interaction of basement uplift and thin skinned thrusting moxa arch and the western overthrust belt wyoming a hypothesis immediately north of big piney in western wyoming the moxa arch is modeled as emplaced during the late cretaceous along an east dipping low angle thrust moxa thrust that has precambrian basement and paleozoic and younger cover rocks in the hanging wall the west verging moxa thrust cut up section from the basement cover contact and flattened to the west along a detachment in lower triassic rocks thaynes detachment during motion on the moxa thrust the leading edge of the hanging wall wedged westward along the thaynes detachment peeling back triassic and younger rocks and thrusting them relatively eastward along the ancestral prospect thrust the 5 3 km of westward movement along the moxa thrust was matched by eastward movement along the west dipping ancestral prospect thrust the ancestral prospect thrust moved an additional 5 1 km during the late paleocene when thrust belt deformation progressed eastward to the moxa arch it appears that in the snider basin area the prospect thrust does not share a ramp with the darby thrust and that the emplacement of the moxa arch and prospect thrust determined the position of the later and more westerly darby thrust

reconstructing the kinematic evolution of curved mountain belts internal strain patterns in the wyoming salient sevier thrust belt u s a analyses of mesoscopic structural and strain patterns in red beds of the triassic ankareh formation and limestones of the jurassic twin creek formation in concert with complementary paleomagnetic studies constrain the three dimensional kinematic evolution of curved fold and thrust systems in the wyoming salient of the sevier thrust belt spaced cleavage fracture and vein networks minor folds and minor faults in limestones and red beds accommodated early layer parallel shortening lps concentrated in front of the growing thrust wedge along with minor strike parallel extension and wrench shear related to the development of orogenic curvature strain estimated using mass balance relations for cleavage seams crinoid ossicles in bioclastic limestone and reduction spots in red beds displays systematic regional patterns principal shortening directions are subperpendicular to structural trend around the salient reflecting a combination of primary curvature and secondary rotation of early lps fabrics lps magnitudes vary from 5 in central parts of the frontal hogsback thrust system where cleavage is absent to 10 30 in the more interior crawford thrust system where cleavage intensity is moderate to strong strain also increases toward the salient ends internal strain is a significant component of total deformation and should be considered when restoring cross sections strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting

paleomagnetic and rock magnetic results from the twin creek formation middle jurassic wyoming abstract a magnetization which passes the fold test has been observed in 73 limestone samples 10 sites from the middle jurassic twin creek formation the pole calculated from the site mean poles is located at 68 4 n 145 0 e k 31 8 a 95 8 7 this pole lies in a segment of the north american apparent polar wander apw path for which there are only a few reliable poles in the literature the results corroborate earlier studies which conclude that the jurassic segment of the apw path does not include the present north pole however the position of the twin creek pole suggests that significantly more apw took place prior to the late jurassic than previous studies indicated

regional shortening fabrics in eastern north america far field stress transmission from the appalachian ouachita orogenic belt paleozoic carbonates of cratonic eastern north america comprise the footwall of the appalachian ouachita fold and thrust belt and contain a layer parallel shortening lps fabric that is preserved by mechanically twinned calcite shortening directions are generally parallel to the appalachian ouachita thrust transport direction in carbonates of the thrust belt proper restored width 400 km and within carbonates up to 1700 km into the foreland giving a pre thrusting sedimentary prism 2100 km wide through which compressive orogenic stresses were transmitted the shortening strain magnitudes 6 and the inferred calcite twinning differential stress magnitudes 90 mpa decrease exponentially away from the orogenic front calcite twinning strain patterns in other adjacent tectonic provinces such as the grenville laramide keweenawan rift and newark basin are distinct from the twinning strains preserved in the cratonic paleozoic carbonates appalachian orogen far field stresses calcite microstructures

displacement of surface monuments vertical motion several measurements of vertical ground motion at pinon flat observatory california indicate the overall weakness and instability of the earth s weathered surface with respect to the underlying rock cumulative long period motions of order 0 5 mm per year dominate these records though smaller elastic deformations caused by precipitation loading atmospheric loading and tidal strains are evident at higher frequencies all of these help to characterize the near surface material the long period records suggest that near surface weathering is the dominant influence on monument motion at least at this site on crystalline rock in a semi arid environment rainfall loading gives an average vertical modulus of 2 6 gpa for the material in the uppermost 26 m of the ground compared with 88 gpa for granite under moderate confining stress atmospheric loading gives similar results but indicates the ground is permeable to airflow at periods longer than a few hours earth tide records show poisson s ratio to be 0 09 in contrast to the normative range of 0 2 0 25 establishing that horizontal strains couple only weakly into vertical ones so that vertical strains near the surface are a poor measure of areal strain the form of the ground surface displacement power spectrum indicates that analyses of geodetic surveys would be improved with the inclusion of a monument positioning error budget that increases with time because of ground instability and the generally small rate of crustal tectonic motions deeply emplaced monuments will be needed for observational programs designed to detect short term changes in crustal deformation over baselengths of order 10 km and less

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

evidence for insolation and pacific forcing of late glacial through holocene climate in the central mojave desert silver lake ca abstract silver lake is the modern terminal playa of the mojave river in southern california usa as a result it is well located to record both influences from the winter precipitation dominated san bernardino mountains the source of the mojave river and from the late summer to early fall north american monsoon at silver lake here we present various physical chemical and biological data from a new radiocarbon dated 8 2 m sediment core taken from silver lake that spans modern through 14 8 cal ka bp texturally the core varies between sandy clay clayey sand and sand silt clay often with abrupt sedimentological transitions these grain size changes are used to divide the core into six lake status intervals over the past 14 8 cal ka bp notable intervals include a dry younger dryas chronozone a wet early holocene terminating 7 8 7 4 cal ka bp a distinct mid holocene arid interval and a late holocene return to ephemeral lake conditions a comparison to potential climatic forcings implicates a combination of changing summer winter insolation and tropical and n pacific sea surface temperature dynamics as the primary drivers of holocene climate in the central mojave desert

jurassic magnetostratigraphy 2 middle late oxfordian of aguilon iberian cordillera northern spain abstract a reproducible magnetic polarity pattern has been obtained from four overlapping stratigraphic sequences of ammonite rich gray pelagic limestones spanning part of the middle and late oxfordian g transversarium toe bimammatum ammonite zones progressive thermal demagnetization removed a present field overprint fold tests both between and within outcrops are positive the mean direction of magnetization is 322 45 95 6 which implies a paleolatitude of 26 n frequent changes of polarity were observed the magnetostratigraphic sequence contains the ammonite zones subzones and horizons of the tethyan faunal realm using the reference horizons provided by this detailed ammonite zonation the polarity zones can be readily correlated among all sections the composite polarity pattern is predominantly of reversed polarity punctuated by a number of relatively shorter duration normal polarity intervals in order to compare this sequence to that of the marine magnetic anomalies we have revised the age estimates for the pre m25 marine magnetic normaly pattern according to the most recent time scale based on those ages the sedimentary record at aguilon suggests a much greater frequency of polarity changes than does either model derived from the two sets of marine magnetic anomalies of this time this sedimentary record probably is a truer approximation of the actual geomagnetic polarity history because of the continuity of sedimentation in these sections and the inherent ambiguities involved in determining a polarity pattern from the very low amplitudes that everywhere characterize this part of the marine record

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

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

the optical anchor a geophysical strainmeter an instrument has been developed to monitor the horizontal displacement of near surface monuments so as to reduce the noise of observatory based strain measurements the device measures the shear strain in the upper 24 m of the earth s crust using an equal path length michelson interferometer the magnitude of the observations 50 m indicates that such measurements are needed to interpret the records produced by precision strainmeters

miocene remagnetisation of carbonate platform and antalya complex units within the isparta angle sw turkey abstract a palaeomagnetic study has been carried out within the mesozoic and tertiary units of the relatively autochthonous carbonate platforms and the allochthonous deep sea volcanics and sediments of the antalya complex exposed around the isparta angle sw turkey the antalya complex is interpreted as a mosaic of carbonate platforms basinal sediments volcanic and ophiolitic rocks which formed within a southerly strand of the neotethyan ocean adjacent to gondwana the results indicate a widespread remagnetisation event negative fold tests show that the remanence at most sites is of secondary origin e g within the cirali lavas the magnetisation is carried by magnetite of presumed authigenic origin the remagnetisation event is believed to have occurred in the early middle miocene burdigalian langhian it was possibly triggered by the migration of orogenic fluids ahead of the advancing lycian nappes during their emplacement onto the carbonate platforms subsequent to remagnetisation a large segment of the isparta angle underwent an anticlockwise rotation of 30 this rotation is attributed to the overall convergence and bending of the hellenic arc and the final stages of emplacement of the lycian nappes during the late miocene in agreement with previous studies previously southerly palaeolatitudes were inferred from late triassic extrusives of the godene zone calbali dag unit the post folding magnetisation identified here within the cirali lavas of the godene zone to the south implies that these low palaeolatitudes result from the inappropriate application of structural tilt corrections the available data cannot be used to substantiate an origin for the antalya units south of the equator in the early mesozoic instead a position close to the northern margin of gondwana is indicated

fluid density distribution in a high temperature stratified thermohaline system implications for saline hydrothermal circulation density distribution within the salton sea geothermal system of fluids ranging from 20 c to 325 c has been computed using chemical and thermal data from geothermal production well tests and curve fit models of nacak chloride solution properties density corrections can easily be made to 0 01 g cm3 for solute effects of each of the dominant chloride salts as well as pressure above vapor saturation field data on dissolved co2 is too sparse to routinely compute a correction however this moderately but variably abundant component can be shown to produce only minor 0 01 g cm3 errors in density estimates fluid density within the shallow cool 20 wt the density of brine climbs rapidly to near 1 0 g cm3 as the salinity increase across this interface overwhelms the thermal effect on fluid density this steep density gradient precludes all but diffusional conductive or perhaps double diffusive convective mass and heat transfer in this transitional regime measurement uncertainties for reservoir depths temperatures and salinities commonly exceed errors of our density model limiting the accuracy of details in our modeled density distribution gross scale relationships of this salinity stratified geothermal system are however apparent permitting rational discussion of extrapolations to conditions beneath the presently explored reservoir and inference of dynamics during the systems evolution

high resolution climatic evolution of coastal northern california during the past 16 000 years holocene and latest pleistocene oceanographic conditions and the coastal climate of northern california have varied greatly based upon high resolution studies ca every 100 years of diatoms alkenones pollen caco 3 and total organic carbon at ocean drilling program odp site 1019 41 682 n 124 930 w 980 m water depth marine climate proxies alkenone sea surface temperatures ssts and caco 3 behaved remarkably like the greenland ice sheet project gisp 2 oxygen isotope record during the balling allerod younger dryas yd and early part of the holocene during the yd alkenone ssts decreased by 3 c below mean bolling allerod and holocene ssts the early holocene ca 11 6 to 8 2 ka was a time of generally warm conditions and moderate caco 3 content generally 4 the middle part of the holocene ca 8 2 to 3 2 ka was marked by alkenone ssts that were consistently 1 2 c cooler than either the earlier or later parts of the holocene by greatly reduced numbers of the gyre diatom pseudoeunotia doliolus 10 and by a permanent drop in caco 3 to 3 starting at ca 5 2 ka coastal redwood and alder began a steady rise arguing for increasing effective moisture and the development of the north coast temperate rain forest at ca 3 2 ka a permanent ca 1 c increase in alkenone sst and a threefold increase in p doliolus signaled a warming of fall and winter ssts intensified higher amplitude and more frequent cycles of pine pollen alternating with increased alder and redwood pollen are evidence that rapid changes in effective moisture and seasonal temperature enhanced el nino southern oscillation enso cycles have characterized the site 1019 record since about 3 5 ka

deformation of remanent magnetism in a synthetic aggregate with hematite a hematite calcite aggregate bonded with portland cement is deformed triaxially at 200 mpa confining pressure and at a constant strain rate of 10 5 s 1 in some cases with a pore fluid pressure of 120 mpa this aggregate deforms in a ductile fashion at room temperature and is a good textural analog of naturally penetratively strained hematite bearing rocks the changes in inclination of an isothermal remanent magnetisation are indistinguishable from the behaviour expected of a homogeneously strained non material line rotating according to wettstein s equation this is encouraging for paleomagnetists who might wish to restore remanence in tectonically strained hematite bearing rocks an unknown combination of rigid grain rotation and intracrystalline stress must be responsible for rotation of the grains that carry remanence and thus for the changes in inclination of remanence the reason that wettstein s equation is approximately valid may be because the large aspect ratios of rigid hematite grains cause them to rotate nearly as quickly as a passive line p hydrostatic compaction wet deformation and dry deformation in that order are progressively more effective in reducing the intensity of remanence however wet deformation is more effective than dry deformation in reducing the intensity of the most coercive components presumably by spinning the finest grains in the cement matrix and scattering their individual moments p where multiple direction components of remanence are present deformation reduces the components with higher coercivities 70 mt more than those with lower coercivities in previous experiments on magnetite bearing samples the lower coercivity components were most reduced thus the natural tectonic deformation of an nrm may be expected to preserve selectively low coercivity components in hematite and high coercivity components in magnetite this may be used to predict the success of preservation of a characteristic nrm and conversely the success of overprinting by syntectonic remagnetisation

a new paleomagnetic pole for the neoproterozoic uinta mountain supergroup central rocky mountain states usa abstract hematite cemented strata of the neoproterozoic age 800 750 ma uinta mountain group umg provide a new well defined mid neoproterozoic paleomagnetic pole for laurentia thermal demagnetization results and rock magnetic experiments reveal the presence of two discrete magnetizations in umg rocks both magnetizations are carried by hematite the dominant characteristic remanent magnetization chrm is an east directed or antipode shallow positive to negative inclination magnetization that is typically unblocked over a narrow range of high laboratory temperatures between 660 and 680 c the second magnetization exclusively of normal polarity is north to northeast directed and has a moderate to steep positive inclination and probably represents a recent or modern day viscous remanent magnetization the response of umg samples to thermal and chemical demagnetization comparison with previously obtained data from umg rocks in the area results from rock magnetic experiments and the inferred low latitude of laurentia during umg deposition in the neoproterozoic are all consistent with the interpretation that the west or east directed shallow magnetization is a near primary remanence the presence of dual polarities of the chrm with the west directed remanence inferred to be of normal polarity and the east directed of reverse polarity suggests that magnetization acquisition in the umg rocks took place over a sufficient amount of time to average secular variation of the geomagnetic field the mean paleomagnetic pole derived from the inferred primary magnetizations from umg strata 0 8 n 161 3 e 95 4 6 n 9 sampling localities consisting of 79 sites is fully consistent with recently reported results from rocks of mid neoproterozoic age for southwest laurentia which define a tight counterclockwise loop located in the south pacific in the apparent polar wander path for laurentia during the neoproterozoic

apparent polar wander paths and the close of late archean crustal transpression northern ontario 1 lamprophyre dikes of the southern superior province of the canadian shield crosscut archean structures including a late archean unconformity and its overlying younger archean timiskaming sedimentary and volcanic rocks the dikes also cut the single schistosity that formed in these rocks nevertheless the lamprophyre dikes show a weak internal tectonic fabric that is approximately parallel to the schistosity in the bedrock and oblique to dike walls the consistent orientation of this internal tectonic fabric has been recognized in every lamprophyre dike using anisotropy of magnetic susceptibility ams thus the lamprophyre dikes provide a useful chronological marker trapping the last pulse of archean tectonism unfortunately our previous attempts to compare the lamprophyre paleopoles with those of the age calibrated apparent polar wander path apwp failed because it was not possible to isolate separate young and old components of magnetization that failure occurred due to the overlap of blocking temperatures or coercivities for the different characteristic vector components borradaile et al 1994a using new cores from the same specimens here we distinguish successfully between consistently oriented vector components by using low temperature demagnetization ltd before thermal demagnetization although the paleopoles appear well defined comparisons with published apwps for the superior province from late archean irving 1979 dunlop 1983 symons et al 1994 to early proterozoic buchan et al 2000 are difficult moreover the earlier parts of the paths are progressively less precise and complete nevertheless a component remanences for the dikes give a paleopole near the path at 2650 ma for a timiskaming volcanic breccia with a u pb age of 2696 ma the a component paleopole may lie between 2600 and 2650 ma b component remanences yield paleopoles in the nw pacific region incompatible with any proposed archean path but perhaps representing some much younger proterozoic remagnetization event e g keweenawan 1100 ma

remagnetization by basinal fluids testing the hypothesis in the viola limestone southern oklahoma migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late paleozoic secondary magnetizations in the rocks of north america paleomagnetic and geochemical results from the ordovician viola limestone in southern oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit the viola limestone contains what we interpret to be a pervasive pennsylvanian synfolding magnetization residing in magnetite and a localized permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins both secondary magnetizations are interpreted as chemical remanent magnetizations crm based on low burial temperatures and the presence of authigenic magnetic phases the relative proportion of the permian crm in hematite gradually decreases whereas the magnetite crm increases with distance from the veins fluid inclusion and sr isotope studies indicate that the vein mineralization calcite with mississippi valley type oxides and sulfides precipitated from basinal fluids which were warm saline and radiogenic the radiogenic 87sr 86sr values of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones the coincidence of the geochemical and remagnetization trends suggests that the permian crm dates the migration of basinal fluids through the veins geochemical results from the viola limestone containing the pervasive crm indicate that it is relatively unaltered with no evidence for basinal fluids the lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive crm need to be tested the results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone

origin of magnetization in the phosphoria formation at sheep mountain wyoming a possible relationship with hydrocarbons hydrocarbon impregnated fenestral dolomites of the phosphoria formation were sampled on both flanks of sheep mountain anticline wyoming demagnetization results indicate one dominant component that is either synfolding laramide in age or predates folding cretaceous in age thermal demagnetization yields stable decay to temperatures between 500 c and 550 c and rock magnetic experiments suggest that the magnetization is dominated by a low coercivity component magnetic extracts contain spheres 15 diameter as well as other authigenic forms energy dispersive analysis demonstrates that fe is the only detectable element present in the spheres and x ray diffraction analysis indicates that magnetite is present in the extracts the spheres are interpreted to be authigenic magnetite the magnetization is interpreted as a chemical remanent magnetization crm residing in this authigenic magnetite chemical conditions created by the hydrocarbons may have caused precipitation of the authigenic magnetite and acquisition of the associated crm

displacement of surface monuments horizontal motion measurements of tectonic deformation depend on both accurate instrumentation and adequate coupling of the apparatus to the earth s crust existing techniques capable of resolving the signals of interest strain rates of 10 n yr are mainly observatory based the limitation in the baselength of these instruments 1 km results in a requirement that the reference monuments be exceptionally stable 10 tzm yr however records from pition flat observatory california show that the actual horizontal displacements for massive near surface monuments eraplaced in competent weathered granite are of the order of 50 tzm yr the low noise level of the strain measurements at this site indicates that this magnitude of monument displacement is abnormally small until high accuracy geodetic techniques are developed sophisticated monuments or monument monitoring devices will be necessary to record faithfully continuous crustal deformation

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

measurements of coseismic deformation in southern california 1972 1982 since 1972 a number of static strain changes have been measured at pinon flat observatory southern california as a result of regional faulting as recorded by a collection of long baselength strainmeters and tiltmeters these signals are in good quantitative agreement with the deformation calculated for a dislocation in an elastic half space in contrast colocated short baselength sensors are found to produce spurious results whose size is much larger than expected theoretically and well correlated with peak site acceleration calculations show that the root mean square rms of the coseismic signals from several different instruments at one site or from several sites at known hypocentral distances provide a good estimate of the seismic moment a result confirmed by the observations static moment estimates for the four largest events in this period are imperial valley 1979 m0 9 0 1018nm mexicali valley 1980 m0 6 1 1018nm westmorland 1981 m0 5 3 1017nm and brawley 1979 m0 2 3 1017 nm these estimated moments generally exceed those determined seismically by about 25 which is as expected since they include some postseismic deformation results from the mexicali valley earthquake are particularly interesting indicating an average fault strike approximately 9 different from the historic fault trace several small to moderate earthquakes ml 3 7 5 2 were also well enough recorded to estimate source parameters for the only other detectable events in the period 1972 1982 a total of three others for which the strain change erms was greater than 0 4 ne high accelerations caused most of the instruments at the site to malfunction no precursory signals were evident for any of the earthquakes recorded at this site

hysteresis properties of magnetic spherules and whole rock specimens from some paleozoic platform carbonate rocks it is widely established that many paleozoic carbonate rocks were remagnetized during the late paleozoic kiaman reversed superchron yet the paleomagnetic recorders of this event have remained elusive magnetic spherules have been candidates for this role because they are probably authigenic therefore we have studied the hysteresis properties of individual magnetic spherules from two formations the remagnetized onondaga devonian new york and the effectively unremagnetized wabash silurian indiana also we have compared the hysteresis properties of spherules to those of remagnetized limestones from the onondaga trenton ordovician new york and helderberg devonian new york formations and from the unremagnetized wabash formation the spherules studied here vary from about 20 m to over 100 m in diameter nevertheless on the basis of hysteresis behavior this spherule population spans the full range of domain states from truly multidomain md to pseudo single domain psd to single domain sd because these spherules are so large it is remarkable that more than half display the highly psd like behavior typical of many remagnetized carbonate rocks equally as remarkable is the lack of dependence on spherule size of various hysteresis properties such as coercive force hc remanent coercive force hcr the ratio of saturation remanence to saturation moment mrs ms and the ratio of remanent coercive force to coercive force hcr hc these findings dispel the preconception that large spherules are too magnetically soft to be stable carriers of ancient magnetization surprisingly onondaga and wabash spherules yield virtually identical suites of hysteresis properties on the one hand this high degree of similarity could indicate that spherules are irrelevant to remagnetization more intriguing however is that this similarity could mean that the wabash formation although effectively unremagnetized nevertheless contains magnetic mineralogical traces of a regional remagnetization event it is significant that for these spherules a bilogarithmic plot of mrs ms versus hcr hc does not define a simple linear trend instead mrs ms increases more rapidly with decreasing hcr hc for spherules in the psd range than in the md range when a power law is fit solely to the hysteresis ratios of psd like to sd like spherules extrapolation to the sd limit gives mrs ms 0 75 similarly a power law fit to the hysteresis ratios obtained here from remagnetized carbonate rocks yields mrs ms 0 72 at the sd boundary these results could indicate that a large percentage of sd like spherules like the sd carriers in remagnetized carbonate rocks are governed by magnetocrystalline anisotropy finally the present data point to values of hc near 350 oe both for sd like spherules and for the sd grains thought to carry remanence in remagnetized carbonates clearly psd and sd like spherules share several key rock magnetic properties with remagnetized carbonate rocks thus spherules still are likely candidates for carrying the remagnetization signal

remagnetisation of a rock analogue during experimental triaxial deformation abstract synthetic specimens containing pseudo single domain magnetite were given a univectorial anhysteretic remanent magnetisation they were deformed in a ductile manner at room temperature under a confining pressure of 150 mpa 1 5 kbar the strain rate was 10 5 s 1 and the maximum strain achieved was 23 shortening a weak 0 035 mt magnetic field was present in the pressure vessel during deformation hydrostatic compaction alone produced pressure demagnetisation of low coercivity components of remanence but did not change remanence directions macroscopic differential stress was needed to deform the specimens and this also permitted syndeformational remagnetisation for values of more than 20 mpa 200 bars this added new components of piezo remanent magnetisation prm approximately parallel to the direction of the magnetic field in the pressure vessel they were acquired in all coercivity intervals but the largest components were added below 35 mt the final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence prm contributions almost completely masked the changes related to strain induced grain rotation at these low strains the directions of the externally imposed stress strain axes may not significantly influence the direction of new remanence components prm acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress history of the magnetite grains in part this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation or thermal or chemical processes

grain scale deformation and the fold test evaluation of syn folding remagnetization the paleomagnetic fold test examines for pre syn or post folding re magnetization generally passive limb rotation is used to restore folded units to their original position and the position of the fisherian maximum deter mines the relationship between folding and magnetization in a more complete analysis the reorientation of material lines in different fold models is used however the magnetic vec tor should not be considered a material line as it represents the sum of vectors in individual magnetic carriers and does not cross grain boundaries therefore the role of grain scale deformation mechanisms was examined using two end mem bers rigid body rotation spin and homogeneous strain from this study it is concluded that 1 the fisherian maximum will in general occur before complete unfolding 2 the sign of the inclination may change during unfolding and consequently cross over unfolding is not representati ve of syn folding remagnetization 3 statistical analysis will not necessarily produce significant results a number of recommendations are given which include stepwise unfol ding and unplunging should be carried out routinely local rather than regional fold tests should be performed both limbs and the hinge of a single folded layer should be exa mined in order to determine the active deformation mecha

continuous measurements of crustal deformation for the 1992 landers earthquake sequence abstract we describe and attempt to interpret continuous measurements of strains and tilts made at pinon flat observatory pfo before during and after the landers and joshua tree earthquake sequences these data show substantial transient deformation following the landers mainshock with a total amplitude of several percent of the co seismic deformation and a decay time of at least several days comparing data from the many types of instruments at pfo allows us to infer possible sources for this deformation the immediate postseismic transient was nearly the same size on three long base strainmeters suggesting either broadscale deformation or local motion near one part of the observatory the latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source possible mechanisms for broad scale deformation include postseismic fault slip time dependent creep in near surface rocks and elastic or thermal responses to water table changes the first two agree best with the observations from pfo but if postseismic fault slip is the source it must have been distributed differently than the co seismic slip and may have included faults other than those that ruptured seismically if one of the other mechanisms is the main source the pfo data imply that the postseismic slip must have been very much smaller than the seismic slip perhaps 2 or less no significant preseismic deformation was observed at a level of 2 10 3 of the co seismic deformation for the days to minutes before the earthquake

scanning and transmission electron microscope observations of magnetite and other iron phases in ordovician carbonates from east tennessee previous paleomagnetic observations for the carbonates of the lower ordovician knox group have indicated that ancient magnetizations in these rocks are of the same age as the late paleozoic alleghenian orogeny rock magnetic properties strongly suggest magnetite as the carrier of the magnetization but the textural and crystalline characteristics sizes morphologies and mineral associations of these magnetites are poorly known we have examined magnetic extracts and iron oxides in thin sections with scanning sem and scanning transmission stem electron microscope techniques to determine whether the observed iron oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers several different forms of magnetite in limestones and dolomites which in places are host to mississippi valley type deposits are documented by imaging and energy dispersive analysis using sem and stem by x ray diffraction and electron diffraction patterns using stem the magnetite is either spherical with a dimpled surface or nonspherical in the form of void filling single grains or grain aggregates most of the iron oxides have the composition of pure end member magnetite but occasional titanomagnetite and hematite including rare zincian hematite have been observed only in limestone wherever found in thin section nonspherical magnetites occur in association with secondary dolomite potassium feldspar illite and quartz some iron oxides have in fact inclusions of k feldspar and quartz some of the magnetite spherical and nonspherical is polycrystalline this implies that the larger observed grains may consist of single domains or pseudo single domains this provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single domain but interacting subgrains on the basis of remanent coercivities and blocking temperatures we interpret the pure end member magnetite to be authigenic having formed at approximately the same time as the k feldspars which in nearby areas have yielded late paleozoic radioisotopic ages 278 322 ma the knox carbonates therefore are inferred to carry a chemical remanent magnetization iron rich clays or original iron titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution precipitation processes these processes require rock fluid interactions which are thought to be related to migrating connate brines during the alleghenian orogeny

timing of deformation in overthrust belt and foreland of idaho wyoming and utah a review of the timing and displacement evidence of the major structures of the western wyoming overthrust belt and foreland shows there is a progression in thrust displacement apparent duration of motion and palinspastic position of thrust traces from west to east those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east however average thrust velocities are all on the order of 0 5 0 5 cm yr 0 2 in yr foreland events are in part synchronous with thrust belt events and had an effect on them although dating precision varies widely on major normal faults present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting

the nature and chronostratigraphy of quaternary pyroclastic accumulations from lake barombi mbo west cameroon abstract the k ar dating of a limburgitic flow indicates that the barombi mbo maar was active a million years ago the morphology of the maar and the unusual high k 2 o na 2 o ratio of the limburgite suggest either that this age is too old or that this flow is not the last activity of the maar twenty three meters of sediments were cored from the bottom core bm6 of the 110 m deep lake which currently fills the maar they are composed of clayey and sideritic laminae with interstratified ash layers in the lower half of the core these ashes composed of slightly vesiculated sideromelane with olivine and labradorite phenocrysts have been dated between 21 000 and 11 000 yr b p they show no alteration and correspond to aerial fallout of alkaline sodic basaltic affinity which is common in the area but are distinct from the mafic magmas which generated the maar the origin of the tilting and uplifting of a 1 5 m sedimentary column in the lower part of the core remains controversial holocene age pyroclastites are rare and consist of a mixture of granulitic xenocrysts and typically transitional tachylite similar to volcanics from the northeast manengouba magnetite microspherules occurring in holocene layers have no volcanic equivalent they could be related to biogenic or pedogenetic processes induced by more humid climatic conditions

hydrologic changes in colorado during the mid holocene and younger dryas abstract the stratigraphy of upper big creek lake ubc an overflowing lake in northern colorado indicates that the lake responded to a series of hydroclimatic changes over the past 13 000 years modern sediments in eight comparator lakes from across wyoming and colorado facilitate the stratigraphic interpretation by showing that littoral sediments have exponentially higher sand content lower loss on ignition loi and lower net accumulation rates than off shore sediments sand layers in two near shore cores from ubc share the characteristics of the modern littoral sediments 80 sand 1 9 m below its outlet from 12 500 to 11 300 and from 7500 to 2100 cal yr bp ground penetrating radar gpr profiles consistent with the low water episodes show correlative sand layers and submerged wave cut scarps around the lake margin the stratigraphic features correlate with those formed in the comparator lakes during extensive droughts at 11 300 6900 6000 and 4800 3400 cal yr bp as well as a series of north south anti phased droughts between 5800 and 1200 cal yr bp lake closure at ubc amplified the local changes and demonstrates that climate changes can increase the extent of intermittent streamflow in the rocky mountains relative to today

interpretations and common challenges of aeolian records from north american dune fields abstract geomorphic and chronologic data for dune fields are evaluated for three contrasting areas of north america 1 the prairie parkland and boreal ecozones of the northern great plains in canada 2 the central great plains of the usa and 3 the deserts of southwestern usa and northern mexico luminescence and radiocarbon ages for periods of dune accumulation and stability are compared with palaeoenvironment proxies to provide an assessment of the boundary conditions of dune system response to changes in sediment supply availability and mobility dune fields in the northern great plains were formed from sediment originating from glaciofluvial or glaciolacustrine sediments deposited during deglaciation 16 11 ka subsequent aeolian deposition occurred in parkland and prairie dune fields as a result of mid holocene 8 5 ka and late holocene despite spatial and temporal gaps in chronometric data as a result of sampling biases the record from north american dune fields indicates the strong influence of sediment supply on dune construction with changes in sediment availability as a result of drought episodes resulting in dune field reactivation and reworking of pre existing sediment

the role of true polar wander on the jurassic palaeoclimate from the late carboniferous until the middle jurassic continents were assembled in a quasi rigid supercontinent called pangea the first palaeomagnetic data of south america indicated that the continent remained stationary in similar present day latitudes during most of the mesozoic and even the palaeozoic however new palaeomagnetic data suggest that such a scenario is not likely at least for the jurassic in order to test the stationary versus the dynamic continent model we studied the jurassic apparent polar wander paths of the major continents that is eurasia africa and north america that all in all show the same shape and chronology of the tracks with respect to those from south america we thus present a master path that could be useful for the jurassic pangea one of the most remarkable features observed in the path is the change in pole positions at 197 ma early jurassic which denotes the cessation of the counter clockwise rotation of pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the early jurassic 185 ma here we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 ma and conclude that true polar wander is the most likely in order to do this we used morgan s tectonophysics 94 123 139 1983 grid of hotspots and performed absolute palaeogeographical reconstructions of pangea for the late triassic and jurassic the palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

remagnetizations and thrusting in the idaho wyoming overthrust belt a study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the idaho wyoming overthrust belt samples were taken from the jurassic twin creek unit of the most easterly thrust the prospect thrust for paleomagnetic study detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole latitude longitude 83 n 286 e located on the tertiary segment of the north american apparent polar wander path apwp the age of the paleopole roughly coincides with the age of thrusting on the prospect thrust as determined by synorogenic deposits the magnetization was found to reside in magnetite spherules previous data from the absaroka and darby thrust sheets that lie to the west of the prospect thrust were reevaluated and they also indicate a synfolding age for the remagnetization the corresponding paleopole 64 n 162 e lies on the late mesozoic segment of the north american apwp for these more interior thrust sheets the apparent age of the remagnetization is older than that for the exterior prospect thrust sheet but again roughly corresponds with the inferred age of thrusting for the sampled thrusts we conclude that remagnetization in the idaho wyoming overthrust belt is temporally related to deformation and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events

experimental stress remagnetization of magnetite abstract pseudo single domain psd and multidomain md magnetite grains remagnetize in weak magnetic field 30 t during experimental triaxial deformation the magnetite is supported in a calcite cement matrix minor remagnetization occurs with hydrostatic stress of 100 mpa significant remagnetization requires hydrostatic pressure of 150 mpa with differential stress of 5 mpa superposed on the sample intergranular differential stresses must be much higher due to amplification at grain asperities stress remagnetization does not need chemical or thermal energy new components of magnetic remanence are added parallel to the remagnetizing field however this only happens in grains or parts of grains with coercivities of remanence 60 mt grains with coercivities of 20 55 mt remember the primary magnetization and are not stress magnetized these coercivity limits do not depend on the differential stress or strain rate of the experiment the spatial distribution of vector components of remanence was isolated by af demagnetization after deforming a magnetized sample the components of remanence spread along a partial great circle between the initial remanence and the direction of the remagnetizing field the directions of the original magnetization and the remagnetizing field are the only factors controlling the course of the remagnetization path triaxial deformation shortened these samples by if the experimental results are transferable to nature it is possible that a pulse of excess crustal stress 25 mpa could partially remagnetize low dislocation density magnetite the experiments show that the directions of the remagnetizing field and the primary magnetization are the only variables that affect the demagnetization vector path we do not need to know the strain history coaxial or noncoaxial finite strains or principal strain directions to detect the paleofield directions unlike paleomagnetic vectors affected by large strains permanent changes in magnetic structure occur as a result of experimental deformation regardless of the differential stress the proportion of grains with remanence coercivities

late quaternary environments in ruby valley nevada abstract palynological data from sediment cores from the ruby marshes provide a record of environmental and climatic changes over the last 40 000 yr the modern marsh waters are fresh but no deeper than 3 m a shallow saline lake occupied this basin during the middle wisconsin followed by fresh and perhaps deep waters by 18 000 to 15 000 yr b p no sediments were recovered for the period between 15 000 and 11 000 yr b p possibly due to lake desiccation by 10 800 yr b p a fresh water lake was again present and deeper than modern conditions lasted until 6800 yr b p the middle holocene was characterized by very shallow water and perhaps complete desiccation the marsh system deepened after 4700 yr b p and fresh water conditions persisted until modern times vegetation changes in ruby valley were more gradual than those seen in the paleolimno logical record sagebrush steppe was more widespread than at present through the late pleistocene and early holocene giving way somewhat to expanded shadscale vegetation between 8500 and 6800 yr b p shadscale steppe contracted by 4000 yr b p but had greater than modern coverage until 1000 to 500 yr ago pinyon juniper woodland was established in the southern ruby mountains by 4700 yr b p

late paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east tennessee evidence for a post ore chemical event thermal demagnetization studies on samples from mineralized and unmineralized rocks in the lower ordovician knox group of east tennessee reveal a mean direction of d i 157 18 95 7 7 k 52 9 after 60 unfolding combining stable endpoint and remagnetization circle data although not significant at the 95 confidence level the fold test is suggestive of a synfolding alleghenian age of the magnetization the magnetization cannot be postfolding i e permian or younger because the in situ directions do not resemble those for post carboniferous times the magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the early permian but unlike poles for pre late carboniferous times characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test the best estimate for the paleopole resulting from this study falls at 40 n 126 e within the late carboniferous segment of the north american paleozoic apparent polar wander path thus the magnetization is interpreted as synfolding and secondary due to widespread kiaman remagnetization blocking temperatures of up to 520 c and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres secondary dolomites have the same magnetic characteristics as the primary limestones and it is inferred that similar magnetite spheres are the carriers of their magnetization because this magnetization and therefore its carrier occurs in sphalerite and secondary dolomite it must be coeval with or younger than the ore which itself is thought to be of middle to late paleozoic age it is argued that the magnetization was acquired during growth of magnetite associated with a late paleozoic chemical event

magnetic hysteresis of limestones facies control abstract the hysteresis properties of 116 non red marine limestones from 92 localities indicate that remanence is carried by magnetite of pseudo single domain psd and small multidomain md size pelagic limestones have paramagnetic matrices and hysteresis properties compatible with larger psd or md grain sizes of magnetite probably associated with detrital clay minerals introduced by pelagic rain out thus they may be less suitable recorders of stable remanence other limestone facies excepting dolomitized examples have diamagnetic matrices they include shallow subtidal limestones which tend to have smaller psd sizes of magnetite as do backreef lagoonal undifferentiated shelf and reef facies it is believed that the wide geographical and temporal range of samples minimizes effects related to post compaction groundwater flow late diagenesis and that the associations recognized should be tested in future studies

jurassic magnetostratigraphy 3 bathonian bajocian of carcabuey sierra harana and campillo de arenas subbetic cordillera southern spain abstract four sections in majocian bathonian middle jurassic pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300 c to in excess of 450 c the polarity patterns indicate that the majority of the bajocian and bathonian is characterized by quite frequent reversals of the magnetic field lengthy periods of constant polarity particularly constant normal polarity were not observed the average frequency of reversals is about 6 per ammonite zone which roughly may be interpreted as a frequency of a reversal every 260 000 years a rate comparable to that of the miocene pliocene paleolatitudes of these sites 25 28 are about 10 south of their present positions variable clockwise block rotations within the subbectic region have rotated these sites relative to stable iberia

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

a paleomagnetic study of tectonically deformed red beds of the lower glarus nappe complex eastern switzerland a paleomagnetic study at 15 sites 315 samples in permo triassic red beds of the helvetic nappes of eastern switzerland isolated stable characteristic and secondary remanent directions thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction secondary directions are scattered but may have been acquired during the alpine deformation in the oligocene and miocene the characteristic directions also show large between site scatter anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage and that there is a north south lineation this anisotropy pattern implies that the magnetic grains were realigned by the alpine deformation it is not possible to explain the characteristic remanent magnetization deviations on a site by site basis but the site mean directions are streaked along a great circle the down dip direction of which is approximately parallel to the regional fold axes 60 20 this spread could be explained by a penetrative simple shear deformation associated with the nappe internal deformation

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

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

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

patterns of hydroclimatic change in the rocky mountains and surrounding regions since the last glacial maximum abstract the paleohydrologic record of western north america since the last glacial maximum reveals a wide range of hydroclimatic variability in time and space to improve the understanding of abrupt hydroclimatic shifts and millennial scale hydrologic changes in the central rocky mountains we reconstruct the lake level histories of two small lakes in the beartooth and bighorn mountains in northern wyoming over the past 17 ka to do so we use ground penetrating radar gpr and sediment cores to track the elevations of shoreline sediments within the lakes through time we compare the stratigraphies with those from four other lakes in wyoming and colorado and find widespread evidence for a terminal pleistocene drought from 15 to 11 ka an early holocene humid period from 11 to 8 ka and mid holocene aridity from 8 to 5 5 ka the northern wyoming lakes also provide evidence of high levels in the pleistocene possibly before ca 15 ka and rapid hydroclimatic changes that may have correlated with heinrich event 1 ca 16 8 ka we place the changes in a broad context by summarizing and mapping water level changes from 107 additional previously studied lakes important patterns include 1 extensive drying across the western u s after 15 ka 2 sub regional differences during the pleistocene holocene transition 3 a north south contrast from 9 to 6 ka consistent with a northward shift in storm tracks as the influence of the laurentide ice sheet diminished and 4 rapid increases in effective moisture across much of western north america from 6 to 4 ka

near field high resolution strain measurements prior to the october 18 1989 loma prieta ms 7 1 earthquake high resolution strain recordings were made in deep boreholes throughout california prior to during and following the october 18 1989 ms 7 1 loma prieta earthquake the nearest dilational strainmeters sensitivity 1010 and 3 component tensor strainmeters sensitivity 10 9 were 37 km to 42 km respectively from the main shock high quality data including details of strain offsets were recorded on both instruments through the earthquake these data have been searched for indications of short intermediate and long term strain redistribution and or fault slip that might have indicated imminent rupture short and intermediate term changes in both tensor strain and dilational strain several nanostrain if any during the minutes to months before the earthquake are at least 1000 times smaller than that generated by the earthquake itself if short term preseismic slip did occur at the nucleation point of the earthquake during the previous week and if the type of slip is similar to that during the earthquake its moment could not be more than 1024 dyne cm stated another way slip equivalent to that expected for a m 5 3 earthquake could have occurred in the hypocentral region without the strainmeters detecting it at these distances and azimuthal positions long term changes appear to have occurred in mid 1988 and mid 1989 these changes were both followed by ml 5 earthquakes in the hypocentral region on june 27 1988 and august 8 1989 respectively and since they correspond approximately to changes in geodetic strain rate over the epicenter may indicate precursory strain redistribution in the epicentral region minor postseismic strain recovery 14 occurred in the month following the main shock

sem stem observation of magnetic minerals in presumably unremagnetized paleozoic carbonates from indiana and alabama abstract the silurian wabash formation in indiana and the mississippian pride mountain formation in alabama appear not to have been affected by a late paleozoic remagnetization event in an attempt to characterize the magnetic mineralogy in these presumably unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates scanning and scanning transmission microscope sem stem observations and rock magnetic investigations were carried out it is possible to recognize differences in magnetic mineralogy in the unremagnetized carbonate from that in remagnetized carbonates 1 1 iron oxides associated with iron sulfides are hematite in this study as a result of replacement of pyrite instead of magnetite as was found elsewhere 2 2 occurrences of large euhedral pure iron oxides of secondary origin are common in the unremagnetized carbonates 3 3 a rare occurrence of fine grained single crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates although the abundance of the fine grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent the lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks for the wabash and the pride mountain formations this may also be true early cementation has significantly reduced the porosity and permeability in the wabash formation in indiana whereas the presence of the impermeable chattanooga shale may have protected the mississippian pride mountain formation in alabama

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

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

origin of magnetite responsible for remagnetization of early paleozoic limestones of new york state scanning electron microscope and scanning transmission electron microscope observations of thin sections and separates of the devonian onondaga and helderberg limestones and ordovician trenton limestone in new york state allow us to identify three types of magnetite pseudoframboids nonspherical magnetite and fine grained magnetite magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes fine grained magnetite consisting of aggregates of one or more rounded single crystals approximately 2000 a 200 run in diameter could not be observed in thin sections of new york carbonates due to its small size but was found in magnetic extracts hysteresis measurements of magnetic extracts verify that fine grained magnetite is capable of carrying remanent magnetizations however pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain sized crystals that also can be carriers of the remagnetization these data in combination with observations from other localities collectively imply that the alleghenian remagnetization is carried by the fine grained magnetite although pseudoframboidal and nonspherical magnetite may also contribute thus the alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite crystallization of magnetite was mediated by fluids with dissolution and crystallization activated by stress during the alleghenian orogeny the origin of such fluids is unknown but they may have originated through crustal scale fluid migration tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids

palaeomagnetic rotations in the eastern betic cordillera southern spain palaeomagnetic declinations from the external zones of the eastern betic cordillera southern spain and an adjacent area of the internal zone indicate variable and locally very large clockwise rotations the rotations occurred after latest oligocene times and probably before the late miocene this overlaps a period of dextrally oblique convergence during the early to middle miocene along the internal external zone boundary and within the external zone rotations in the external zone are recorded by palaeomagnetic results from 27 sites in upper jurassic ammonitico rosso facies limestones from the subbetic zone and one site of a similar age from the prebetic the sites show either two or three component behaviour the low temperature component is coincident with the present field direction the intermediate temperature component was probably acquired during miocene folding the high temperature component passes fold tests miocene age and a conglomerate test eocene early oligocene age palaeomagnetic declinations from individual tectonic blocks in the subbetic are consistent but indicate large differential rotations between blocks these blocks are underlain by low angle thrust faults which probably accommodated much of the rotation the largest rotations occur on relatively small isolated blocks of jurassic carbonates in a highly deformed triassic evaporite sequence the single site in the prebetic has not rotated significantly relative to stable iberia in malaguide rocks of the sierra espun a in the adjacent internal zone stable palaeomagnetic components were measured at one site in upper miocene sedimentary rocks at three sites in upper oligocene lower miocene red marls at one site in oligocene marls at one site in jurassic limestones and at two sites in permo triassic red beds the palaeomagnetic results suggest that about 60 of clockwise rotation occurred in the latest oligocene earliest miocene and a further 140 of clockwise rotation subsequently one site in late miocene sedimentary rocks yields unrotated declinations which if not representing an overprinted direction indicate that the rotation was complete by the end of the miocene

influence of overthrusting on maturation of hydrocarbons in phosphoria formation wyoming idaho utah overthrust belt geochemical analyses were performed on 11 phosphoria formation samples collected throughout the wyoming idaho utah overthrust belt maturation models incorporating the thermal effects of thrust faulting with lopatin s techniques were then used to interpret these geochemical data in particular four of the sample sites were selected for detailed thermal modeling these four sites include one sample site in an area unaffected by thrusting two sites located in the hanging walls of major thrusts and one site in the footwall of the darby thrust the detailed thermal modeling shows overthrusting did affect the thermal history of the phosphoria formation the effect of thrusting was to cool the overriding sheet and warm the sediments being overridden thus the hanging wall samples had a different post thrusting thermal history than the footwall sample cooling of the hanging wall slowed post thrusting maturation in the two thrust sheet samples in contrast the footwall sample was exposed to higher temperatures after thrusting and the maturation process was accelerated results of this study indicate that tectonism influences the thermal history of the rocks and should be included in any reliable source rock evaluation program involving overthrust strata a simple evaluation based only on the thickness of the overburden in general will not be sufficient to explain fully the maturation data in the absence of actual geochemical information such as ro tai or cai the models described in this paper appear to be sufficiently accurate to give a reasonable estimate of thermal maturity of a potential source bed

mid holocene drying of the u s great basin recorded in nevada speleothems lake level records point to dramatic changes in great basin water balance over the last 25 ka but the timing and pace of holocene drying in the region remains poorly documented here we present stable isotope and trace metal data from two lehman caves nv speleothems that provide a well dated record of latest pleistocene to mid holocene hydroclimate in the u s great basin together the stalagmites span the interval between 16 4 ka and 3 8 ka with a hiatus from 15 0 ka to 12 7 ka mg ca and d 13 c covary throughout the records consistent with control by the extent of degassing and prior calcite precipitation pcp measurements of modern cave and soil waters support pcp as the primary control on drip water trace element composition we therefore interpret mg ca and d 13 ca s reflecting infiltration rates with higher values corresponding to drier periods both mg ca and d 13 c indicate a wet period at the beginning of the record 12 7e8 2 ka followed by pronounced drying after 8 2 ka this mid holocene drying is consistent with records from around the western united states including a new compilation of great basin lake level records the strong temporal correspondence with the collapse of the laurentide ice sheet over hudson bay suggests that this drying may have been triggered by northward movement of the winter storm track as a result of ice sheet retreat however we cannot rule out an alternative hypothesis that wet early holocene conditions are related to equatorial pacific sea surface temperature regardless our results suggest that great basin water balance in the early holocene was driven by factors other than orbital changes

southern california permanent gps geodetic array error analysis of daily position estimates and site velocities we analyze time series of daily positions estimated from data collected by 10 continuously monitoring global positioning system gps sites in southern california during the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes each time series exhibits a linear tectonic signal and significant colored noise spectral power at frequencies in the range 5 yr 1 to 0 5 d 1 is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise in this short period range attributed by others to geodetic monument motions estimating a single slope for the time series power spectra suggests fractal white noise processes with spectral indices of about 0 4 site velocity uncertainties assuming this fractal white noise model are 2 4 times larger than uncertainties obtained assuming a purely white noise model a combination white noise plus flicker noise spectral index of 1 model also fits the data and suggests that the velocity uncertainties should be 3 6 times larger than for the white noise model we cannot adequately distinguish between these two noise models nor can we rule out the possibility of a random walk signal at the lowest frequencies these questions await the analysis of longer time series in any case reducing the magnitude of low frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments otherwise rate uncertainties estimated from continuous gps measurements may not be improved significantly compared to those estimated from infrequent campaign mode measurements

occurrence of secondary magnetite within biodegraded oil samples of solid bitumen from the thornton quarry illinois and the cynthia quarry mississippi were found to be strongly magnetic and to have rock magnetic properties suggesting that the magnetizable grains present are magnetite studies of magnetic isolates revealed that magnetite is present primarily as spherical crystal aggregates that appear identical to magnetite spherules isolated from re magnetized paleozoic carbonate units from other localities organic geochemical analyses of the solid bitumen suggest an origin by microbial attack on what once was liquid crude oil the occurrence of secondary magnetite as inclusions within solid bitumen suggests a relationship between crude oil biodegradation and development of that mineral in our samples we infer that secondary magnetite in other geologic environments may be related to the presence of hydrocarbons the discovery of a natural association of secondary magnetite and hydrocarbons has important implications for paleomagnetism and for petroleum exploration

layer parallel shortening across the sevier fold thrust belt and laramide foreland of wyoming spatial and temporal evolution of a complex geodynamic system abstract varying patterns of layer parallel shortening lps and vertical axis rotations from the thin skin sevier fold thrust belt to the thick skin laramide foreland of wyoming are quantified from integrated structural anisotropy of magnetic susceptibility ams and paleomagnetic analyses within the sevier belt widespread early lps was accommodated by spaced cleavage fracture sets minor folds and minor faults lps directions are subperpendicular to structural trends of systematically curved thrust sheets of the wyoming salient reflecting a combination of primary dispersion and secondary rotation during thrusting within the laramide foreland limited lps was accommodated mostly by minor faults with conjugate wedge and strike slip geometries lps directions in gentler fold limbs vary from perpendicular to acute with structural trends of variably oriented anastomosing basement cored arches steep forelimbs display more complex relations including younger fault sets that developed during evolving stress states and localized vertical axis rotations although internal strain is limited weak ams lineations defined by kinked and rotated phyllosilicates are widely developed and consistently oriented perpendicular to measured lps directions palinspastically restored lps directions corrected for paleomagnetically determined vertical axis rotations vary on average from w e in the sevier belt to wsw ene in the laramide foreland in detail lps directions display deflections related to primary sedimentary wedge geometry and basement fabrics lps in the sevier belt is interpreted to partly reflect stress transmitted from the hinterland through the growing orogenic wedge and topographic stress along the front of the wedge lps in the laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath thick cratonic lithosphere with spatial temporal variations in stress trajectories related to basement heterogeneities and evolving fault systems evidence for significant late n s laramide shortening as proposed by multi stage shortening models is not observed

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

strainmeters and tiltmeters despite steady effort over the last century continuously recording tiltmeters and strainmeters have not yet been successful except for earth tide measurements this article reviews the techniques and instrument designs that have been developed in this field the continuous measurement of true tectonic motions which have rates of 10 14 s 1 requires extremely high instrument stability but more rapid motions tides and seismic waves apart are even smaller and not much more easily detectable much past effort has been spent improving the transducers used in these instruments which usually measure small displacements the commonest choices are optical the most accurate capacitive the most sensitive and inductive the most rugged nowadays building a transducer is the easiest part a much harder task equally important to a good design is attaching the instrument to the ground so that it can detect the signal wanted this part of the design includes choosing a location a large underground opening is thermally stable but distorting a surface installation is easy to build but noisy because of soil weathering and a borehole is inaccessible to weather but also to the operator all such installations and indeed the whole field of strain and tilt measurement of course rely on the assumption that the deformations of interest can be measured over baselengths of a few hundred meters or less this is true for tides and seismic waves but unproven for tectonic motions appropriate attachment techniques differ for each location and many mundane details such as cementing techniques for borehole instruments remain to be worked out the goal of measuring tectonic and tidal tilt has created many tiltmeter designs most are relatively small instruments that use some form of pendulum or bubble level a few use a liquid surface to measure tilts over a long baseline repeated experience has shown the latter type when well built to be superior to the former proving end monument motions to be a dominant source of noise the same appears to hold true for strainmeters the best results coming from long base laser instruments though strainmeters using rigid or flexible material length standards are useful for special purposes and borehole instruments show promise because achieving a good design has been so difficult it is important to select the goals an instrument must meet and be prepared to compromise on others the difficulty of validating results from this type of instrument makes comparison tests essential such tests as have been made so far show that good results cannot be gotten at low cost or without careful attention to details

syndeformational magnetization in the ordovician bigfork chert at black knob ridge western ouachita mountains southern oklahoma abstract paleomagnetic and rock magnetic results from unweathered samples of the ordovician bigfork chert at black knob ridge in southern oklahoma indicate the presence of a pervasive magnetization that resides in magnetite the magnetization has a steep southeast direction small and medium scale folds acquired magnetization during or after carboniferous folding the declination in sites along black knob ridge also shows a north to south 28 counter clockwise shift the bigfork chert at black knob ridge was folded and thrusted along the ti valley fault system in the late paleozoic a pervasive late paleozoic magnetization with a shallow inclination occurs in the laterally equivalent viola limestone in front of the thrust and in the arbuckle mountains this direction is used as a reference for comparison with the bigfork chert formation at black knob ridge the steep inclinations in the rocks at black knob ridge are interpreted to be primarily the results of rotation around a horizontal axis as a result of thrusting the declination change along the ridge is also interpreted to be the result of rotational movements during thrusting the pervasive magnetizaion at black knob ridge is interpreted to have been acquired during the deformation the results of fold tests and an incremental test correcting for both strike and dip suggest remanence acquisition during late stage folding and during an early stage of thrusting the low thermal history of the bigfork chert suggests that the pervasive component is a chemical remanent magnetization crm although a strain related mechanism cannot be definitely ruled out the proposed crm may be related to fluid migration through fractures produced during folding and thrusting alternatively an in situ chemical process e g maturation of hydrocrabons may have caused the remagnetization

are magnetite spherules capable of carrying stable magnetizations to determine if magnetite spherules extracted from ancient carbonate rocks can carry ancient remanent magnetizations electron microscope observations and magnetic data have been obtained for more than 100 individual spherules extracted from the jurassic twin creek limestone wyoming and the mississippian leadville limestone colorado two categories of surface textures smooth and patterned correlate with different hysteresis parameters spherules with patterned surfaces group a have ratios of saturation remanent magnetization to saturation magnetization msr ms 3 most spherules with smooth surfaces group b have 0 1 msr ms 0 3 and hcr hc 3 the patterned surfaces consist of 1 parallel dendritic nets with units 1 m wide 2 platelets with widths of 1 to 3 m 3 mosaic surface units with diameters between 2 and 5 m each of these surface elements consists of a single crystal as verified by selected area electron diffraction patterns showing sharp spots whereas the spherules as a whole show multicrystalline patterns by x ray diffraction the msr ms and hc values of group a increase with decrease in grain size these group a magnetite spherules are undoubtedly multi domain in character and are unlikely to carry stable ancient remanences the smooth surfaced group b spherules are not very abundant 8 of the entire population and have pseudo single domain psd or multi domain md magnetic properties they may carry ancient remanences although we infer that single domain sd non spherical submicrometer magnetite is the principal carrier of remanence in most carbonates