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

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

paleomagnetism of rocks from burin peninsula newfoundland hypothesis of late paleozoic displacement of acadia criticized the posttectonic st lawrence granite 360 5 ma and associated subvertical dikes have two magnetizations n and h n points north with an intermediate upward inclination declination 005 inclination 50 alpha 95 10 paleopole 12 n 120 e we consider n to have been acquired close to the time of initial cooling the h magnetization points south with low inclination h is also the dominant magnetization in the nearby permo carboniferous spanish room formation h is considered to be an overprint acquired chemically during the kiaman reversed superchron late carboniferous and permian the n magnetization corresponds to a paleolatitude of 31 7 s very different from the near equatorial values expected from the commonly accepted late devonian and earliest carboniferous tournaisian reference paleofields for cratonic north america this is generally consistent with the hypothesis of a southerly displaced acadia proposed by kent and opdyke although the displacement is much greater 30 rather than 15 however we show that all inclinations observed from middle to upper devonian and tournaisian rocks from the craton are not significantly different either from the kiaman paleofield or from the h overprints observed from newfoundland some of these cratonic studies are not very detailed but it is noteworthy that the declination observed in the most comprehensive of them catskill red beds also agrees with the kiaman paleofield and with the h overprints from newfoundland declinations in some other cratonic studies show small departures of only a few degrees with one exception based on limited sampling and two from the colorado plateau which may have been rotated which could be caused by experimental inaccuracies moreover the directions obtained from cratonic studies are 97 reversed as they are in rocks with kiaman magnetizations we suggest therefore that the commonly accepted late devonian to earliest carboniferous reference paleofield for north america is either entirely kiaman in age or has been seriously affected by kiaman overprinting consequently the 30 of southerly displacement which the n magnetization of the st lawrence granite apears to indicate and the 15 invoked by kent and opdyke we consider to be illusory three late precambrian rock units 623 to 606 ma have scattered declinations but similar inclinations yielding paleolatitudes of about 35 the scattered declinations presumable were caused by local pre carboniferous relative rotations there is little or no evidence of kiaman overprinting in precambrian rocks indicating that overprinting is comparatively superficial and is probably related to late paleozoic groundwater circulation

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

a cratonic middle jurassic paleopole callovian oxfordian stillstand j 2 cusp rotation of the colorado plateau and jurassic north american apparent polar wander 1 paleomagnetic results have been obtained from two sequences of the upper middle jurassic upper callovian summerville formation which are located on the north american craton north central new mexico the thicker of the two sections trujillo exhibits a well defined dual polarity remanence forming a magnetostratigraphy of at least 12 relatively short polarity intervals the high reversal frequency is consistent with that recorded by the oceanic crust at this time the second section romeroville is thin gypsiferous and poorly indurated consequently only a short interval 5 m was successfully sampled the characteristic magnetization was wholly normal polarity in the rapid reversals of the thicker section that thickness of normal polarity is unique and thus forms the basis of magnetostratigraphic correlation between the two sections a cratonic late middle jurassic paleomagnetic pole at 148 2 e 57 3 n k 30 3 95 6 0 was obtained from the larger trujillo sequence despite large confidence limits the small sample population of the romeroville sequence displayed the same pole throughout the western united states the summerville formation is overlain with little or no discordance by the morrison formation although the two stratigraphic units are commonly reported to be separated by an unconformity designated j 5 the new mexico paleopoles of the summerville and overlying lower morrison formations are statistically identical at the 95 confidence level a similar situation exists between summerville and lower morrison paleopoles of the colorado plateau the presence of the same geomagnetic field direction in both the summerville and the lower morrison formations suggests that the j 5 unconformity which separates them may encompass little time perhaps representing only a minor break in deposition the two newly investigated summerville sites are located east of the rio grande rift hence representative of the north american craton their paleopoles are displaced counterclockwise relative to those of the colorado plateau and in amounts consistent with other coeval craton colorado plateau pole pairs together six coeval paleopoles indicate that the colorado plateau has rotated clockwise by about 9 0 3 since late pennsylvanian time the cratonic summerville paleopole also removes the uncertainty over the jurassic north american apparent polar wander path the statistically identical summerville and lower morrison paleopoles form a substantial data set five paleopoles which indicate that na apw traced a path roughly along 60 latitude the five paleopoles stand in stark contrast to the paleopole from the slightly older 5 8 ma moat volcanics spreading rates in the central atlantic do not allow na plate motion that would include both paleopoles positions because the weight of the data five paleopoles from widely spaced localities and different tectonic blocks the moat volcanics pole cannot be representative of north america in the middle jurassic

paleomagnetism of the permian wallowa terrane implications for terrane migration and orogeny a sequence of volcanic astic sedimentary and pyroclastic rocks of permian age late leonardian or early guadalupian 260 ma in the wallowa terrane of oregon and idaho has been sampled for paleomagnetic study forty six sites 222 cores were collected from folded laminated siltstone mudstone and tuffaceous sandstone beds exposed along the snake river in hells canyon several stratigraphic sections within the 1500 to 2500 m thick hunsaker creek formation were sampled over a stratigraphic interval of about 700 m thermal demagnetization revealed three components of magnetization the a component has the lowest unblocking temperatures generally less than about 200oc has normal polarity and fails the fold test the b component is removed between heating steps of about 200oc and 550oc is of normal polarity and also fails the fold test the c component observed between heating steps of approximately 550oc and 660oc is always of negative inclination the grouping of this component improves slightly upon unfolding with the optimum grouping occurring at 95 unfolding using sites with n 3 and k 10 k95 ko l 18 or 100 using sites n 3 k95 k0 l 51 this component is interpreted as the characteristic remanent magnetization acquired during deposition of the hunsaker creek formation based on the following observations the better grouping of the high temperature magnetic component after unfolding and the consistent reversed polarity seen in samples from the different sections the resulting paleolatitude calculated using inclination only statistics of 24o 12 o n n 3 k 10 or 26o 9on n 3 sites implies a northern hemisphere position of the wallowa terrane during the early permian and possible southward transport of this terrane with respect to north america if the wallowa terrane was attached to the wrangellia superterrane our result indicates a northern hemisphere location for that superterrane during the permian

paleomagnetic study of the eastern klamath terrane california and implications for the tectonic history of the klamath mountains province paleomagnetic study of permian through jurassic volcanic and sedimentary strata of the eastern klamath terrane has shown the remanent magnetization of many of these rocks to be prefolding and most likely primary similarities in magnetic declinations recorded by coeval strata over a broad area are consistent with the hypothesis that the terrane in general has behaved as a single rigid block paleomagnetic data indicate that the volcanic island arc represented by this terrane the nucleus of the province was facing toward the present southwest during late paleozoic time although its orientation during earlier periods is unknown whether the arc was separated from the north american craton by a small marginal basin or originated far offshore cannot be determined from paleomagnetic data the declination anomalies for both permian and triassic strata are similar average 106 12 so we infer that clockwise rotation of the late paleozoic arc did not begin until latest triassic or earliest jurassic time the arc may have completed its initial rotation with respect to stable north america by middle jurassic time after some retrograde motion the arc was again facing west by the late jurassic by which time some of the more westerly terranes of the province had become attached to the eastern klamath terrane the composite klamath mountains terranes continued to rotate until the final 60 of clockwise rotation was nearly complete by the early cretaceous coincidence of the waning stages of rotation at about 136 ma with the beginning of deposition of the basal great valley sequence onto the klamath basement probably represents the completion of accretion of the klamath mountains terranes to the north american continent nearly all the rotation occurred while the klamath mountains terranes were part of a converging oceanic plate with only about 20 of rotation in mid tertiary time during basin and range extension no data currently available show evidence for any significant latitudinal displacement of any klamath mountains terranes relative to cratonic north america

paleomagnetic poles for the carboniferous brush creek limestone and buffalo siltstone from southwestern pennsylvania abstract we report partial results of a larger project being undertaken in the appalachian basin to determine the character of the geomagnetic field in the carboniferous the brush creek limestone which contains abundant terrigenous matter yields reliable results from three different sites while the overlying buffalo siltstone appears at one locality af demagnetization curves irm acquisition curves thermomagnetic analysis and optical reflection microscopy indicate that the remanence carriers are magnetites or titanomagnetites which are probably primary in origin stepwise af demagnetization reveals that the best demagnetizing fields are 15 20 and 20 30 mt for the limestone and siltstone units respectively a total of 93 specimens from 20 samples oriented hand samples and field drilled cores yields a paleopole at s 36 1 w 55 7 with an alpha 95 of 4 2 and a k of 13 1 for the brush creek limestone the buffalo siltstone paleopole is located at s 27 4 w 57 0 with alpha 95 6 1 and k 13 0 using 45 specimens from 31 oriented hand samples these results agree fairly well with the mean reversed carboniferous paleopole and noltimier s coal pole but are sufficiently different to raise the possibility of doing magnetostratigraphic correlations

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

polar wander and paleomagnetic reference pole controversies the purpose of this paper is to review the progress made over the past four years in studies of phanerozoic and precambrian polar wander with an emphasis on the contributions made by u s scientists we perceive a general theme embracing the notion that the accuracy of cratonic reference poles and therefore of apparent polar wander apw paths is not as good as was generally believed four years ago a renewed interest in research concerning cratonic reference poles is largely the result of paleomagnetic studies in orogenic belts namely the western cordillera and appalachians of north america the andes of south america and elsewhere which seek to document the existence or non existence of displacements of suspect terranes i e rotation and or translation with respect to their presently associated cratons some of the results of this research are reviewed in this volume by hillhouse and mcwilliams an important realization of which paleomagnetists have long been aware but which was demonstrated recently by a number of sobering examples for both the paleozoic and mesozoic apw paths for north america is that the reliability of concordance discordance parameters that define terrane displacement is dependent not only on the precision and accuracy of the individual paleomagnetic study but also on the precision and accuracy of the appropriate cratonic reference pole

the late cretaceous paleomagnetic field in north america a south american perspective determining the late cretaceous paleomagnetic pole for north america has been difficult because of the lack of suitable rocks of that age in cratonic areas to provide the necessary data as an alternative different studies have appealed to paleomagnetic data from rocks in western north america using paleopoles from stable areas in neighboring continents it is suggested that the available late cretaceous paleomagnetic record in western north america should be analyzed in terms of rigid body deformations rather than be used to represent the cratonic reference field

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

tests of the rigid plate hypothesis and bounds on intraplate deformation using geodetic data from very long baseline interferometry the geodetic results of ma et al 1994 from very long baseline interferometry are used to assess rates of intraplate deformation we analyze the resulting set of velocities of 18 plate interior sites nine on the north american plate four western european sites on the eurasian plate three on the pacific plate and two on the australian plate we formulate three alternative error budgets based partly on the work of ryan et al 1993a an optimistic error budget a conservative error budget and an upper bound error budget all three budgets assume the existence of a previously unmodeled error that is inversely proportional to the time span of observations at a site use of the conservative error budget indicates no significant motion between sites on any one of the four plates use of the upper bound error budget designed to be precisely large enough to be consistent with rigid plates places a usefully small upper bound on how fast any site might be moving relative to the rest of its plate speeds faster than 2 mm yr are excluded at the 95 confidence level for seven of the 18 plate interior sites adjustments of the site velocities for motion due to postglacial rebound as predicted from the model of tushingham and peltier 1991 give an insignificant improvement to the fit of the horizontal velocities to the rigid plate model the velocities of six sites near the western margin of the interior of the north american plate were also analyzed use of the conservative error budget indicates that a site near fairbanks alaska has an upper bound of 3 mm yr on its speed a site in nome alaska has an upper bound on its speed of 8 mm yr and a site in penticton british columbia has an upper bound on its speed of 4 mm yr of three sites on the colorado plateau two move marginally significantly and one moves insignificantly relative to stable north america upper limits on their speeds relative to stable north america range from 4 to 5 mm yr we conclude that the data do not yet demonstrate significant velocities between sites in the stable interior of any one plate but do provide useful and small upper bounds of 2 mm yr or less on the speeds of the best observed intraplate sites six sites near the western margin of the north american interior have upper bound speeds of 3 to 8 mm yr

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

paleomagnetic results from the late carboniferous early permian casper formation implications for northern appalachian tectonics abstract paleomagnetic samples were collected from 190 m of the late carboniferous early permian casper formation in southeastern wyoming a total of 549 samples was drilled near the vicinity of horse creek station at an average stratigraphic interval of 33 cm all samples were reversely magnetized rock magnetic analyses indicate that the primary carrier of remanence in the formation is hematite a selection criterion applied to the partial demagnetized data restricted the sample population to 233 resulting in a paleomagnetic north pole located at 47 4 n 127 4 e p 0 7 m 1 4 the casper pole agrees well with other late carboniferous early permian poles for cratonic north america the tight clustering of these paleomagnetic poles suggests that little apparent polar motion with respect to north america occurred during this time comparing the stable north american poles with paleomagnetic poles from late carboniferous early permian strata of the new england canadian maritime region acadia indicates that this region did not reach its present position relative to north america until at least the early permian

the pictou red beds pennsylvanian pole could phanerozoic rocks in the interior united states be remagnetized the pennsylvanian red beds of the pictou group on prince edward island canada retain a stable remanence in hematite that gives a paleomagnetic pole position at 42 n 131 e dp 2 dm 4 positive conglomerate and contact tests along with dual polarities show that the remanence is primary and of stephanian age 286 306 ma for elongated pole distributions through time bingham statistics is more suitable than fisher statistics bingham statistics shows that there is a consistent discordance in the pennsylvanian to late triassic poles from the appalachian orogenic belt and interior craton of north america this discordance is most readily explained by younger partial remagnetization or possibly complete remagnetization in many rock units especially red beds of the interior craton bingham analysis also shows that there is a significant difference between the colorado plateau and interior craton that is most readily explained by a laramide 5 5 2 9 clockwise rotation of the plateau relative to the craton

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

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

segmentation in gravity and magnetic anomalies along the u s east coast passive margin implications for incipient structure of the oceanic lithosphere segmentation is a characteristic feature of seafloor spreading along the global mid ocean ridge system while segmentation of active spreading centers has been the focus of much recent research the process by which a rifted continental margin develops into a segmented mid ocean ridge is still poorly understood in this study we investigate the segmentation character of the u s east coast margin through a modeling study of the margin basement structure magnetics and gravity anomalies the east coast margin is of particular interest because it is one of several rifted continental margins that display thick sequences of high seismic velocity igneous crust presumably formed during rifting the east coast magnetic anomaly ecma a distinct total field magnetic high running offshore along the margin is commonly located seaward of the thickest sections of the high velocity crust and displays segmentation on length scales 100 120 km similar to the segmentation observed at the mid atlantic ridge mar isostatic gravity anomalies were calculated by removing from free air gravity the predicted effects of seafloor sediments and a crust mantle model assuming local isostatic compensation the resultant residuals show a corridor of high anomaly running along the margin situated close to the maximum thickness of the high seismic velocity crust as determined from the two available seismic refraction lines reduction to the pole r t p of the total field magnetic anomaly shows that after the removal of skewness from the ecma the location of the isostatic gravity high is closely correlated to the ecma the isostatic gravity high is also segmented but in two distinct wave bands 100 150 km and 300 500 km the short wavelength 100 150 km segmentation in the r t p magnetic and isostatic gravity anomalies is similar in wavelength to segmentation in magnetization and mantle bouguer anomaly observed along the present day mar the 300 500 km segmentation in the along margin isostatic gravity anomaly is similar in length scale to both intermediate wavelength tectonic segmentation observed in the south atlantic and variations in lithospheric strength observed along the african margin furthermore two of the intermediate wavelength 300 500 km isostatic gravity lows correspond to the early traces of the kane and atlantis fracture zones suggesting that these two fracture zones may define boundaries of a single tectonic corridor in the north atlantic we hypothesize that the direct cause of the intermediate wavelength segmentation may be along margin variations in both the amount of underplated igneous crust and the strength of the lithosphere although the relative importance of these two effects remains unresolved our results imply that segmentation is an important feature of margin development and that segmentation at mature oceanic spreading centers may be directly linked to segmentation during continental rifting

late cretaceous paleomagnetism and clockwise rotation of the silver bell mountains south central arizona late cretaceous ash flow volcanism in the silver bell mountains of southern arizona 32 3 n 248 5 e was associated with caldera formation and porphyry copper mineralization oriented samples from 34 sites in volcanic volcaniclastic and intrusive units related to this episode of igneous activity 73 69 ma yield a mean paleomagnetic direction of i 61 2 d 24 0 95 7 6 primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping 95 confidence level of site mean directions for a widespread tuff unit after structural correction a comparison of this direction with a direction at silver bell calculated from contemporaneous units for stable north america i 58 8 d 342 3 95 7 7 indicates a late cretaceous paleolatitude anomaly of 2 4 7 6 not significant at the 95 confidence level and a declination anomaly of 41 7 14 3 for the silver bell mountains previously determined paleomagnetic data for southeastern arizona suggest that this apparent clockwise rotation in the silver bell mountains is a local phenomenon although preliminary the average paleomagnetic direction for oligocene and lower miocene rocks in the silver bell area i 43 8 d 357 3 95 16 5 is similar to that calculated for stable north america i 50 2 d 352 2 95 3 9 implying that the observed rotation in the silver bell mountains occurred before 26 ma and was most likely associated with dextral strike slip movement along the ragged top and related wnw trending faults bounding the silver bell mountain block these data in conjunction with plate reconstructions and other paleomagnetic data from southwestern north america imply that wnw trending strike slip faults may have played an important role during late cretaceous to early tertiary deformation in southern arizona

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

canadian cordilleran displacement palaeomagnetic results from the early jurassic hazelton group terrane i british columbia canada summary the early jurassic telkwa formation comprising the base of the hazelton group was sampled at three areas in the central part of the stikine terrane british columbia detailed alternating field and thermal step demagnetization on samples from nine sites from the red canyon area and 10 sites from the zymoetz river area both of which are in the bulkley ranges and eight sites from the telkwa range area isolate stable remanence magnetization components the components are interpreted to be primary magnetizations because of the presence of two polarities and the much improved agreement of their inclinations after full tilt correction of the sites from the bulkley ranges within locality declinations agree well between locality declinations are discordant the expected cratonic declination is 340 whereas declinations from this study are 265 227 and 328 for the red canyon zymoetz river and telkwa range areas respectively these results are similar to those of monger irving 1980 and we concur that the discordant declinations are the result of differential rotation about vertical axes between sample localities our observed mean inclination of 52 is in good agreement with monger irving s 1980 result of 54 they interpreted this to indicate 1300 km of northward displacement since the early jurassic and their interpretation was substantiated by several later studies however with revisions to the geologic time scale and the early jurassic reference pole for north america the early jurassic canadian cordillera palaeomagnetic data base appears latitudinally concordant this indicates that terrane i and southern wrangellia were in much the same latitudinal position relative to the north american craton as they are now tectonic displacement models developed in view of discordant cretaceous palaeomagnetic data from the canadian cordillera must now consider latitudinally concordant permian to early jurassic data and middle eocene to recent data this tectonic senario appears more reconcilable with the regional tilt hypothesis however this hypothesis cannot account for discordant cretaceous data from bedded volcanic rocks in terrane i it is therefore probable that cretaceous cordilleran tectonics were characterized by both regional tilting and moderate latitudinal displacement

paleomagnetism of the late pennsylvanian and permian a test of the rotation of the colorado plateau coeval formations on both the north american craton and the colorado plateau have been sampled for the comparison of paleomagnetic pole positions the latest pennsylvanian earliest permian laborcita and the early permian abo formations of eastern new mexico and the latest pennsylvahian wescogame formation of the supai group of northwestern arizona were studied thermal demagnetization demonstrated that the majority of the samples in all three formations were remarkably free of significant secondary overprinting computation of pole positions used only samples which exhibited univectorial decay to the origin over at least three or more temperature steps the paleopole of the wescogame formation is 119 2 e 47 3 n n 45 samples k 41 6 dp 1 7 and dm 3 4 the laborcita formation paeopole is 130 6 e 41 8 n n 114 samples k 42 9 dp 1 0 and dm 2 0 and the overlying abo formation paleopole is 118 3 e 49 7 n n 84 samples k 54 9 dp l l and dm 2 1 comparison of the wescogame laborcita abo and other paleopoles of this age shows the plateau poles displaced clockwise confirming a clockwise rotation of the colorado plateau with respect to stable north america the new data could be interpreted to indicate a 7 4 rotation of the plateau however the direction of displacement of pennsylvanian and permian paleopoles is significantly different from that exhibited by the early and the late triassic poles the current body of data is best fit by not one but two rotations and suggests that the colorado plateau rotated in late permian time prior to a later presumably laramide rotation separate calculations of the late permian rotation from late pennsylvanian data and from early permian data both indicate a rotation of about 9 5 2 7 northward and 5 4 2 7 westward this study thus suggests that at least two rotations of the colorado plateau have occurred prior to the most recent rotation involving the opening of the rio grande rift this history of rotations implies a quasi independent plate motion behavior for the colorado plateau microplate

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

sr nd os evidence for a stable erosion regime in the himalaya during the past 12 myr modern erosion of the himalaya the world s largest mountain range transfers huge dissolved and particulate loads to the ocean it plays an important role in the long term global carbon cycle mostly through enhanced organic carbon burial in the bengal fan to understand the role of past himalayan erosion the influence of changing climate and tectonic on erosion must be determined here we use a 12 myr sedimentary record from the distal bengal fan deep sea drilling project site 218 to reconstruct the mio pliocene history of himalayan erosion we use carbon stable isotopes 13c of bulk organic matter as paleo environmental proxy and stratigraphic tool multi isotopic sr nd and os data are used as proxies for the source of the sediments deposited in the bengal fan over time 13c values of bulk organic matter shift dramatically towards less depleted values revealing the widespread late miocene ca 7 4 ma expansion of c4 plants in the basin sr nd and os isotopic compositions indicate a rather stable erosion pattern in the himalaya range during the past 12 myr this supports the existence of a strong connection between the southern tibetan plateau and the bengal fan the tectonic evolution of the himalaya range and southern tibet seems to have been unable to produce large re organisation of the drainage system moreover our data do not suggest a rapid change of the altitude of the southern tibetan plateau during the past 12 myr variations in sr and nd isotopic compositions around the late miocene expansion of c4 plants are suggestive of a relative increase in the erosion of high himalaya crystalline rock i e a simultaneous reduction of both transhimalayan batholiths and lesser himalaya relative contributions this could be related to an increase in aridity as suggested by the ecological and sedimentological changes at that time a reversed trend in sr and nd isotopic compositions is observed at the plio pleistocene transition that is likely related to higher precipitation and the development of glaciers in the himalaya these almost synchronous moderate changes in erosion pattern and climate changes during the late miocene and at the plio pleistocene transition support the notion of a dominant control of climate on himalayan erosion during this time period however stable erosion regime during the pleistocene is suggestive of a limited influence of the glacier development on himalayan erosion

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

paleomagnetic evidence for post jurassic stability of southeastern mexico maya terrane the tectonic evolution of southeastern mexico has been a subject of major controversy not only in regard to past geometry but also in the timing of proposed geological events as well for the past 10 years most if not all investigators agree that the gulf of mexico basin was formed by late jurassic time and that the maya terrane was in its current location prior to the cretaceous in order to gain further insight into the drift history of the maya terrane we have undertaken a paleomagnetic study of the uppermost jurassic lower cretaceous tithonian lower neocomian san ricardo formation in southeastern mexico at 93 7 w 16 8 n the sampling site is located east of the isthmus of tehuantepec on the southwest side of the maya block at the base of the yucatan peninsula a suite of 133 samples was collected in stratigraphic succession from a 114 m thick sequence of red shales and sandstones near cintalapa chiapas mexico after progressive thermal demagnetization of all samples at six steps from 350 c to 630 c 89 samples were selected for final paleopole analysis on the basis of their magnetic stability four different polarity intervals were observed the sequence being from bottom to top n r n r which assists in the assessment of the reliability of the observations the mean pole position obtained 160 0 e 69 8 n agrees with the mean pole position of the upper part of the morrison formation of colorado a unit of virtually identical age these results indicate that no discernible rotation or displacement of the maya block has occurred since at least early neocomian times

paleomagnetism of middle proterozoic diabase sheets from central arizona paleomagnetic results from 1090 ma diabase sheets of the southwestern u s a diabase province exposed in central arizona yield two distinct remanent magnetizations herein termed adn and adr in accordance with the findings of previous investigations magnetization adn is well defined and has an in situ mean direction of d 283 3 i 45 1 k 17 7 95 8 7 n 17 independent observations a mean pole after correction of paleomagnetic site means for a net 5 clockwise rotation of the colorado plateau and transition zone is located at 22 7 n 179 3 e k 21 9 a95 7 8 the second magnetization adr gives an in situ mean direction of d 161 1 i 87 5 k 22 2 95 19 9 n 4 independent observations with a poorly defined pole at 37 6 n 247 6 e k 6 5 a95 38 9 rock magnetic and alternating field and thermal demagnetization characteristics indicate the adn and adr magnetizations are both carried by low ti titanomagnetite both magnetizations are interpreted to be primary

paleomagnetism of a polarity transition in the lower triassic chug water formation wyoming a total of 156 oriented specimens taken at stratigraphic intervals of 3 to 20 cm have been collected from the red peak member of the chugwater formation at three localities approximately 25 km southeast of dubois wyoming the stratigraphic interval containing the polarity transition in the remanent magnetic field is about 1 0 m all specimens were thermally demagnetized at 500 550 620 and 660 c and show directions of magnetization of normal reversed and transitional character as well as a remarkable decrease of the remanent intensity in the transition zone the section sampled is divided into five intensity periods two periods recording the dipolar field one recording a dipolar field of decreasing intensity one recording a dipolar field of increasing intensity and a period of low intensity recording the field present during the 180 directional swing a virtual geomagnetic pole vgp path obtained for a n r polarity transition makes a fairly smooth transit with the path confined to a sector of longitude between 85 w and 110 e the reversal s onset is first indicated by a slow movement of the vgp across northern latitudes the vgp then lingers to two positions 60 n and 40 n 35 e to 70 e and 20 n and 40 n 335 e to 5 e suggesting brief periods of field stability or nondipolar field dominance the onset of the transition appears to be more rapid than the recovery the vgp path observed at a northern hemisphere site is located about 90 from the site the pole position derived from stable normal and reversed portions of this study 115 5 e 46 4 n 95 4 1 dp 2 2 dm 4 2 lies approximately in the center of the main group of poles reported for other triassic units

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

identifying the sedimentary imprint of high frequency holocene river floods in lake sediments development and application of a new method abstract in an effort to improve the understanding of floods and how they relate to climate we have developed a method that objectively identifies the sedimentary imprint of individual river floods in downstream lake basins the method is applied to a high resolution lake sediment core retrieved from southern norway resulting in a detailed record of holocene river flood activity covering the last 10 000 years including floods also recorded by instrumental and historical data the minimum number of individual floods recorded for this period is 100 on multidecadal to centennial timescales significant change in flood frequency is observed that arguably can be attributed to large scale climatic changes such as the varying amount of winter precipitation and summer rainstorms flood frequency was low during the early holocene 9770 7700 cal yr bp and even lower for the period that followed lasting until 5500 cal yr bp for the next 2500 years a modest increase in flood activity followed this trend was truncated 2500 cal yr bp by a sudden shift towards increased flooding frequency with the exception of a short interval around 1000 cal yr bp when the number of floods again was low this tendency of increased flood activity prevailed until present including stor ofsen a large flood that occurred in ad 1789 and also three other historically documented river floods

plate motions crustal and lithospheric mobility and paleomagnetism prospective viewpoint the shift in viewpoint from a static to a mobilistic solid earth as brought about by paleomagnetism opened many new directions of research that continue to be exciting today and hold prospects for an exciting future here five topics of current research on earth s mobilistic surface are reviewed and their future directions discussed first dividing earth s solid surface into two domains 1 nearly rigid plate interiors and 2 plate boundaries some of them very wide leads to a review and discussion of the kinematics of wide plate boundaries many more adjustable parameters will be needed to describe the complex kinematics of plate boundaries which cover 15 of earth surface than the few dozen parameters needed to describe the kinematics of plate interiors which cover 85 of earth s surface second the question of the degree of rigidity of plate interiors is discussed the integral of the velocity gradient across plates interiors is at least a few hundredths or tenths of millimeters per year and in several well determined cases less than 2 or 3 mm yr future investigations will no doubt be aimed at narrowing these limits and to expanding the area of earth s surface over which limits are known third space geodetic data mainly from very long baseline inter ferometry satellite laser ranging and the global positioning system have demonstrated a remarkable similarity of velocities of stable plate interiors averaged over a few to a dozen years with plate velocities averaged over a few million years already however significant differences of a few millimeters per year are emerging and the tectonic and dynamic significance of these differences need to be evaluated fourth despite more than two decades of investigation the question of how fast hotspots move relative to one another is still a contentious issue one group of researchers maintains that maximum speeds are 3 mm yr whereas another maintains that speeds are 10 20 mm yr or more the differences cannot be reconciled until the uncertainties in plate reconstructions relative to hotspots are systematically and properly incorporated into analyses of hotspot motion fifth paleomagnetists have estimated true polar wander over the past 200 myr by equating it with the apparent polar wander of hotspots however the possible motion among hotspots and the neglect of the uncertainties in reconstructions relative to the hotspots have left these analyses unconvincing further progress requires the incorporation of these uncertainties a possible exception is the apparent polar wander of the hotspots over the past 10 20 myr or less which requires only small adjustments for plate motion and has presumably smaller uncertainties due to plate reconstructions the cause of this apparently significant geologically recent shift of the pole is poorly understood compared with the rate and direction of the secular shift of the pole observed this century from the international latitude service and for 1976 to 1994 from space geodetic data the paleomagnetically observed shift is unlikely to be due to the removal of northern hemisphere midlatitude ice sheets which is the commonly accepted explanation for the shift observed over the past century i speculate instead that the apparently rapid uplift of the tibetan plateau sometime during the past 10 20 myr is at least partly responsible for the geologically recent shift of the pole finally i conclude that the theory of plate tectonics has provided a framework that leads naturally to further quantification of the kinematics and deformation of earth s solid surface chiefly because of the key assumption of the rigidity of plate interiors which permits specific predictions to be made this revolution in quantification still has far to go and holds exciting prospects for future tectonic studies

fracture network of the ferron sandstone member of the mancos shale east central utah usa abstract the fracture network at the outcrop of the ferron sandstone member of the mancos shale was studied to gain an understanding of the tectonic history of the region and to contribute data to studies of gas and water transmissivity related to the occurrence and production of coal bed methane about 1900 fracture readings were made at 40 coal outcrops and 62 sandstone outcrops in the area from willow springs wash in the south to farnham dome in the north of the study area in east central utah two sets of regional vertical to nearly vertical systematic face cleats were identified in ferron coals a northwest striking set trends at a mean azimuth of 321 and a northeast striking set has a mean azimuth of 55 cleats were observed in all coal outcrops examined and are closely spaced and commonly coated with thin films of iron oxide two sets of regional systematic joint sets in sandstone were also identified and have mean azimuths of 321 and 34 the joints of each set are planar long and extend vertically to nearly vertically through multiple beds the northeast striking set is more prevalent than the northwest striking set in some places joints of the northeast striking set occur in closely spaced clusters or joint zones flanked by unjointed rock both sets are mineralized with iron oxide and calcite and the northwest striking set is commonly tightly cemented which allowed the northeast striking set to propagate across it all cleats and joints of these sets are interpreted as opening mode mode i fractures abutting relations indicate that the northwest striking cleats and joints formed first and were later overprinted by the northeast striking cleats and joints burial curves constructed for the ferron indicate rapid initial burial after deposition the ferron reached a depth of 3000 ft 1000 m within 5 2 million years m y and this is considered a minimum depth and time for development of cleats and joints the sevier orogeny produced southeast directed compressional stress at this time and is thought to be the likely mechanism for the northwest striking systematic cleats and joints the onset of the laramide orogeny occurred at about 75 ma within 13 7 m y of burial and is thought to be the probable mechanism for development of the northeast striking systematic cleats and joints uplift of the ferron in the late tertiary contributed to development of butt cleats and secondary cross joints and probably enhanced previously formed fracture sets using a study of the younger blackhawk formation as an analogy the fracture pattern of the ferron in the subsurface is probably similar to that at the surface at least as far west as the paradise fault and joe s valley graben farther to the west on the wasatch plateau the orientations of ferron fractures may diverge from those measured at the outcrop