocks and the neoproterozoic mafic dikes in the yili block nw china magnetite and hematite were proven to be the principal magnetic remanence carriers demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity the magnetic remanence of only the early permian strata turned out to be primary based on positive fold tests meanwhile the magnetic remanence age of the mafic dikes is ambiguous accordingly the first early permian paleomagnetic pole for the yili block is calculated at 81 5 n 256 5 e n 11 and a95 10 9 comparisons of this new pole with published ones from the yili tarim and south junggar blocks provide new quantitative constraints on late paleozoic kinematic evolution of the sw central asian orogenic belt 1 between the yili and tarim blocks significant relative movement took place along major strike slip faults during the late carboniferous to early permian 580 290 km and the early to late permian 585 340 km and the displacement rate increased from the late carboniferous to early permian 19 3 9 7 mm yr to the early to late permian 29 3 17 0 mm yr 2 a significant relative rotation of 28 3 18 3 in the late permian and a lateral displacement of 630 295 km after the late permian occurred between the yili and south junggar blocks the significant strike slip movements played an important role in the formation of the central asian orogenic belt and should be considered with great attention in tectonic and paleogeographic reconstructions

thermal demagnetization of vrm and ptrm of single domain magnetite no evidence for anomalously high unblocking temperatures stepwise thermal demagnetization of viscous remanent magnetization vrm and partial thermoremanent magnetization ptrm of single domain magnetite with mean grain size 37 nm reveals only minor deviations from the predictions of neel theory three different initial states were used following 1 alternating field demagnetization to 100 mt 2 thermal demagnetization to 610 c and 3 modified thermal demagnetization consisting of zero field heating to 610 c followed by zero field cooling to the vrm or ptrm acquisition or blocking temperature tb vrms were produced by applying a 0 1 mt field for 3 5 hr starting from initial states 2 3 and 1 in that order at tb 283 c and from states 3 2 and 1 at tb 404 c in a final experiment ptrm was produced by applying 0 1 mt during cooling from tb1 414 c to tb2 404 c starting from state 3 in all but one case vrm began to unblock significantly 90 of initial intensity 15 c below tb dropped to 50 at tb and then tailed off dropping below 10 40 c above tb 283 c or 25 c above tb 404 c partial trm behaved in the same way except that demagnetization was essentially complete by tb1 414 c the maximum unblocking temperatures tub observed are exactly as predicted by the theory of pullaiah et al 1975 although very long acquisition times cannot be tested we find no evidence for anomalously high tubs of vrm in single domain grains in previously reported cases of rocks with unexpectedly high tubs the vrm was probably carried by multidomain grains

on the resolution of multivectorial remanences abstract low temperature demagnetization ltd promotes single domain sd like alternating field af and thermal demagnetization properties of anhysteretic remanent magnetization arm and thermoremanent magnetization trm by eliminating low coercivity or low unblocking temperature fractions of remanence we have investigated the effectiveness of ltd in improving the independence of partial arms parms independence of parms and of partial trms ptrms is crucial because multivectorial directional analyses and paleointensity determinations are otherwise compromised in normalized vector plots of af demagnetization of orthogonal parms ltd treated samples showed universal improvement in the degree of independence the greatest improvement was for synthetic sd and pseudo single domain magnetites and natural lake sediments on the other hand independence barely improved for synthetic multidomain magnetite granites and gabbros in general ltd provides a better resolution of superimposed vectors in multicomponent remanence the direction of the original arm corresponding to primary nrm in nature is perfectly recovered however the parm the overprint is not perfectly resolved especially for overprinting coercivities

paleointensity determination using continuous thermal measurements by a high temperature vibrating thermomagnetometer a vibrating thermomagnetometer which measures magnetization m continuously at high temperature is used to test a new method of paleointensity determination involving single rather than double heatings loss of natural remanent magnetization nrm is recorded by the average of 20 25 measurements at the peak temperature t achieved in a zero field heating step partial thermoremanent magnetization ptrm is calculated from the average m after in field cooling to room temperature t0 for comparison with data taken at t0 values of m measured at t are multiplied by ms t0 ms t with the aid of the thermomagnetic or ms t curve measured on a chip of the sample nrm versus ptrm results from 11 heating cooling steps on a diabase containing both single domain magnetite inclusions in plagioclase and coarser multidomain magnetite grains reproduce the features of thellier double heating paleointensity results for samples from the same site the nrm ptrm plot is nonlinear with convex down curvature for a stringent validation of the single heating method a truly single domain sample needs to be tested

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

paleointensity determination on the late precambrian tudor gabbro ontario paleointensity determination by the coe modified thellier method was carried out on the 1100 ma tudor gabbro of southern ontario the tudor has a bivectorial natural remanent magnetization nrm the c nrm is a grenvillian uplift thermoviscous remagnetization which overprints the a nrm the surviving primary thermoremanent magnetization trm of the tudor intrusion a sharp junction at 530 c between the a and c vectors in laboratory thermal demagnetization matches the peak metamorphic reheating temperature of 480 500 c in nature when the difference in time scales is taken into account the unblocking temperature distributions of the a nrm and of laboratory trm also match a is carried by single domain or small pseudo single domain grains probably magnetite rods exsolved in plagioclase or pyroxene at temperatures above 580 c arai plots of nrm versus partial trm are linear from 520 c or 530 c to 580 c for 45 specimens of 19 samples from 9 sites the 45 reliable paleointensity values from these specimens give a virtual axial dipole moment vadm of 4 6 0 8 1022 a m2 around 1100 ma this is about one half of the modern field intensity but is similar to the mean cretaceous cenozoic intensity and to most archean and proterozoic vadms keweenawan rocks whose ages are within 20 myr of the tudor gabbro have a vadm of 11 4 1022 a m2 the difference between the tudor and keweenawan vadms is similar to differences recorded over similar time intervals in the cretaceous and seems to represent normal variation in geomagnetic field strength throughout earth s history

magnetic overprinting in southeastern australia and the thermal history of its rifted margin the magnetizations of many rock units in southeastern australia have been severely overprinted interpretation of early work was hindered by this and one of the aims of the current research is to ascertain the nature of the overprint mechanism its geological significance and its regional extent results from three igneous bodies from the sydney basin which have been studied previously in a reconnaissance fashion have provided a new insight into the reason for such widespread magnetic overprinting the hornsby breccia hb and milton monzonite mm yield similar magnetic signatures both containing discrete high and low temperature magnetizations with directions of dec 72 inc 83 95 4 and dec 13 inc 76 95 2 respectively for hb and dec 60 inc 78 95 4 and dec 348 inc 79 95 6 respectively for mm the corresponding pole positions are 29 s 166 e a95 7 and 59 s 139 e a95 4 for hb and 22 s 171 e a95 7 and 55 s 158 e a95 11 for mm the paleomagnetic direction and pole from the third body studied the mogo hill basalt mh are dec 241 inc 81 95 5 and 41 s 130 e a95 9 this pole position and the two low temperature overprint pole positions from hb and mm are consistent with ages ranging from 100 m y to approximately 70 m y when compared to the late mesozoic apparent polar wander path we relate the overprinting to uplift erosion and rapid supracrustal cooling of the southeastern margin of australia associated with initial phases of marginal rift development prior to seafloor spreading in the tasman sea the temperatures of rocks now exposed may have reached 200 c or higher before the rapid cooling blocked and stabilized the overprint magnetizations

paleomagnetism and paleothermometry of the sydney basin 1 thermoviscous and chemical overprinting of the milton monzonite the early triassic 245 ma milton monzonite of the sydney basin australia has four distinct components of natural remanent magnetization nrm with only slightly overlapping ranges of unblocking temperatures the low temperature lt component the first to be thermally demagnetized is thought to be a late cretaceous 100 ma thermoviscous overprint acquired in slow cooling during uplift the high temperature ht component the second to be demagnetized is probably the primary thermoremanent magnetization trm of the milton intrusion but could possibly be a jurassic overprint lt and ht are usually carried by magnetite and occasionally by pyrrhotite samples from nine sites have a further nrm component which unblocks at higher temperatures than ht but below the magnetite curie temperature of 580 c this component is argued to be a chemical remanent magnetization crm because of its discrete range of high unblocking temperatures above those of the thermal components ht and lt and is called crm1 crm1 has almost the same direction as lt and is likely carried by authigenic magnetite produced during uplift 100 ma samples from five sites have a fourth nrm component with a direction resembling that of ht but carried by hematite this fourth component could be a primary trm but is more likely a crm and is therefore called crm2 the ht crm2 mean direction is d 50 i 75 5 defining a paleopole at 16 s 172 e the ht crm2 paleopole falls near 150 ma on the australian apparent polar wander path but is a considerable distance from paleopoles of permian and early triassic age there is no known tectonic or other remagnetizing event in the sydney basin around 150 ma for this reason we propose that the ht crm2 paleopole defines a new triassic segment of the australian polar wander path the lt crm1 mean direction is d 348 i 79 with a paleopole falling at 56 s 1580 e near 100 ma on the polar wander path this age is consistent with uplift and cooling related to initial rifting of the tasman sea

absolute velocity of north america during the mesozoic from paleomagnetic data we use paleomagnetic data to map mesozoic absolute motion of north america using paleomagnetic euler poles pep first we address two important questions 1 how much clockwise rotation has been experienced by crustal blocks within and adjacent to the colorado plateau 2 why is there disagreement between the apparent polar wander apw path constructed using poles from southwestern north america and the alternative path based on poles from eastern north america regarding 1 a 10 5j clockwise rotation of the colorado plateau about a pole located near 35jn 102jw seems to fit the evidence best regarding 2 it appears that some rock units from the appalachian region retain a hard overprint acquired during the midcretaceous when the geomagnetic field had constant normal polarity and apw was negligible we found three well defined small circle apw tracks 245 200 ma pep at 39 2jn 245 2je r 81 1j root mean square error rms 1 82j 200 160 ma 38 5jn 270 1je r 80 4j rms 1 06j 160 to f125 ma 45 1jn 48 5je r 60 7j rms 1 84j intersections of these tracks the cusps of gordon et al tectonics 3 1984 499 are located at 59 6jn 69 5je the 200 ma or j1 cusp and 48 9jn 144 0je the 160 ma or j2 cusp at these times the absolute velocity of north america appears to have changed abruptly north america absolute motion also changed abruptly at the beginning and end of the cretaceous apw stillstand currently dated at about 125 and 88 ma j geophys res 97 1992b 19651 during this interval the apw path degenerates into a single point implying rotation about an euler pole coincident with the spin axis using our pep and cusp locations we calculate the absolute motion of seven points on the north american continent our intention is to provide a chronological framework for the analysis of mesozoic tectonics clearly if apw is caused by plate motion abrupt changes in absolute motion should correlate with major tectonic events this follows because large accelerations reflect important changes in the balance of forces acting on the plate the most important of which are edge effects subduction terrane accretion etc some tectonic interpretations 1 the j1 cusp may be associated with the inception of rifting of north america away from land masses to the east the j2 cusp seems to mark the beginning of rapid spreading in the north atlantic 2 the j2 cusp signals the beginning of a period of rapid northwestward absolute motion of western north america motion of tectonostratigraphic terranes in the westernmost cordillera seems likely to have been directed toward the south during this interval 3 the interval f88 to f80 ma saw a rapid decrease in the paleolatitude of north america unless this represents a period of true polar wander terrane motion during this time should have been relatively northward d 2003 elsevier b v all rights reserved

constraining the intracontinental tectonics of the sw central asian orogenic belt by the early permian paleomagnetic pole for the turfan hami block in order to better understand the kinematics and geodynamics of the late paleozoic to early mesozoic transition from accretionary to intracontinental tectonics in the southwest central asian orogenic belt we performed a paleomagnetic study on lower permian strata in the turpan hami basin nw china magnetite and hematite are shown to be the principal magnetic remanence carriers a typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study stable characteristic remanent magnetizations all of reverse magnetic polarity are revealed by step demagnetizations remanence age is determined to be of early permian affinity based on a fold test remanence behavior of minerals and geochronologic data with inclination shallowing corrections the first early permian paleomagnetic pole for the turpan hami block is acquired as 75 7 n 276 3 e a 95 5 7 and n 10 sites comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region the turpan hami and yili blocks have kept their relative positions since the early permian significant latitudinal movement of 14 7 7 2 occurred between the turpan hami and south junggar blocks and could be mainly accommodated by late paleozoic crustal shortening of at least 530 km across an intra arc basin in the bogda belt the turpan hami block has experienced large magnitude relative rotations of 41 1 7 1 a nd 21 2 9 4 with respect to tarim and south junggar since the early permian respectively corresponding to dominant intracontinental tectonics characterized by large magnitude strike slip displacements along megashear zones

effect of grain size and domain state on thermal demagnetization tails thermal demagnetization of viscous remanence vrm and partial thermoremanence ptrm of 20 and 135 m natural magnetites reveals a broad spectrum f tub of unblocking temperatures tub both and tb the blocking temperature in contrast 0 04 m single domain sd grains demagnetize sharply at tub tb high and low tub tails of f tub were wider for 135 m multidomain md grains than for 20 m psd grains 10 mt alternating field demagnetization rendered the vrm of the 20 m magnetite more sd like in subsequent thermal cleaning selectively erasing both low and high tub tails of f tub primary and secondary remanences are then cleanly separated and non linear thellier paleointensity determination becomes linear our results agree well with tb tub data for 105 year thermoviscous overprints in the milton monzonite over the range of log time common to the two studies because tub av tb for psd and md as well as sd grains the pullaiah et al paleothermometry method will work if tub av values are used instead of tub max values

rock magnetic properties of remagnetized palaeozoic clastic and carbonate rocks from the ne rhenish massif germany summary several rock magnetic properties are reported for remagnetized biohermal carbonate rocks platform carbonate rocks and siliciclastic rocks from the ne rhenish massif germany thermal demagnetization of a triaxial isothermal remanent magnetization irm applied at room temperature identifies a variety of magnetic minerals in the sample set magnetite is the predominant magnetic phase and carries a late palaeozoic remagnetization component and can be accompanied by haematite and pyrrhotite as the carrier of remagnetization high haematite contents are characteristic for samples carrying a remagnetization of triassic age zero field heating of remanence acquired at 10 k indicates significant contribution from magnetic material with blocking temperatures between 10 and 100 k the verwey transition at 120 k is much more pronounced in siliciclastic rocks than in carbonate rocks and is presumably related to the presence of detrital multidomain md magnetite samples from biohermal carbonate rocks have hysteresis properties with high m rs m s and h cr h c ratios similar to those from remagnetized carbonate rocks from north america in those lithologies the intensity decrease of a low temperature irm during heating from 10 to 100 k combined with the absence of paramagnetic minerals high hysteresis ratios magnetic viscosity and frequency dependence of susceptibility give strong evidence for the presence of superparamagnetic sp magnetite this supports the hypothesis of a chemical remagnetization event in the ne rhenish massif during the late carboniferous the rock magnetic properties of siliciclastic rocks are characterized by high amounts of paramagnetic material and md magnetite md material obscures the contributions of other material to the bulk magnetic properties completely in most siliciclastic rocks and partly in platform carbonate rocks

recognizing magnetostratigraphy in overprinted and altered marine sediments challenges and solutions from iodp site u1437 core disturbance drilling overprints post depositional acquisition of remanence authigenic growth of magnetic iron sulfides and alteration all contribute challenges to recognizing the primary magnetostratigraphy in marine sediments we address these issues in a sequence of tuffaceous muds and volcaniclastics at international ocean discovery program site u1437 and produce the longest continuous magnetic polarity stratigraphy in the history of scientific ocean drilling remanence measurements were filtered to remove intervals affected by fluidization plastic sediment disturbance and core biscuiting drilling overprints are concentrated in the disturbed annulus surrounding intact core material bioturbation was limited to a vertical extent of at most 15 cm changes in sediment color stiffness and magnetic hysteresis all suggest that remanence was locked in within a few meters of the sediment water interface we did not observe any systematic offset between magnetostratigraphic and biostratigraphic datums authigenic growth of greigite in response to both initial sulfate reduction in the upper 50 m of the sediment column and to deeper resupply of sulfate has led to magnetic overprinting anomalous polarity artefacts extending 5 m and occurring within about 20 m below a real polarity transition appear to be due to a chemical remanence acquired by greigite produced during early diagenesis diagenetic magnetic mineral alteration resulted in the progressive loss of fine grained magnetite which enhanced susceptibility to drilling and post drilling overprints and increased the resistance of these overprints to removal by conventional demagnetization we recovered the magnetostratigraphic record from many samples with resistant overprints through low temperature demagnetization through the verwey transition this article is protected by copyright all rights reserved

triassic paleomagnetism from the western pyrenees revisited implications for the iberian eurasian mesozoic plate boundary since the pioneering studies of van der voo tectonophysics 7 1969 5 and van der voo and boessenkool j geophys res 78 1973 5118 paleomagnetism of permo triassic redbeds and volcanics from the western pyrenees has furnished important contributions for delineating the mesozoic boundary between the iberian and eurasian plates in this paper we present a new paleomagnetic study focussed on triassic red beds 23 sites of the paleozoic basque massifs pbm the aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the western pyrenees two stable magnetic components have been isolated 1 a dual polarity pre folding magnetisation carried by specular hematite and 2 a secondary normal polarity component also carried by hematite our data confirm both the origin and the rotation pattern of the primary remanence described in previous works nevertheless field tests performed on the secondary component do not confirm the earlier interpretations by schott and peres tectonophysics 156 1988 75 as they indicate a synfolding nature of the remagnetisation instead of a post folding origin we consider that the secondary component is better explained if a cretaceous age is considered the presence of such remagnetisation in the western pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern iberia in connection with the extensional tectonic events that occurred during cretaceous times a comparison of the rotations recorded by the triassic component and by the remagnetisation indicate that the paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired most likely in connection with the counterclockwise rotation of iberia with respect to eurasia these results favour that the mesozoic plate boundary between the iberian and eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary

paleomagnetic dating of fracturing using breccia veins in durness group carbonates nw scotland abstract a paleomagnetic study of red fault related breccia veins in the cambro ordovician durness group in nw scotland was conducted to determine the time of brecciation the origin of the veins and the nature and timing of associated fluid related diagenetic alteration the veins contain brecciated fragments of the host durness group and strike either east west or north south clasts of breccia cemented by calcite suggest multiple brecciation events the host durness limestone is a gray dolomite and contains a devonian chemical remanent magnetization crm that resides in magnetite the veins contain magnetizations that reside in hematite and are interpreted as crms the breccias in north south veins contain a triassic crm whereas the veins with east west strikes contain a jurassic crm authigenic hematite is common in the breccias along growth planes in the calcite cements the two crms within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite the brecciation and fluid flow events are interpreted to be related to extension in the mesozoic which is consistent with the extensional history of the northern atlantic margins

paleomagnetism of the mount stuart batholith revisited again what has been learned since 1972 we have collected 20 new paleomagnetic sites from the mount stuart batholith and the adjacent beckler peak stock using thermal and low temperature demagnetization and rock magnetic tests we have found that the remanence in most of the batholith is carried by single domain magnetite the mean of the new mount stuart batholith sites is d 354 2 i 46 2 k 87 2 a95 4 6 n 11 and is similar to those of the beck and noson 1972 and beck and others 1981 studies examination of ar geochronology of hornblende and biotite from the mount stuart batholith finds that the southern part of the batholith cooled through the blocking temperature of magnetite at 91 ma the northern part cooled through the blocking temperature of magnetite at 86 ma and of pyrrhotite at 83 ma from these combined results we conclude that the paleomagnetic directions from the southern part of the batholith were acquired within 0 5 to 1 0 ma of the time at which the ah barometers cooled below their closure temperature use of the ah barometry to establish paleohorizontal for these rocks is thus well justified correcting the mount stuart direction for the tilt indicated by the aluminum in hornblende ah barometry the mean becomes d 355 9 i 50 6 this result places the mount stuart batholith at a latitude of 31 3 3 8 3 4 n at 91 ma by identifying the carrier of remanence in the mount stuart batholith as being single domain sd magnetite we have removed the major remaining uncertainty in interpreting the paleomagnetism of these rocks the characteristic magnetization repeatedly found in the mount stuart rocks clearly supports the microplate tectonic model of beck and noson 1972 presently manifest as the baja british columbia baja bc hypothesis

curie temperatures of titanomagnetite in ignimbrites effects of emplacement temperatures cooling rates exsolution and cation ordering pumices ashes and tuffs from mt st helens and from novarupta contain two principal forms of titanomagnetite homogeneous grains with curie temperatures in the range 350 500 c and oxyexsolved grains with similar bulk composition containing ilmenite lamellae and having curie temperatures above 500 c thermomagnetic analyses and isothermal annealing experiments in combination with stratigraphic settings and thermal models show that emplacement temperatures and cooling history may have affected the relative proportions of homogeneous and exsolved grains and have clearly had a strong influence on the curie temperature of the homogeneous phase the exsolved grains are most common where emplacement temperatures exceeded 600 c and in laboratory experiments heating to over 600 c in air causes the homogeneous titanomagnetites to oxyexsolve rapidly where emplacement temperatures were lower curie temperatures of the homogeneous grains are systematically related to overburden thickness and cooling timescales and thermomagnetic curves are generally irreversible with lower curie temperatures measured during cooling but little or no change is observed in room temperature susceptibility we interpret this irreversible behavior as reflecting variations in the degree of cation ordering in the titanomagnetites although we cannot conclusively rule out an alternative interpretation involving fine scale subsolvus unmixing short range ordering within the octahedral sites may play a key role in the observed phenomena changes in the curie temperature have important implications for the acquisition stabilization and retention of natural remanence and may in some cases enable quantification of the emplacement temperatures or cooling rates of volcanic units containing homogeneous titanomagnetites

magnetostratigraphy and tectonic implications of paleogene neogene sediments in the yinchuan basin western north china craton abstract the yinchuan basin one of the cenozoic faulted basins in the western margin of the north china craton ncc was filled by thick cenozoic fluvial lacustrine sediments which documented tectonic sedimentary processes however many debates were highlighted in the cenozoic tectonic evolution processes due to a lack of ages of the cenozoic sediments in the basin in this paper we provide a high resolution magnetostratigraphic records of the yinchuan basin based on the xl15 1 01 borehole in the southwestern basin detailed magnetostratigraphy of the 850 m thick sedimentary sequences record c2an 1n to c8n 2n of the geomagnetic polarity timescale gpts with the age span of 26 06 ma to 2 77 ma corresponding to the middle oligocene pliocene which delimits the depositional periods of the qingshuiying zhangnengbu and ganhegou fomations in the yinchuan basin the upper age limit of the qingshuiying formtion is 21 32 ma the zhangnengbu formation extends from 21 32 to 10 17 ma and the ganhegou formation spans the interval of 9 58 to 2 77 ma the sedimentary discontinuity between the ganhegou formtion and the zhangnengbu formation in the yinchuan basin initiated at 10 17ma which is interpreted as the initial time for the northeastward expansion of the tibetan plateau impact on this region and continued for 0 59 myr the depth age correlation determined from magnetostatigraphy reveals a distinct increase in sedimentation rate at 9 58 ma responding to an intensive uplift of the tibetan plateau

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

the demagnetization temperature necessary to remove viscous remanent magnetization recent experimental studies have shown that the temperature necessary to demagnetize viscous remanent magnetization vrm in magnetite bearing rocks is significantly higher than predicted by standard neel single domain theory the walton time temperature equation which has been invoked by some to explain the enhanced thermal stability actually relates intensities of vrms acquired at different temperatures rather than describing the unblocking of a previously acquired vrm we show that the low field approximation lfa used in all previous formulations which predicts equal vrm acquisition and decay rates is not valid for a magnetite assemblage in the earth s field a medium field approximation mfa predicts significantly faster vrm acquisition and therefore greater thermal stability in decay for the larger particles in the assemblage conventional theory does apply to red bed acquisition decay experiments since the lfa is valid for hematite

low temperature behavior of single domain through multidomain magnetite abstract we have investigated the low temperature behavior of a suite of grown synthetic and natural magnetites that span single domain sd and multidomain md behavior synthetic samples had been grown in the laboratory either in an aqueous medium or in glass natural samples included sd magnetites occurring in plagioclase and truly md magnetites in the form of large octahedra in all experiments a sample was first given a saturation remanence at room temperature next moment was measured continuously during cooling and warming between 230 k and 60 k similar to results reported earlier by other workers magnetic memory is large in sd samples whereas truly md samples are almost completely demagnetized by cycling between room temperature and 60 k pseudo single domain samples exhibit behavior that is intermediate with respect to that of the sd and truly md states when data from this study are combined with data obtained by hartstra 10 from sized natural magnetites it is found that the percentage of total remanence that survives cycling between room temperature and 60 k decreases linearly with the logarithm of grain size and thus with increasing number of domains this relation suggests that memory can provide a reasonable estimate of grain size in those magnetite bearing rocks for which these samples provide good analogues remarkably some of the large natural octahedra provide a magnified view of md response to low temperatures and thus reveal two surprising and intriguing types of behavior first below approximately 180 k these octahedra demagnetize through a series of large barkhausen jumps second near 117 k these same octahedra exhibit a wild zone where magnetic moment executes large random excursions we interpret these two phenomena as direct evidence for the unpinning and irreversible displacement of domain walls in response to the drop in coercivity and possibly the broadening of domain walls as temperatures drop toward the isotropic point one implication of this behavior is that cooling to progressively lower temperatures could provide an effective method for stepwise removal of paleomagnetic components carried by md grains even without passage through the isotropic point of magnetite

testing the independence of partial thermoremanent magnetizations of single domain and multidomain grains implications for paleointensity determination 1 the thellier method of paleointensity determination requires that partial thermoremanent magnetizations ptrms be additive mutually independent and reciprocal in their thermal blocking and unblocking we tested the independence law by thermally demagnetizing a sum of orthogonal ptrms produced by replacing part of an original trm tc t0 h by ptrm t t0 h with h rotated 90 between the trm and the ptrm tc curie temperature t0 room temperature the composite remanence simulates thermal remagnetization in nature for single domain grains thermal demagnetization resolved the orthogonal ptrms cleanly two remagnetization temperatures were tried t 400 c and 550 c the latter gave a better test because the two ptrms were almost equal directions were recovered to within 5 for three of the four ptrms trajectories on vector diagrams intersected at exactly 400 c or 550 c and the arai plots were linear and yielded the correct field intensity for both ptrms however multidomain magnetite violated the independence law a remagnetization temperature t 400 c produced almost equal ptrms but in thermal cleaning because of overlap of their unblocking spectra they no longer appeared orthogonal each was displaced 13 15 toward the direction of the other as judged by quasi linear segments on a vector diagram the angle between the fields that produced the ptrms thus appeared to be 60 not 90 arai plots for both ptrms were strongly curved and yielded no usable paleointensity estimate phenomenological modeling correctly predicted the main results of the independence experiments

orogenic remagnetizations in the front ranges and inner foothills of the southern canadian cordillera chemical harbinger and thermal handmaiden of cordilleran deformation abstract paleomagnetic studies of carbonate rocks in mountain belts have been helpful to the understanding of orogenic mational origin with only one exception the a component has normal polarity in the front ranges and reverse processes the timing of deformation and the nature of fluid flow in actively deforming foreland basins most studies on shallow water carbonates indicate that their remanence is not primary but rather they record remagnetization events contemporaneous with orogeny by considering the remanent directions fold test results and rock magnetic properties and tying these results to complementary structural geochronological and geochemical studies we are able to interpret deformation history hydrological environment and orogenic diagenesis we study the relationship between remagnetization and deformation of paleozoic carbonates of the front ranges and inner foothills of the southern canadian rockies the magnetic properties are remarkably constant along 500 km strike length as sampled at 124 sites through 4 transects primary paleozoic remanent directions which would have shallow inclination are never observed rather the paleomagnetic signal is dominated by geographically persistent remagnetizations characterized by steep inclination as well as a soft present field overprint we observed two distinctive secondary magnetizations named the a and b components carried by fine grained magnetite pervasive diagenesis induced the a component a total chemical remanent remagnetization poles for the a component are better concentrated after bedding correction indicating a pre or early syndefor polarity in the inner foothills pole positions polarity and geological and thermal constraints indicate that the a component was acquired diachronously in advance of the eastward migrating cordilleran tectonic wedge subsequently an intermediate temperature partial thermo remanent remagnetization the b component was superimposed on large regions of the front ranges and inner foothills b component directions are brought into optimal concentration by differential untilting of 0 to 50 indicating that the component was acquired after the rocks were incorporated into the orogenic wedge but before the end of contractional deformation the b component is strongest within a couple of kilometers of the frontal thrust of the front ranges the relative magnitude of the b to a components and the maximum unblocking temperature of the b component decrease away from the frontal thrust over about 30 km both to the west and to the east the b component thermal overprint was attained by 250 c heating in response to tectonic or possibly sedimentary loading it was preserved by a rapid cooling accompanying a differential uplift and erosion of up to 8 km in the vicinity of the frontal thrust late in or post dating its local tilting history the likely cause was uplift of the exposed structural panel by contraction of younger underlying thrust structures

beyond neel s theories thermal demagnetization of narrow band partial thermoremanent magnetizations partial thermoremanent magnetization ptrm was imparted over a narrow temperature interval t 370 350 c to a suite of crushed and annealed natural magnetite samples ranging in grain size from 1 to 125 150m and in domain state from small pseudo single domain psd to multidomain md in this way effectively a single blocking temperature tb o f ptrm was activated stepwise thermal demagnetization did not erase the ptrms sharply at tb as for single domain sd grains demagnetization began well below 350 c and continued above 370 c with a median unblocking temperature tub close to 360 c the largest grains deviated most from sd behavior their ptrm demagnetized over the entire interval from room temperature to the curie point in accordance with predictions for md grains in terms of the unblocking temperature distribution f tub or slope dm dt of the thermal demagnetization curve sd grains have a sharp spectrum t ub t b m d grains were observed to have a broad roughly symmetrical spectrum centered on tb and intermediate size grains in the psd range had non gaussian spectra combining a central peak near tb with broad tails above and below tb in this respect psd grains display a superposition of sd and md remanences not a blend between the two practical implications of these observations are that thellier s law of reciprocity t ub t b will be increasingly violated as grain size increases in the psd range the low tub part of f tub produces anomalously large demagnetization of nrm in low temperature heating steps of thellier type paleointensity determinations and a sagging shape of the arai plot the high tub part of f tub results in undemagnetized remanence at and above tb in thermal demagnetization among pre treatments designed to make remanence more sd like in subsequent thermal cleaning alternating field af pre cleaning sharpened f tub more effectively than low temperature demagnetization for the 20m sample 2001 elsevier science b v all rights reserved

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

thermal demagnetization of partial thermoremanent magnetization three well characterized glass ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations ptrms in the temperature interval 400 350 c for varying lengths of time 10 min to 5 days and in magnetic fields from 0 5 to 5 oe 0 05 0 5 mt the thermal demagnetization behavior of the ptrms changes with domain state in the sense that with decreasing coercivity increasing grain size demagnetization curves become less steep and may not completely demagnetize until heated to the curie temperature numerical calculations based on neel s single domain sd theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures we observe that experimental results do not match any of the theoretical predictions there is no discernible field dependence of demagnetization curves for the sd and pseudo single domain samples and even for the soft multidomain md sample in the field range investigated an important consequence for thermally derived paleointensity data is that they may not be entirely reliable especially if the sample remanence is due to md grains

paleothermometry of the sydney basin evidence from overprinting of magnetizations of late permian and mesozoic rocks and from the rank of permian coals and mesozoic phytoclasts coal particles suggests that surface rocks in the sydney basin eastern australia have been raised to temperatures of the order of 200 sup 0 c or higher as vitrinite reflectance an index of coal rank or coalification is postulated to vary predictably with temperature and time estimates of the paleotemperatures in the sydney basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin these estimates give maximum paleotemperatures of present day surface rocks in the range 60 249 sup 0 c depending on factors such as location in the basin the thickness of the sediment eroded and the maximum paleogeothermal gradient higher coal rank and consequently larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the tasman sea on the basin s eastern margin a theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperaturesmore in the sydney basin this estimate places the maximum paleotemperature in the range 250 300 sup 0 c along the coastal region both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures which in places exceed 200 sup 0 c and the loss of sediment thicknesses in excess of 1 km due to erosion less

redbeds and thermoviscous magnetization theory thermal demagnetization character istics of a brunhes age viscous overprint in appalachian redbeds and a thermoviscous component acquired in the laboratory at moderate tempera ture agree well with relaxation time blocking temperature relations for hematite proposed by pullaiah et al 1975 supporting evidence was obtained from redbeds associated with an igneous dike intrusion although the interpretation of these data are complicated by magnetochemical alterations paradoxically experimental data for magnetite in some limestones were shown to agree more closely with an alternative theory of walton 1980 the hematite in redbeds is predominantly singledomain sd whereas magnetite in the lime stones studied extends well into the multidomain md range thus experiment and theory could be reconciled if it is assumed that pullaiah eta applies strictly to sd material whereas waltoh s theory somehow describes thermoviscous effects dominated by larger md grains

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments titanomagnetite fe3 xtixo4 0 x 1 is a common naturally occurring magnetic mineral critical to many paleomagnetic studies underlying most interpretations is the assumption that lacking chemical alteration curie temperature tc remains constant however recent work has demonstrated that tc of many natural titanomagnetites varies strongly as a function of thermal history independent of chemical alteration this is inferred to arise from reordering of cations and or vacancies in the crystal structure and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments because changes take place at t tclose will have their original tb spectrum truncated at t tclose predicted behavior during thellier type paleointensity experiments results in only modest deviations in nrm lost or ptrm gained from the non reordering case much larger deviations are predicted for ptrm checks compared to paleointensity results from titanomagnetite bearing pyroclastic deposits modeled non ideal behavior occurs in the same temperature intervals but is much more systematic reordering is likely one contributing factor to failure of paleointensity experiments this article is protected by copyright all rights reserved

paleomagnetism and paleothermometry of the sydney basin 2 origin of anomalously high unblocking temperatures the milton monzonite of southeastern australia was thermoviscously remagnetized as a result of cretaceous burial and uplift thermal demagnetization separates the low unblocking temperature lt overprint from the high unblocking temperature ht primary remanence with a relatively sharp junction between lt and ht components in vector projections for single domain grains the junction temperature tl between two such vectors corresponds to the maximum blocking temperature tr reactivated in nature apart from a correction for the difference between natural and laboratory timescales however measured tl values are distributed over an implausibly wide range 250 c for burial remagnetization of an untilted intrusion like the milton monzonite furthermore many tl values are anomalously high compared to the predictions of single domain theory multidomain grains are the cause of these anomalies samples pretreated before thermal demagnetization by zero field cycling to liquid nitrogen temperature so as to erase multidomain remanence and isolate single domain remanence do have the theoretically expected tl values in these samples realistic remagnetization time and temperature tr tr conditions in nature are predicted using the t t contours of pullaiah et al 1975 the anomalously high tl values before low temperature treatment are due to multidomain grains which carry 50 of the lt overprint the lt thermal demagnetization curve in samples dominated by multidomain grains is quasi exponential in shape with a high temperature tail extending almost to the curie point as predicted by multidomain theory these high lt unblocking temperatures which are much greater than plausible remagnetization temperatures reached in nature overlap and mask the lower part of the ht unblocking temperature spectrum driving up tl values and leading to inflated estimates of tr although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the milton monzonite chemical overprinting may be a factor in other lithologies and tectonic settings

thermoviscous remanent magnetism of columbia river basalt blocks in the cascade landslide we studied sixteen basalt samples from a landslide in the columbia river gorge to determine if they had acquired a thermoviscous remanent magnetism tvrm since the slide was emplaced about 800 years ago all samples were thermally demagnetized at 20 heating steps until 360 c where a large change in susceptibilit y was noted analysis of the directional changes during demagnetization indicates that the samples contain up to four tvrm components in addition to an nrm component the tvrm components with the lowest blocking temperatures are tightly clustered around the present field direction while the nrm directions are consistent with a random distribution as expected for a landslide deposit measurements of hysteresis parameters and thermomagnetic analyses of the samples demonstrate that the dominant magnetic mineral in the basalt is single domain magnetite the temperature at which the first tvrm component was removed ranges from 70 c to 100 c this result can be compared to nomographs that relate the time and temperature used to demagnetize a tvrm with the time and temperature at which the tvrm was acquired our results are more consistent with the nomograph of pullaiah et al 1975 than with the nomograph of middleton schmidt 1982