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

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

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 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

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 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

new early jurassic paleopole from france and jurassic apparent polar wander abstract a new early jurassic paleopole for stable europe obtained from a doleritic dyke system in brittany france is given at latitude 61 n and longitude 79 e k 36 anda95 10 2 based on seven equivalent sites and more than a hundred samples demagnetized by alternating field and thermal treatments d 41 i 62 k 66 95 7 5 radiometric datings provide ages between 190 and 200 ma this result represents the first indication obtained from european igneous data for a very westerly cusp in the apparent polar wander path around 200 ma the european data also tend to support the hypothesis of a higher latitude track for the jurassic apwp as already proposed from north american data

tufted microbial cyanobacterial mats from the proterozoic stoer group scotland upward thinning stellate and polygonal carbonate structures are described and compared to modern and ancient stromatolites although tufted microbial mats are common in modern settings they have rarely been described from ancient rocks owing to their poor preservation potential the preservation of these from the stoer group is due to early replacement by calcite before their original tufted relief could be obliterated by compaction associated limestones with some similar features are interpreted as flat to mamillated microbial mats

remagnetization of jurassic volcanic rocks in the santa rita and patagonia mountains arizona implications for north american apparent polar wander paleomagnetic poles for the jurassic corral canyon sequence and glance conglomerate in southern arizona have been used to construct apparent polar wander apw paths for the north american plate but they are controversial and conflict with higher latitude poles from new england lower jurassic dacites and ash flow tuffs of the mount wrightson formation in the santa rita mountains were initially sampled to provide an additional paleopole for southern arizona these rocks however have a predominantly reversed polarity characteristic magnetization in situ i 47 d 154 95 9 which is statistically indistinguishable from that for the nearby latest cretaceous elephant head pluton i 48 d 165 95 8 although magnetizations of both polarities are observed in the ash flow tuffs they are mostly carried by hematite and dual polarity components are observed within some specimens moreover widespread mineralization and a k ar age of 67 ma for altered rocks of the mount wrightson formation imply that these rocks were subjected to a prolonged episode greater than one polarity interval of low temperature alteration and remagnetization middle jurassic ash flow tuffs of the corral canyon sequence in the patagonia mountains to the south have a similar geologic setting they are exposed along strike amid mineralized rocks on the eastern flank of a mountain range cored by paleocene intrusions hematite is also the dominant remanence carrier in most of the corral canyon sequence and its predominantly normal polarity direction in situ i 51 d 326 95 9 is indistinguishable from that for the nearby patagonia granodiorite i 49 d 342 95 8 rocks of the corral canyon sequence therefore are likely remagnetized as well problems also exist with the glance conglomerate pole these rocks are situated within a caldera structure and have been potassium metasomatized this potassic alteration could have occurred shortly after emplacement or at a later time postdeformation the low latitude jurassic apw path for north america and j 2 cusp therefore are not well supported and may need revision

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

rotation of the colorado plateau an analysis of paleomagnetic data hamilton 1981 has proposed that the colorado plateau has rotated clockwise relative to cratonic north america by a total of 5 7 since mid cretaceous time a rotation this small is unresolvable by most paleomagnetic studies in this paper we present a new method for simultaneously comparing all of the available paleomagnetic data from on and off the plateau in order to estimate rotation of the colorado plateau our new method examines the effect of various hypothetical plateau rotations on the dispersion of data fit to a smooth model of apparent polar wander a maximum likelihood criterion is used to estimate the amount of rotation confidence limits are found using an f ratio test assuming the plateau craton euler pole to be located at 37 n 103 w we applied this method to north american paleomagnetic data of carboniferous to cretaceous age the best paleomagnetic estimate of the rotation is 3 9 clockwise with a 95 confidence interval of 1 4 6 6 clockwise in good agreement with geologic estimates of rotation because of the dependence of published north american apparent polar wander paths on data from the colorado plateau the existence of such a rotation requires a minor revision in the apparent polar wander path the largest change being a 3 9 eastward shift of the cusp connecting the carboniferous permian triassic and jurassic cretaceous tracks

geological modeling of the new champ magnetic anomaly maps using a geographical information system technique 1 reliable global crustal field anomaly maps produced from magnetic measurements of the champ satellite mission now allow for quantitative geological studies of crustal structure and composition we have developed a gis based forward modeling technique to model these anomaly maps on the basis of the geologic and tectonic maps of the world laboratory susceptibility values of the occurring rock types and the seismic thickness of the crust a vertically integrated susceptibility grid is generated in the gis system in addition a remanent magnetization grid is computed for the oceanic crust using a digital isochron map of the ocean floor and rotation models of the paleoplates combining the global vis and remanent magnetization grids the vertical magnetic field anomaly is computed at satellite altitude and compared with the corresponding champ magnetic anomaly map over the oceans induced and remanent magnetization explains well the prominent observed anomalies over the cretaceous quiet zones we also find a good agreement between predicted and observed anomalies over the continents remaining discrepancies between the predicted and observed anomalies can be used to adjust poorly known boundaries and the composition of the buried precambrian provinces until the recomputed anomalies fit the observed anomalies the feasibility of this approach is demonstrated on greenland the west african craton bangui in central africa and the kolyma omolon block in siberia we conclude that quantitative information on the lateral extent the composition and the thickness of the lower crust within a precambrian province can thus be inferred from the new satellite magnetic anomaly maps

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

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

anomalous mesozoic thermal regime central appalachian piedmont evidence from sphene and zircon fission track dating a total of 17 sphene and 45 zircon fission track ages fta are reported from the piedmont province of pennsylvania delaware and maryland the reading prong of new jersey and the newark basin of pennsylvania with the exception of one sphene sample fta fall within a relatively restricted age range sphene 190 217 ma average 199 ma and zircon 143 218 ma average 184 ma near concordance of fta over a large tract of crystalline basement sim20 000 km 2 and within upper triassic sediments in the newark basin indicates late triassic early jurassic total resetting of the fission track clocks followed by regional cooling between sphene sim275 pm 25 circ c to zircon sim 220 pm 40 circ c closure temperatures at rates of 4 8 c ma fourteen rb sr whole rock mica ages together with previously reported 40 ar 39 ar biotite and hornblende ages generally indicate no regional mesozoic resetting thus the maximum temperature experienced by the samples was 300 c sphene fta clo

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

paleomagnetism of early jurassic rocks watchung mountains newark basin evidence for complex rotations along the border fault a paleomagnetic study of late triassic to early jurassic rocks from the watchung syncline provides useful new constraints on the complex development of the newark basin independent of existing structural data a baked contact test indicates stable remanent magnetizations are primary results from the eastern limb of the fold are used to calculate an early jurassic pole at long 115 1 e lat 62 2 n alpha 95 10 5 k 41 4 which is similar to previous newark basin results after results from the western limb were corrected for folding 32 of clockwise discordance remained between data from the western and eastern limbs this cryptic rotation of newark basin strata appears to be localized to the border fault zone and may be due to the presence of left lateral secondary brittle shear zones running parallel to strata of the western limb of the watchung syncline

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement