tectonic implications of a late mesozoic fold and thrust belt in northwestern nevada the luning fencemaker fold and thrust belt of northwestern nevada formed east of the mesozoic sierran arc in a back arc setting where noncontinental crust localized marine deposition and subsequent deformation the fold thrust belt was active in middle or late jurassic and early cretaceous time and evolved during major northwest southeast contraction comparable shortening was not developed in the sierran arc and for much of the mesozoic the arc and the back arc regions were at least partially decoupled and separated by the regionally extensive pine nut fault system which underwent substantial left lateral displacement development of the fold thrust belt and associated strike slip fault is attributed to oblique subduction the shortening axis of the fold thrust belt and inferred displacement of the strike slip fault are incompatible with right oblique subduction and call for an interval of left oblique convergence during part of jurassic cretaceous time

palaeomagnetism and magnetostiatigraphy of triassic strata in the sangre de cristo mountains and tucumcari basin new mexico usa summary we report palaeomagnetic data and a composite magnetic polarity sequence for middle and upper triassic rocks assigned to the anton chico member of the moenkopi formation and chinle group respectively exposed along the eastern flank of the sangre de cristo mountains and in the tucumcari basin of eastern and northeastern new mexico thermal demagnetization isolates a well defined dual polarity characteristic magnetization carried in most cases by haematite and interpreted as an early acquired chemical remanent magnetization crm characteristic magnetizations from 74 palaeomagnetic sites one site one bed are used to define a magnetic polarity sequence which we correlate with previously published triassic data obtained from both marine and non marine rocks preliminary correlation suggests that the resolution of magnetostratigraphic data derived from continental strata is not necessarily of lesser quality than that from marine rocks on the basis of the magnetostratigraphic data a profound unconformity is believed to separate lower middle norian and upper norian rhaetian strata of the chinle group palaeomagnetic poles derived from selected sites in steeply dipping 85 strata for the middle triassic anisian 240 ma 50 n 121 e n 8 late carman early norian 225 ma 53 n 104 e n 16 and late norian rhaetian 208 ma 59 n 77 e n 8 are in relatively good agreement with previously published data for the moenkopi formation and chinle group and related strata in southwest north america none the less comparison with palaeomagnetic poles obtained from gently dipping or flat lying triassic strata from this study anisian 46 n 112 e n 13 late carnian 54 n 87 e n 12 and previously published triassic poles in southwest north america suggest that a modest apparent rotation not greater than about 5 affects declinations from steeply dipping rocks the distribution of palaeomagnetic poles indicates 25 angular distance of apparent polar wander between about 240 and 208 ma

late cretaceous paleomagnetism of the tombstone district and vicinity evidence for a rotational domain boundary in southeastern arizona paleomagnetic data are presented for late cretaceous volcanic and intrusive rocks in the tombstone district and dos cabezas mountains of southeastern arizona the resultant paleopole 73 n 224 e a95 8 is similar to a late cretaceous reference pole for north america 82 n 210 e a95 7 implying that these rocks have not been significantly rotated about vertical or near vertical axes relative to the continental interior generally concordant results have also been published for pennsylvanian terra rosa paleosols within the black prince limestone at localities in the nearby gunnison hills whetstone mountains and naco hills together however these carboniferous data indicate a 9 7 clockwise rotation with respect to north america similar to that determined for the colorado plateau 5 14 since the mid cretaceous published paleomagnetic data for rocks south and west of the tombstone vicinity indicate much larger clockwise rotations ranging from 25 to 60 implying that a rotational domain boundary runs between the mustang mountains and canelo hills to the southeast and between the silver bell and tucson mountains to the northwest this boundary roughly coincides in part with the sawmill canyon fault zone and is parallel to other northwest trending strike slip faults in the region that were active during the laramide orogeny the clockwise rotations in southern arizona are likely part of a larger pattern of dextral shear along the western margin of north america during the late cretaceous and early tertiary caused by northward oblique subduction of oceanic plates relative to the stable craton

paleomagnetism of the front range colorado morrison formation and an alternative model of late jurassic north american apparent polar wander a paleomagnetic study of the upper jurassic morrison formation in the front range of central colorado yields high unblocking temperature dual polarity magnetizations with respect to known paleohorizontal the inclinations absolute mean 57 3 degree 95 confidence interval 52 3 degree to 63 1 degree n 8 sites pass tilt and reversal tests whereas the dispersion in declinations can be attributed to apparent or real tectonic rotations and sedimentary processes the site centered colatitudinal locus of possible front range morrison poles partially overlaps the upper pole but it excludes the lower pole from the morrison formation on the colorado plateau as well as the 151 ma glance conglomerate pole from the basin and range province of southeastern arizona the authors offer various explanations for these disparities and suggest an alternative model of late jurassic north american apparent polar wander through approximately 70 degree n which is supported by late jurassic european poles with positive stability tests transferred to north american coordinates

paleomagnetic data from triassic strata zuni uplift new mexico further evidence of large magnitude triassic apparent polar wander of north america upper carnian 225 ma strata of the bluewater creek formation of the chinle group of the colorado plateau western new mexico contain predominantly reverse polarity characteristic magnetizations of high coercivity and distributed unblocking temperature up to 695 c data from a syndepositional structure suggest acquisition of a stable remanence during or soon after deposition the mean paleomagnetic pole for 13 virtual geomagnetic poles vgp accepted of 17 sites collected is located at 55 2 n 87 5 e a95 6 7 k 39 7 the angular distance between this pole and the mean of recently published middle and late norian 210 ma poles from overlying strata is 12 5 supporting the hypothesis that significant apparent polar wander occurred during late triassic time furthermore the bluewater creek pole lies to the west 6 1 angular distance of the cratonic reference pole for carnian time supporting the hypothesis of a small colorado plateau rotation since the early mesozoic anisian strata of the moenkopi formation contain dual polarity magnetizations of high coercivity and distributed unblocking temperatures up to 690 c the mean vgp of six sites yields is located at 53 1 n 96 3 e a95 6 4 k 109 2 which is similar to previously determined middle triassic poles triassic paleomagnetic poles from southwest north america from strata in continuous stratigraphic succession both on and off the colorado plateau are not consistent with the hypothesis of a late triassic stand still of the geomagnetic pole

the elkhorn mountains revisited new data for the late cretaceous paleomagnetic field of north america the late cretaceous portion of north america s apparent polar wander apw path is poorly known due to a dearth of suitable post 85 ma reference poles a paleomagnetic re investigation of elkhorn mountains volcanics 80 ma was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of apw during this time interval correlates with known late cretaceous tectonic events a total of 249 paleomagnetic samples 30 sites were collected from three separate areas within the elkhorn mountains using both alternating field and thermal demagnetization a characteristic direction of magnetization was isolate at 15 sites a95 15 which passes a fold test at the 95 confidence level the mean pole calculated from site virtual geomagnetic poles is located at 80 3 n 189 5 e n 15 a95 9 6 k 16 8 interestingly the elkhorn mountains pole is not concordant with the cretaceous stillstand poles but is nearly coincident with the paleocene reference pole and the recently published pole for the adel mountains dated at 76 ma these new data suggest the onset of apw which brought the pole from its cretaceous position to near its early tertiary position occurred prior to 80 ma and the transition was exceedingly rapid the onset of this rapid apw now appears to coincide more closely in time with many of the significant late cretaceous tectonic events e g the laramide orogeny however since the elkhorn mountains volcanics are floored by a major east directed thrust and have undergone crustal scale deformation the data should be viewed with caution until more results are available for this time interval

paleomagnetism of jurassic volcanic rocks in the patagonia mountains southeastern arizona implications for the north american 170 ma reference pole paleomagnetic study of middle jurassic volcanic and volcaniclastic rocks in southeastern arizona yields a paleomagnetic pole located at 61 8 n 116 0 e k 49 6 95 6 2 these rocks from corral canyon represent an autochthonous segment of the jurassic cordilleran magmatic arc and were deposited directly on upper paleozoic carbonates of the cratonic miogeocline rock magnetic experiments demonstrate that the natural remanent magnetization of these rocks is dominantly a single component magnetization carried in both magnetite and hematite phases and is interpreted as a primary thermoremanent magnetism rb sr isotope geochemistry on whole rock samples from eight welded ash flow tuffs yields an isochron age of 172 2 5 8 2 ma the corral canyon pole falls between poles from newark trend group ii 179 3 ma and glance conglomerate 151 2 ma

north american jurassic apparent polar wander the anwser from other continents abstract jurassic paleomagnetic data from north america which come essentially from the ne and sw usa outline two contradictory apparent polar wander paths apwps following besse and courtillot j geophys res 96 4029 4050 1991 we derive synthetic apwps for north america based on data from other atlantic bordering continents without resorting to the north american data themselves we discuss variations in the data bases and in the kinematics used for the reconstructions and find that a set of some 12 predicted apwps is well constrained to lie within a band of 5 half width along 75 n latitude the main uncertainties occur along the track i e longitude but not perendicular to it i e latitude we conclude that as has been suggested before most of the jurassic north american data show the effect of either partial remagnetization in a recent field or tectonic rotation and that until more reliable data become available the synthetic transferred paths from other atlantic bordering continents offer more reliable estimates of jurassic north american paleogeography and polar wander

late cretaceous polar wander of the pacific plate evidence of a rapid true polar wander event we reexamined the late cretaceous early tertiary apparent polar wander path for the pacific plate using 27 paleomagnetic poles from seamounts dated by 40ar 39ar geochronology the path shows little motion from 120 to 90 million years ago ma northward motion from 79 to 39 ma and two groups of poles separated by 16 to 21 degrees with indistinguishable mean ages of 84 2 ma the latter phenomenon may represent a rapid polar wander episode 3 to 10 degrees per million years whose timing is not adequately resolved with existing data similar features in other polar wander paths imply that the event was a rapid shift of the spin axis relative to the mantle true polar wander which may have been related to global changes in plate motion large igneous province eruptions and a shift in magnetic field polarity state

stability of the earth with respect to the spin axis for the last 130 million years abstract recently continental paleomagnetic data rotated into a hotspot reference frame have been used to suggest the earth rotated 18 with respect to the spin axis at 110 ma we test this true polar wander tpw hypothesis using paleomagnetic data from widespread granitic rocks of cratonal north america these data conflict with tpw predictions leading to rejection of the hypothesis the calculated polar displacement is instead consistent with motion of the atlantic hotspots during the mid cretaceous this analysis suggests that the time averaged position of the spin axis has deviated by no more than 5 over the last 130 million yr indicating that mantle mass heterogeneities have not changed rapidly enough to drive tpw

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

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

cretaceous magnetizations in northwestern nevada and tectonic implications paleozoic and mesozoic arc related rocks of the black rock desert region of northwestern nevada have acquired a very stable postfolding secondary magnetization probably in late cretaceous time the remagnetization which appears to be of regional extent probably accompanied emplacement of granitic plutons in a northeast trending belt across northwestern nevada near the end of mesozoic time the region has experienced no appreciable tectonic rotation since that time

paleomagnetism of parts of the late triassic diabase dike system associated with the trans new brunswick aeromagnetic lineament a paleomagnetic study was conducted on dike samples taken from three widely separated localities along the northeasterly trending caraquet dike system new brunswick in order to define their age relationship this dike system is recognized on the basis of a well defined aeromagnetic lineament stable magnetization directions were isolated after alternating field 15 40 mt and thermal 300 550 c treatments of the samples taken from the three localities c 353 56 n 353 50 and t 14 37 the virtual pole derived from dike t 82 e 62 n agrees well with previously determined late triassic early jurassic 200 190 ma poles for the appalachians on the other hand the other two virtual pole positions from dikes c 143 e 78 n and n 133 e 74 n fall in the general area of average paleopoles for the middle late jurassic 180 170 ma age the possibility that the dike at t differs in age from that of the dikes at localities c and n by at least 20 ma is suggested in the absence of relia

paleomagnetism of late jurassic rocks in the northern canelo hills southeastern arizona the canelo hills volcanics are exposed in the canelo hills a northwest trending range in santa cruz county southeast arizona the formation is composed of silicic tuffs and flows as well as volcaniclastic conglomerates and sandstones strikes of the rocks are generally to the northwest with moderate dips to the southwest and northeast apparent age results from the sequence studied paleomagnetically include two published isotopic dates of 147 6 ma k ar biotite and 149 11 ma whole rock rb sr and a rb sr isochron age reported here which indicates an age of 151 2 ma paleomagnetic samples were collected from 17 cooling units in the northern canelo hills samples from most of these units responded to alternating field af demagnetization and secondary components were generally erased by peak af between 10 and 50 mt samples from five sites showed no response to af demagnetization thermal demagnetization of samples from these units produced no significant changes in direction of natural remanent magnetization nrm although within site clustering of nrm directions was improved data from two sites were rejected because of failure to isolate a well determined characteristic nrm of the remaining 15 sites 10 sites were of normal polarity while five sites showed reversed polarity intensities of the characteristic nrm ranged from 4 10 3 to 3 10 1 a m the data from these 15 cooling units yield a formation mean direction of i 29 9 d 334 9 with k 33 4 and 95 6 7 the resulting paleomagnetic pole is at 62 2 n 130 3 dp 4 1 dm 7 4 this pole is between poles obtained from the summerville and lower morrison formations the canelo hills pole is thus consistent both in position and age with the late jurassic episode of rapid apparent polar wander originally defined by paleomagnetic data from the summerville and morrison formations

evidence for late paleozoic and mesozoic non dipole fields provides an explanation for the pangea reconstruction problems abstract paleomagnetic pole positions have long been calculated with the assumption that the ancient geomagnetic field was purely that of a geocentric dipole but in recent years it has become a concern that this assumption is not always valid because of long term contributions from non dipole fields in particular octupole fields we tested for the presence of a zonal octupole field in the late carboniferous permian mesozoic and early tertiary by comparing the observed paleomagnetic paleolatitude distributions for the laurentian north america and greenland and european landmasses with those predicted from the mean paleopoles such a test is possible for a good sized continent not subsequently deformed when numerous and reliable paleomagnetic results exist from sites that cover a wide range of paleolatitudes if the field were purely dipolar coeval results should form a coherent dataset such that a regression line through the individual points in an observed versus predicted paleolatitude plot has a slope of 1 if the slopes of regression lines are systematically different from 1 0 for successive time intervals and granted that the earth s radius stayed the same then it is likely that non dipole fields can be held responsible regression line slopes standard errors for three intervals analyzed are 0 78 0 04 200 300 ma 0 93 0 11 120 200 ma and 0 82 0 06 40 120 ma as might be expected with a non dipole contribution to the total geomagnetic field paleomagnetic results from the most northerly areas of laurentia and europe i e ellesmere greenland and svalbard show clearly anomalous paleolatitudes because of the agreement between the equatorial locations determined independently by paleomagnetism and equatorial rain forest occurrences d v kent p e olsen earth planet sci lett 179 2000 311 324 we argue that long term quadrupole fields are less likely than octupole fields to be the cause of the paleolatitude discrepancies estimates of the magnitude of the octupole dipole field ratio are not very precise but center around 0 1 which could cause errors in conventional paleopole determinations using the dipole formula of 7 5 because of the antisymmetry of octupole fields a comparison of paleomagnetic poles from mid northern and mid southern hemisphere locations could thus be off by as much as 15 the well known misfit between the paleomagnetic results from the laurentia european and gondwana continents in a classical pangea a configuration could be explained by such errors due to octupole fields this explanation would negate the need to seek tectonic pangea b type solutions for the misfit these solutions have generally remained unacceptable in terms of the geology of the gondwana laurussia borderzone

long term performance of a mudrock seal in natural co2 storage the ability of mudrock seals to prevent co 2 leakage is a major concern for geological storage of anthropogenic co 2 the long term performance of a mudrock seal which provides a natural analogue in the north sea miller oil fi eld has been evaluated this mudrock seal is immediately above a natural co 2 rich reservoir the paper reports the stable isotopes of carbon from carbonate minerals in the mudrock that have precipitated in contact with co 2 during 4 km of burial a well defi ned linear trend of upward decreasing 13 c traces the progressive penetration of free phase co 2 causing dissolution and reprecipitation of carbonate minerals the co 2 was emplaced ca 70 80 ma and has only penetrated 12 m vertically in this case the infi ltration rate is estimated as 9 8 10 7 g cm 2 yr 1

paleobotanical evidence for cool north polar climates in middle cretaceous albian cenomanian time mid cretaceous albian cenomanian floras are abundant and diverse on the north slope of alaska the older floras consist of conifers cycadophytes ferns ginkgophytes and sphenophytes horsetails angiosperms appeared in latest albian time and rapidly diversified the preserved floras consist entirely of deciduous plants with the exception of a microphyllous conifer ferns and sphenophytes deciduousness is evidence for strong seasonality which for these floras might be variations in either light or temperature or both cool temperatures are suggested by the prevalence of toothed leaves among the angiosperms and the presence of large leaved conifers the paleobotanical evidence points to a mid cretaceous climate that was no warmer than cool temperate on the north slope of alaska

carboniferous through jurassic paleomagnetic data and their bearing on rotation of the colorado plateau small yet systematic differences between paleomagnetic poles derived from strata on the colorado plateau and paleopoles determined from rocks on the north america craton have been interpreted to support the hypothesis of modest post late cretaceous clockwise rotation of the plateau as a quasi rigid body with respect to the craton using an iterative search for the best fit euler pole and rotation angle comparison of the best quality late carboniferous through late jurassic paleomagnetic poles from the colorado plateau and the north america craton gives a cumulative rotation estimate based on a rotation pole at 34 n 105 w of 7 4 3 8 95 confidence limits a similar comparison using subsets of the cratonic database from localities in 1 northeast north america and 2 the craton platform interior give larger 8 8 3 6 and smaller 5 1 3 8 estimates respectively reflecting the fact that poles from localities in northeast north america in particular those from triassic rift basins indicate a larger rotation as concluded in direct pole to pole comparisons the euler pole as determined by the paleomagnetic data only can lie anywhere within a relatively large area that encompasses locations in the western united states previously proposed from geological observations paleomagnetic data and geologic observations together or independently do not support the hypothesis of a large colorado plateau rotation of 11 to 15 if geologically reasonable previous estimates of significant 20 km dextral slip along the eastern margin of the plateau require a position for the euler pole east of the 105 w meridian

a middle triassic paleomagnetic pole for north america two stratigraphic sequences were sampled through the early anisian anton chico member of the moenkopi formation in north eastern new mexico two polarities of magnetization are present a normal polarity interval succeeded by a reversed polarity interval and followed by a short normal and reversed polarity couplet detailed thermal demagnetization 10 to 17 steps was employed to separate magnetic vectors the secondary magnetization is largely a direction similar to the present day and or axial field directions demagnetization above 520 c reveals a near horizontal characteristic magnetization the lithology of the stratigraphic sequence is such that the reversed polarity is contained almost exclusively within coarse sandstone lithologies and hence the reversed polarity characteristic magnetization direction is rarely completely separated from the secondary magnetization because of this the paleopole was calculated from only the lower normal polarity portion of the section the pole calculated from the samples of two localities is located at 121 4 e 43 2 n 95 5 3 this middle triassic paleopole is in good agreement with published triassic paleomagnetic poles for cratonic north america and statistically overlaps the early triassic paleopoles the large scale relative motion during the triassic between the magnetic pole and the north american plate is constrained by this study to have begun after early middle triassic time it suggests that as much as 10 of apparent polar wander occurred between the late anisian ladinian late middle triassic and the middle to late carnian early late triassic a time interval of between 11 and 14 m y

paleomagnetism of middle late triassic plutons in southern maine fang wu and van der voo r 1988 paleomagnetism of middle late triassic plutons in southern maine tecionophysics 156 51 58 a paleomagnetic study of two small syenitic to alkali granitic plutons abbott and agamenticus southern maine yields paleopoles at 48o n 92 e and 48o n 99 o e with k ar ages of 221 f 8 and 228 5 ma respectively a third pluton the litchfield was much more weathered and did not contain stable magnetizations dual polarity magnetizations a dike contact test and excellent statistical parameters obtained after detailed thermal and alternating field demagnetization support our claim that the magnetizations and paleopoles are representative of the middle to late triassic paleomagnetic field however the two paleopoles fall about 10 to the southwest of previously published triassic apparent polar wander path segments for cratonic north america unless this small deviation can be ascribed to post triassic tilt of the plutons this requires a modification of the polar wander path to bring the poles into agreement a tilt correction of some 15 20 needs to be applied about a strike which is perpendicular to the area s geologic trends and the main coast line which appears to be highly unlikely given the stabilized tectonic setting of this area in post triassic time a small but significant revision of the north american cratonic apparent polar wander path may therefore be suggested by these results but this needs further corroboration the more southerly paleolatitudes indicated by these triassic results for north america would imply lesser displacements than previously proposed on the basis of triassic paleomagnetism for exotic terranes in the western cordillera

paleomagnetism of the moenkopi and chinle formations in central new mexico implications for the north american apparent polar wander path and triassic magnetostratigraphy in central new mexico red sedimentary rocks unconformably overlying permian carbonates of the san andres formation have been correlated with the early middle triassic moenkopi and late triassic chinle formations of the colorado plateau paleomagnetic samples from triassic sections exposed on basement cored uplifts along both the east and west side of the rio grande rift near albuquerque yield upon thermal and chemical demagnetization well defined high unblocking temperature dual polarity magnetizations carried by hematite the characteristic magnetization is interpreted as an early acquired chemical remanent magnetization based on a positive intraformational microconglomerate test and bedding parallel magnetization polarity zonation the moenkopi and lowermost chinle formations produced paleomagnetic poles respectively at 57 6 n 100 3 e n 36 sites k 74 1 a95 2 8 and 60 8 n 88 9 e n 17 sites k 130 3 a95 3 1 these data plus previously published and additional results from the underlying permian strata suggest that portions of central new mexico have experienced a small clockwise rotation i e less than 10 similar to that of the colorado plateau with respect to the north american craton the paleomagnetic directions of the chinle formation and related strata in eastern new mexico document about 12 great circle distance of rapid apparent polar wander during mid carnian to late norian times along a track which contains other cratonic poles of similar age we present a preliminary magnetic polarity time scale for the triassic that incorporates the present new mexico data and previously published data mostly form continental red bed sequences this magnetic polarity scale provides a basic framework which can be tested with future data from triassic sections where additional biostratigraphic control exists

paleomagnetism of the sassamansville diabase newark basin southeastern pennsylvania support for middle jurassic high latitude paleopoles for north america a paleomagnetic study of the sassamansville diabase which intrudes the passaic formation red beds of the newark basin in southeastern pennsylvania was conducted to constrain the age of the diabase9s magnetization nine sites were collected from the diabase around the sassamansville syncline a tenth site was collected at an associated dike 12 km to the north eight to ten individually oriented cores were drilled at each site alternating field af and thermal demagnetization isolated a stable internally consistent characteristic remanence at eight sites using the bedding tilt of the nearest baked or unbaked passaic formation sedimentary rocks for a fold test the site means showed best clustering at 65 unfolding this magnetization is a statistically significant at 93 confidence synfolding magnetization the diabase9s magnetization fails a baked contact test conducted at one of the sites indicating that the magnetization is secondary and not a primary thermal remanent magnetization trm isothermal remanent magnetization irm and partial anhysteretic remanent magnetization parm acquisition experiments and of demagnetization results indicate that the magnetization is carried by high coercivity magnetite anisotropy of magnetic susceptibility ams and anhysteretic remanence aar results indicate that this magnetite has a different fabric from the northeast southwest horizontally lineated fabric carried by low coercivity magnetite grains that do not carry the remanence the lineated fabric may have been caused by flow of the diabase from the north east to the southwest during emplacement when the rock magnetic results are considered in light of previous petrographic observations davidson and wyllie 1968 of these rocks the rock magnetics suggest that the magnetization is carried by secondary magnetite which has grown as rims on primary magnetite the secondary magnetite may have formed during a hydrothermal event proposed by sutter 1988 based on geochronologic evidence and dated at 175 ma if the diabase magnetization is considered to be coeval to a widespread remagnetization b component of the newark basin sedimentary rocks witte and kent 1991 the diabase magnetization indicates 15 of counter clockwise rotation for the sassamansville syncline this is consistent with either left lateral motion along the border fault or along the intrabasinal chalfont fault the presence of counterclockwise block rotations along the border fault could weaken previously reported evidence that the newark basin b component remagnetization occurred after the newark basin strata had acquired most of their north westerly tilt witte and kent 1991 correc tion for the tilt would move the newark b component remagnetization paleopole to lower paleolatitudes but necessitate significant post 175 matectonic activity in the newark basin this would contradict the long sedimentary record of rifting and border fault activity for the preceding 50 m y a partial tilt correction may be the best geologic and paleomagnetic interpretation because it would minimize the amount of post 175 ma tectonic activity required while bringing the newark b component remagnetization paleopole into paleolatitudinal agreement with a paleopole derived from the rotation of european south american and african data into north american coordinates van der voo 1992

rotation of the colorado plateau consistent displacement of its paleomagnetic pole positions suggests that the colorado plateau has rotated with respect to cratonic north america since pole positions derived from the colorado plateau form a significant portion 35 of the pre cretaceous population of north american paleomagnetic data the rotation has significant impact on the continent s data base comparison of paleopoles from nearly coeval strata on and off the plateau shows that the plateau pole positions are always displaced clockwise from non plateau counterparts in amounts varying from 9 to 14 the time periods for which such comparisons are possible are the late triassic the early triassic the late pennsylvanian and the middle to late devonian other time periods contain data which appear to be consistent with a clockwise displacement of plateau pole positions but poor age control rapid apparent polar wander and late paleozoic remagnetization all hamper comparisons only one of the time periods examined the permian does not show net rotation of the plateau observation of the rotation of the plateau has been obscured by the fact that a great many of the plateau poles are from times of rapid apparent polar wander apw and the displacement effected by rotation parallels the major apw trends this renders the discrepancy of plateau poles relatively inconspicuous and is only readily observable between poles from the same age strata the rotation of the plateau has particular impact on the early and middle mesozoic paleopoles to which the plateau had contributed heavily in particular for the late jurassic plateau poles are the only data available for the analysis of western north america allochthonous terrane movements

a new late cretaceous paleomagnetic pole from the adel mountains west central montana north america s apparent polar wander path has been poorly defined between the mid cretaceous and paleocene reference pole positions existing data allowed 13 of apparent polar motion over about 22 m y 87 65 ma roughly coinciding with the beginning of laramide deformation 80 ma we report on a paleomagnetic study of the adel mountain volcanic rocks to refine the north american apparent polar wander path for this interval the shonkinite rocks of the adel mountain volcanic field are on the eastern edge of the cretaceous paleocene fold and thrust belt some of these structures disturb the western edge of the volcanic pile we obtained two new k ar dates from the adel rocks one date on biotite from a shonkinite dike that crosscuts most of the volcanic rocks is 71 2 2 7 ma the other a whole rock date from a flow deep in the volcanic pile is 81 1 3 5 ma we collected six to nine paleomagnetic samples from each of 34 sites in roadcuts and natural outcrops of flows dikes and laccoliths positive fold and conglomerate tests along with alternating field and thermal demagnetization indicate that our characteristic remanent directions are primary magnetizations acquired before late cretaceous to paleocene thrust belt deformation averaging the virtual geomagnetic poles from 26 reliable sites all of normal polarity yields a paleopole at 82 2 n 209 9 e 95 6 80 k 18 38 this pole is concordant with the paleocene reference pole 82 0 n 170 2 e 95 3 5 k 18 6 diehl et al 1983 and is 11 6 from the globerman and irving 1988 mid cretaceous pole at 71 n 196 e the youngest information in the cretaceous stillstand pole is from the niobrara formation shive and frerichs 1974 at about 85 89 ma if we take the average age of the adel mountain volcanics to be 76 ma then 12 of apparent polar motion occurred between 87 ma and 76 ma thus rapid apparent polar motion correlates well with the onset of laramide deformation

magnetostratigraphy paleomagnetism and remanence acquisition in the triassic chugwater formation of wyoming the magnetic reversal sequence determined at five sections of the chugwater formation can be correlated over a distance of more than 200 km a detailed study of 203 cores constituting a grid of samples taken from a normal to reverse transition zone at the red grade section shows that the transition is characterized by a fairly smooth 180 direction change accompanied by an intensity decrease in about 80 cm of section the transition occurs in the same way in all vertical columns within the grid and in other columns at the same stratigraphic level up to 80 m from the grid and appears identical to the transition reported by herrero bervera and helsley at the same lithologic horizon about 1 km from the grid thermal and chemical demagnetization of grid samples removes a secondary remanence in the general direction of the earth s present magnetic field and does not reveal the presence of normal and reverse components in the same sample the transition appears to have been magnetized during a reversal of the earth s field samples probably became magnetized in a few hundred years or less while they were close to the deposition surface paleomagnetic pole positions for all five sites are very closely clustered and there appears to be no consistent long term polar wander from bottom to top in the sections the mean pole position 46 6 n 113 5 e 95 1 88 is significantly different from pole positions for the early triassic moenkopi formation from the colorado plateau suggesting that post triassic rotation has occurred between the plateau and the basement uplifts of wyoming

jurassic paleopole controversy contributions from the atlantic bordering continents the early early middle jurassic and early cretaceous mean paleopoles for north america europe south america and africa are very well grouped in appropriate continental reconstructions but the intervening late middle and late jurassic segment of the apparent polar wander path apwp is poorly defined and controversial the available paleopoles reconstructed for the partial opening of the central atlantic ocean form a scattered grouping with no coherent age patterns illustrating that they do not constitute a robust data set uncertainties in the reconstruction parameters between north america and europe also play a role however excellent paleomagnetic results exist for tectonic elements near the margin of west gondwana that are unlikely to have been significantly displaced with respect to cratonic africa and south america these results have not previously been used for apwp reconstructions because local rotations are thought to have deflected the paleopoles in many cases the inclinations of such results however can be used to determine a locus of paleopole positions paleopole loci for about 150 and 170 ma were determined from results from spain italy lebanon and the chilean andes and these were rotated with appropriate parameters to give locus intersections in north american coordinates a late middle jurassic early callovian best estimate of the paleopole in north american coordinates is located at about lat 70 n long 135 e and a late jurassic kimmeridgian best estimate is located at about 70 n 155 e the resulting jurassic early cretaceous apwp follows roughly the 70th parallel passing through the middle of the scattered individual paleopoles from the cratonic parts of the atlantic bordering continents

jurassic and cretaceous apparent polar wander relative to south america some tectonic implications late mesozoic apparent polar wander apw with respect to stable south america has been difficult to define it appears that this observation has a simple explanation very little apw occurred and none at all during a 50 m y period 165 to 115 ma it is possible to compute a stillstand pole for this interval 88 8 s 72 4 e k 244 7 a95 4 9 from five reliable studies comprising a total of 523 sites a second stillstand of much shorter duration 70 80 ma is defined by four poles 52 sites this is located at 80 8 s 346 7 e k 457 7 a95 4 3 apw of 9 2 occurred during the interval 115 80 ma a comparable stillstand is found in the north american record e g van fossen and kent 1992 this has a similar degree of scatter k 421 and spans at least 36 m y 124 to 88 ma thus north america and south america both were rotating due westward during a large part of the cretaceous this has some important implications for global tectonics

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

a middle carnian to early norian 225 ma paleopole from sediments of the newark basin pennsylvania paleomagnetic study of the middle carnian stockton formation upper carnian lockatong formation and the lower part of the norian passaic formation from 24 sites along 3 traverses perpendicular to strike in the newark basin of eastern pennsylvania shows that two hematitic magnetizations can be isolated from these rocks thermal demagnetization experiments reveal a distributed unblocking temperature typically 300 to 680 c magnetization with uniformly downward and northerly directions and a high unblocking temperature 660 c and above magnetization with shallow northerly normal polarity and southerly reversed polarity directions on the basis of a correlatable magnetic polarity stratigraphy between 3 traverses spanning 40 km along strike and within site directional scatter similar to that expected from paleosecular variation the high temperature magnetization was most probably acquired at or near the time of deposition the pole calculated from the high unblocking temperature magnetization 53 6 n 101 6 e a95 4 8 is consistent with other late triassic poles and indicates a paleolatitude of 3 8 n 3 0 the lower unblocking temperature magnetization is an overprint acquired at about the same time as the jurassic newark trend igneous n2 pole magnetization

paleomagnetism of the middle jurassic summerville formation east central utah the paleomagnetism of the late callovian summerville formation was analyzed to obtain a late middle jurassic paleomagnetic pole for north america a total of 281 samples were collected from 35 sedimentary horizons sites in a single locality in the san rafael swell area of east central utah fifteen site mean characteristic remanent magnetization chrm directions pass the reversals test and define at least five polarity zones within 52 m of stratigraphic section suggesting that the chrm was acquired upon or soon after deposition magnetizations of some specimens are complex and several horizons yield anomalous site mean directions data analysis included filtering to provide different combinations of virtual geomagnetic poles for calculation of the paleomagnetic pole however editing the data did not change the pole position by more than 5 the preferred paleomagnetic pole position is 56 3 n 133 4 e a95 7 2 n 11 sites the summerville formation paleomagnetic pole is located near the 172 ma corral canyon pole and is statistically indistinguishable from the 151 ma glance conglomerate and 149 ma lower morrison poles the paleomagnetic pole from the summerville formation is located at a much lower latitude and more easterly longitude than the paleomagnetic pole obtained from the 165 ma moat volcanics of new england we propose that the jurassic north american apparent polar wander path is an age progressive band at 55 n to 65 n latitude extending from 11 e longitude at 172 ma to 150 e longitude at 149 ma

impact of tensile strength and incident angles on a soil slope under earthquake sv waves abstract the tensile strength and incident angles play important roles in seismically induced slope failures in this paper a new criterion for tension shear coupling is proposed based on several sets of relevant lab data and is verified using the experimental data provided by previous researchers this criterion can perfectly depict the tension region as well as the compression region in the entire stress space moreover the tension shear criterion was converted into an expression that could be directly used in a numerical method and it was verified by a numerical example in addition the sv waves were input using the equivalent nodal force method based on a viscous spring artificial boundary this process was implemented by the abaqus software and two numerical examples were used to verify this method subsequently based on the proposed new criterion and input method the impacts of the strength criterion and incident angles on an analysis of seismically induced slope failures were studied the results indicated that the evaluation of the tension shear region is extremely important in the analysis of the seismically induced slope failures the tensile strength of the seismically induced landslides was overestimated by the calculation of mohr coulomb criterion owing to the ambiguous description of the tensile failure or the tension shear failure the assessment of the slope stability was thus not conservative the incident angles of seismic waves affected the travel path and the directivity of slope seismic responses the sliding displacement of the slope side near the epicenter rose with increasing angles of wave incidence the critical slip surface deepened and the hazard aggravated therefore it is important to introduce both the tensile strength and incident angles when evaluating the stability of slopes subjected to seismic loads

paleomagnetism of the chinle and kayenta formations new mexico and arizona paleomagnetic data were obtained from 22 sites 6 10 samples site in the upper shale member of the chinle formation 43 sites in the owl rock member of the chinle formation and 35 sites in the kayenta formation thermal demagnetization and data analyses indicate that within site dispersion is an important criterion for selecting sites which retain a high unblocking temperature characteristic remanent magnetization chrm site mean directions define at least four antipodal polarity zones within each member formation suggesting the chrm was acquired soon after deposition fifteen site mean virtual geomagnetic poles vgps from the upper shale member of the chinle formation yield an early norian paleomagnetic pole position of 57 4 n 87 8 e k 60 a95 5 0 eighteen site mean vgps from the owl rock member of the chinle formation yield a middle norian paleomagnetic pole position of 56 5 n 66 4 e k 183 a95 2 6 twenty three site mean vgps from the kayenta formation yield a pliensbachian pole position of 59 0 n 66 6 e k 155 a95 2 4 combined with paleomagnetic poles from the moenave formation and the shinarump member of the chinle formation these data record 30 m y of north american apparent polar wander apw within a regional stratigraphic succession during the camian and norian stages of the late triassic chinle poles progress westward during the hettangian through pliensbachian stages of the early jurassic the pattern of apw changed to an eastward progression even after correction for 4 clockwise rotation of the colorado plateau a sharp comer in the apw path j1 cusp is resolved near the pole from the hettangian sinemurian 200 ma moenave formation 59 4 n 59 2 e amongst other implications the sharp change in the apw path at the j1 cusp implies an abrupt change from counterclockwise rotation of pangea prior to 200 ma to clockwise rotation thereafter

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

paleomagnetic results from the mogollon datil volcanic field southwestern new mexico and a refined mid tertiary reference pole for north america miocene oligocene rocks of the mogollon datil volcanic field in southwestern new mexico have yielded paleomagnetic data giving 61 site mean unit vgps and a paleomagnetic pole located at 81 9 n 143 6 e a95 4 5 k 17 6 this pole is indistinguishable from other mid tertiary reference poles for north america combining the mogollon datil data with vgp data from other studies of mid tertiary 20 40 ma rocks yields a paleomagnetic pole at 81 5 n 147 3 e n 239 a95 2 4 k 15 4 when divided into normal and reversed polarity subsets the data yield paleomagnetic poles that are statistically different at the 95 confidence level mcfadden and lowes 1981 the paleopole determined from reversely magnetized units located at 78 6 n 138 5 e n 125 a95 3 4 k 14 5 is 6 8 on the far side of the paleopole determined from normally magnetized units positioned at 84 2 n 166 4 e n 114 a95 3 3 k 17 0 when viewed from a common site location i e the western united states similar pole positions an differences are also seen in 10 ma subsets of this data set incomplete removal of a normal overprint on the reversely magnetized units appears unlikely to explain these results rather we suggest that the differences are consistent with the concept that differing harmonics may be associated with the two polarity states over the long term merrill and mcelhinny 1977 merrill et al 1979

did a large departure from the geocentric axial dipole hypothesis occur during the eocene evidence from the magnetic polar wander path of eurasia abstract low paleolatitudes are very frequent in eocene to oligocene formations particularly in the tethyan mobile belt these anomalies and their possible sources are discussed they are neither due to unreliable results nor to inclination errors in sedimentary rocks neither can large tectonic movements explain these discrepancies the most probable causes having been rejected therefore the best explanation consists in an alternative magnetic pole position for eurasia located at 69 n 215 e it is proposed that the mean magnetic field of the earth was dipolar but that its axis has swung from one stable position to another during the middle tertiary and was therefore sometimes different from the rotation axis

testing the fixed hotspot hypothesis using ar 40 ar 39 age progressions along seamount trails abstract hotspots and their associated intra plate volcanism producing seamount trails have become an accepted fact in geology from a conceptual theory the azimuths and age progressions of these seamount trails provide the only means to determine absolute plate motions with respect to an independent reference frame of fixed hotspots however the presumed fixity of hotspots is in disagreement with recent paleomagnetic studies and global circuit plate reconstructions for the hawaiian emperor seamount trail in this study we provide independent evidence suggesting that hotspots are not fixed relative to each other we use a straightforward test that compares the observed 40ar 39ar age progressions along pacific seamount trails 0 140 myr with the pacific plate velocities as predicted by their poles of plate rotation i e euler poles in most of these comparisons the age progressions were found incompatible with published euler poles or with a new set of euler poles as derived in this study using discrete seamount locations digitized from the bathymetry maps of smith and sandwell eos 77 1996 315 science 277 1997 1956 1921 we conclude that the relative motion between hotspots may be required to reconcile the observed age progressions with the predicted plate velocities from their modeled euler poles on average the pacific hotspots may show motion at 10 60 mm yr over the last 100 myr partly attributed to individual hotspot motion whereas systematic motion of these hotspots due to true polar wander may account for the remainder

a barremian aptian early cretaceous north american paleomagnetic reference pole four intrusive complexes from southern maine containing rocks ranging in composition from olivine gabbro to granodiorite were sampled to determine an early cretaceous reference pole for the north american plate sampling included the lebanon diorite the tatnic complex alfred complex and the cape neddick complex dated radiometrically by the 40ar 39ar or k ar method on biotite between 125 and 120 ma the majority of samples have unblocking temperatures between 550 c and 580 c and curie temperatures between 550 c and 570 c indicating that the dominant remanence carrier is titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete c1 c3 grains as exsolution or oxidation exsolution lamellae in the silicates as a product of magmatic reactions forming magnetite ilmenite symplectites with pyroxene or olivine or as ultrafine dust hemo ilmenite is present as a minor component in the lebanon diorite and in this case the remanence direction carried by the hemo ilmenite is indistinguishable from the titanomagnetite detailed petrographic observations and rockmagnetic experiments demonstrate that despite the coarse average grain size of the oxides in these rocks which might lead one to expect unstable magnetic remanence the magnetic remanence in most samples is extremely stable and caused by single domain or pseudo single domain behavior in magnetite containing fine oxidation exsolution lamellae of ilmenite the tatnic and alfred intrusions are of normal polarity d 333 i 62 pole 70 5 n 210 3 e a95 3 7 n 24 sites the cape neddick and lebanon intrusions are of reversed polarity d 161 i 57 pole 73 7 n 185 7 e a95 5 0 n 17 sites together these intrusions yield a mean paleomagnetic pole at 72 2 n 198 9 e a95 3 3 n 41 sites the well characterized reversed polarity recorded in the precisely dated cape neddick complex may represent the best age for the magnetic polarity chron m0r the last reversal before the onset of the cretaceous normal superchron at the barremian aptian stage boundary

paleomagnetism of the brushy basin member of the morrison formation implications for jurassic apparent polar wander the paleomagnetism of the 147 ma tithonian brushy basin member of the morrison formation was analyzed to obtain a late jurassic paleomagnetic pole for north america a total of 200 samples were collected from 25 sedimentary horizons sites at norwood hill in southwest colorado at montezuma creek in southeast utah 184 samples were collected from 26 sites detailed thermal demagnetization up to nine temperature steps between 600 c and 680 c and principal component analysis were required to confidently isolate characteristic remanent magnetization chrm directions carried by hematite demagnetization behavior for many horizons is erratic and does not allow isolation of a high unblocking temperature chrm data selection criteria required sample chrm directions to be defined by three or more thennal demagnetization steps and maximum angular deviations of sample chrm directions to be 20 eight sites from the norwood hill location and 10 sites from the montezuma creek location passed these criteria the 18 site mean virtual geomagnetic poles yield a paleomagnetic pole position from the brushy basin member of 68 3 n 156 2 e a95 4 8 k 53 this pole position is within 2 of the paleomagnetic pole which steiner and helsley 1975a reported for the upper morrison formation at norwood hill colorado a second paleomagnetic pole was calculated after excluding sites with site mean 95 20 and sites with fewer than three samples that passed the above selection criteria this additional editing did not significantly change the paleomagnetic pole position at the 95 confidence level along with other paleomagnetic poles from the continental interior the paleomagnetic data from the brushy basin member of the morrison formation are interpreted to indicate that the late jurassic part of the north american apparent polar wander path progresses from a late middle jurassic 160 ma position at 60 n 135 e toward the mid cretaceous pole position at 72 n 191 e

age and magnetism of diabase dykes and tilting of the piedmont abstract we have conducted a paleomagnetic and radiometric study of diabase dikes in the south carolina piedmont where such data is lacking our virtual geomagnetic pole position for 20 of 24 diabase sites in the piedmont is 83 9 e 66 1 n and lies near the older 195 m y age group of smith and noltimier 1979 single k ar apparent ages of twelve diabase sites indicate that one half of the diabase dikes studied have ages near 195 m y two of the sites whose pole positions lie well to the east of our mean value have apparent ages in the 180 m y range thus while two groups appear to be present in south carolina it is the older group that seems to predominate we combined our pole position with other data from diabase in the southern piedmont watts 1975 bell et al 1979 and have calculated a mean paleopole position of 86 1 e 66 4 n based on 50 sites in georgia south carolina and north carolina the southern diabase paleopole position plots slightly off the north american apparent polar wander path but is not significantly different for the 190 m y average pole of irving 1982 these results do not provide an indication of the degree of tilting of the appalachians in post early mesozoic times however of the two possible scenarios that can be envisioned 1 1 injection into an updomed area or 2 2 tilting of the appalachian surface due to post early mesozoic differential erosion and isostatic rebound adjustment to account for a 10 westerly tilting of the piedmont surface about the appalachian trend brings our southern pole position more nearly into coincidence with the north american cratonic pole for early jurassic times

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

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

late cretaceous true polar wander not so fast using recalculated paleopoles from seamount anomaly modeling sam sager and koppers 1 1 proposed an episode of rapid late cretaceous true polar wander tpw a critical review of the data used however suggests that sager and koppers have underestimated the effects that errors and data