tracking the late jurassic apparent or true polar shift in u pb dated kimberlites from cratonic north america superior province of canada different versions of a composite apparent polar wander apw path of variably selected global poles assembled and averaged in north american coordinates using plate reconstructions show either a smooth progression or a large 30 gap in mean paleopoles in the late jurassic between about 160 and 145 ma in an effort to further examine this issue we sampled accessible outcrops subcrops of kimberlites associated with high precision u pb perovskite ages in the timiskaming area of ontario canada the 154 9 1 1 ma peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5 of the reverse polarity magnetization of the 157 5 1 2 ma triple b kimberlite the combined 156 ma triple b and peddie pole 75 5 n 189 5 e a95 2 8 lies about midway between igneous poles from north america nearest in age 169 ma moat volcanics and the 146 ma ithaca kimberlites showing that the polar motion was at a relatively steady yet rapid 1 5 myr pace a similar large rapid polar swing has been recognized in the middle to late jurassic apw path for adria africa and iran eurasia suggesting a major mass redistribution one possibility is that slab breakoff and subduction reversal along the western margin of the americas triggered an episode of true polar wander

the drift history of adria and africa from 280 ma to present jurassic true polar wander and zonal climate control on tethyan sedimentary facies article i nfo the orogenic belts surrounding the undeformed adriatic sea represent the margins of an area known as adria the african promontory we have undertaken a critical appraisal of paleomagnetic data from regions of adria considered parautochthonous relative to africa obtained either from biostratigraphically dated sed imentary rocks corrected for inclination shallowing or from igneous rocks that are regarded as free from any inclination shallowing bias paleomagnetic directions were used to calculate paleomagnetic poles for compar ison with coeval and inclination flattening free paleomagnetic poles from stable africa visual coherence of paleopoles for several time slices from the early permian to the eocene supports the construction of a com posite apparent polar wander path apwp valid for parautochthonous adria and stable africa this compos ite apwp is compared to previous apwps finding good agreement with the global apwp of kent and irving 2010 both apwps show a remarkable and rapid polar shift of 40 in the jurassic that other apwps tend to underestimate we interpret this shift to represent a major episode of true polar wander tpw from 183 ma in the early jurassic to 151 ma in the late jurassic using a simple zonal climate model the drift motion of adria attached to africa appears to be consistent with the distribution of permian cretaceous sed imentary facies on adria

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

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

reply to comment by robert f butler steven r may and david r bazard on high latitude paleomagnetic poles from middle jurassic plutons and moat volcanics in new england

mesozoic alpine facies deposition as a result of past latitudinal plate motion the fragmentation of pangaea as a consequence of the opening of the atlantic ocean is documented in the alpine mediterranean region by the onset of widespread pelagic sedimentation1 shallow water sediments were replaced by mainly pelagic limestones in the early jurassic period radiolarian cherts in the middle late jurassic period and again pelagic limestones in the late jurassic cretaceous period1 during initial extension basin subsidence below the carbonate compensation depth ccd is thought to have triggered the transition from early jurassic limestones to middle late jurassic radiolarites1 it has been proposed that the transition from radiolarites to limestones in the late jurassic period was due to an increase in calcareous nannoplankton abundance when the ccd was depressed below the ocean floor1 but in modern oceans sediments below the ccd are not necessarily radiolaritic here we present palaeomagnetic samples from the jurassic cretaceous pelagic succession exposed in the lombardian basin italy on the basis of an analysis of our palaeolatitudinal data in a broader palaeogeographic context we propose an alternative explanation for the above facies tripartition we suggest that the lombardian basin drifted initially towards and subsequently away from a near equatorial upwelling zone of high biosiliceous productivity our tectonic model for the genesis of radiolarites adds an essential horizontal plate motion component to explanations involving only vertical variations of ccd relative to the ocean floor it may explain the deposition of radiolarites throughout the mediterranean and middle eastern region during the jurassic period

a record of the jurassic massive plate shift from the garedu formation of central iran modern generations of apparent polar wander paths apwps show the occurrence in north american and african coordinates of a major and rapid shift in pole position plate shift during the middle to late jurassic 175 145 ma that alternative curves from the literature tend to underestimate this jurassic massive polar shift jmps of vast and as yet unexplored paleogeographic implications is also predicted for eurasia from the north atlantic plate circuit but jurassic data from this continent are scanty and problematic here we present paleomagnetic data from the kimmeridgian tithonian upper jurassic garedu formation of iran which was part of eurasia since the triassic paleomagnetic component directions of primary pre folding age indicate a paleolatitude of deposition that is in excellent agreement with the latitude drop predicted for iran from apwps incorporating the jmps moreover we show that paleolatitudes calculated from these apwps used in conjunction with simple zonal climate belts better explain the overall stratigraphic evolution of iran during the mesozoic as iran drifted from the tropical arid belt to the mid latitude humid belt in the late triassic carbonate platform productivity stopped while widespread coal bearing sedimentation started whereas as iran returned to arid tropical latitudes during the jmps carbonate platform productivity and evaporitic sedimentation resumed these results illustrate 1 the potent but often neglected control that plate motion continental drift and or true polar wander across zonal climate belts exerts on the genesis of sedimentary facies and 2 the importance of precisely controlled paleogeographic reconstructions for tectonic interpretations especially during times of fast plate motion like the jurassic as a suggestion for future research we predict that the adoption of eurasian reference paleopoles incorporating the jmps may lead to a reconciliation or reinterpretation of existing geologic and paleomagnetic data regarding the deformation history of central asia

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

large scale motion between pacific and atlantic hotspots studies of true polar wander tpw the rotation of the solid earth with respect to the spin axis1 have suggested that there has been 10 15 of relative motion over the past 130 myr refs 2 4 in such studies the orientation of the spin axis is recovered from continental palaeomagnetic poles corrected for relative plate motions and compared with a deep mantle reference frame defined by hotspot locations but deducing relative plate motions becomes increasingly difficult for older mesozoic time periods hindering tests of tpw on timescales comparable to those of large scale mantle convection moreover the assumption of hotspot fixity is controversial5 6 we examine here a more direct approach7 8 using palaeolatitudes derived from pacific guyots contrary to predictions from tpw models these data suggest only minor latitudinal shifts of pacific hotspots during the cretaceous period instead of tpw relative motion between the atlantic and pacific hotspot groups9 is required at a velocity of approximately 30 mm yr 1 more than 50 larger than previously proposed5

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

linearity spectrum analysis of multi component magnetizations and its application to some igneous rocks from south eastern australia summary a method that enables the objective resolution of almost parallel multi component magnetizations is described and demonstrated a feature distinguishing this method from others is its simultaneous analysis of demagnetization data from a group of specimens rather than the analysis of data from one specimen at a time the only prerequisite is that the specimens are derived from a homogeneous source thus for a formation carrying a simple single component magnetization all specimens from the formation may be simultaneously reduced for a more complicated two component magnetization it is shown that only specimens from a particular site can be considered homogeneous and for a complex three component system each sample often requires undivided attention thus the workload is proportionally increased to achieve analyses of comparable reliability from data of variable quality new pole positions from mesozoic intrusions of the sydney basin nsw are from the marsden park breccia pipe 48 s 127 e a95 6 the st marys breccia pipe 46 s 150 e a95 8 the prospect dolerite 60 s 142 e a95 13 and 53 s 180 e a95 6 and from the dundas breccia pipe 58 s 162 e a95 36 and 31 s 195 e a95 16 the last two formations possess multi component magnetizations these pole positions are consistent with previous results from south eastern australia

baked contact test demonstrates primary nature of dominant n1 magnetisation of nipissing intrusions in southern province canadian shield abstract nipissing diabase sills cone sheets and dykes define a major episode of igneous intrusion in the southern province of the canadian shield at about 2200 ma the dominant n1 remanence of these intrusions has been established in several palaeomagnetic studies and hence is a key tie point for the north american apparent polar wander path yet the age of this magnetisation remains very controversial positive baked contact tests reported for the n1 remanence have often been dismissed as inconclusive and the n1 palaeopole attributed to overprinting during the hudsonian orogeny 1900 1800 ma due to its position on a widely accepted polar wander path for north america previous baked contact studies utilise sediments of the huronian supergroup as host rock the remanence of these sediments is often difficult to interpret in this study a positive baked contact test is described for an n1 nipissing sill or cone sheet using stably magnetized mafic dykes of the matachewan swarm as host thus the n1 remanence is primary and no longer supports the time calibration of polar wander paths which assume that it is a 1900 1800 ma overprint

palaeomagnetic study of the el quemado complex and marifil formation patagonian jurassic igneous province argentina summary the upper jurassic el quemado complex was sampled at 36 sites from five localities in the cordilleran foothills of southern patagonia between lago argentino and lago posadas sierra colorada and the middle jurassic marifil formation at 12 sites in the somuncura massif near camarones the main lithology was ignimbrite with minor tuff and lava petrographical and sem observation show that the el quemado rocks suffered an intense high temperature alteration which resulted in transformation of most primary ti magnetite in pseudobrookite rutile and minor ti haematite and fe hydrated oxides a similar less pronounced alteration occurred in the marifil rocks 40ar 39ar dating of el quemado was possible for one sample from sierra colorada and yielded an age of 156 5 1 9 ma magnetic mineralogy measurements isothermal remanence hysteresis loop curie balance show that the remanent magnetization is dominated by psd low ti magnetite often associated to a minor high coercive mineral haematite secondary magnetization components are usually absent or weak at el quemado sites strong at marifil they were completely erased by thermal and af demagnetization and a characteristic remanence chrm stable up to temperatures higher than 550 c or peak field values of 100 mt was successfully isolated the virtual geomagnetic pole vgp from the marifil formation 83 s 138 e is in agreement with the literature data for jurassic rocks from stable south america the el quemado vgps fall in two groups the localities to the north of latitude 48 s lago posadas sierra colorada yield a vgp 81 s 172 e close to that of marifil whereas those south of latitude 49 s lago san martin lago argentino show a highly elongated vgp distribution consistent with counter clockwise block rotation about vertical axes these rotations were likely caused by thrust sheets which were rotating counter clockwise at the same time they were advancing towards the foreland the amount of rotation varies according to the location of the sampling sites in the thrust and fold belt

experimental determination of trace element partitioning between pargasite and a synthetic hydrous andesitic melt in order to more fully establish a basis for quantifying the role of amphibole in trace element fractionation processes we have measured pargasite silicate melt partitioning of a variety of trace elements rb ba nb ta hf zr ce nd sm yb including the first published values for u th and pb experiments conducted at 1000 c and 1 5 gpa yielded large crystals free of compositional zoning partition coefficients were found to be constant at total concentrations ranging from 1 to 100 ppm indicating henry s law is oparative over this interval comparison of partition coefficients measured in this study with previous determinations yields good agreement for similar compositions at comparable pressure and temperature the compatibility of u th and pb in amphibole decreases in the order pb th u partial melting or fractional crystallization of amphibole bearing assemblages will therefore result in the generation of excesses in238u activity relative to230th similar in magnitude to that produced by clinopyroxene the compatibility of pb in amphibole relative to u or th indicates that melt generation in the presence of residual amphibole will result in the long term enrichment in pb relative to u or th in the residue this process is therefore incapable of producing the depletion in pb relative to u or th inferred from the pb isotopic composition of morb and oib comparison of partition coefficients measured in this study with previous values for clinopyroxene allows some distinction to be made between expected trace element fractionations produced during dry cpx present and wet cpx amphibole present melting rb ba nb and ta are dramatically less compatible in clinopyroxene than in amphibole whereas th u hf and zr have similar compatibilities in both phases interelement fractionations such as dnb dba are also different for clinopyroxene and amphibole changes in certain ratios such as ba nb ba th and nb th within comagmatic suites may therefore offer a means to discern the loss of amphibole from the melting assemblage elastic strain theory is applied to the partitioning data after the approaches of beattie and blundy and wood and is used to predict amphibole melt partition coefficients at conditions of p t and composition other than those employed in this study given values of dca dti and dk from previous partitioning studies this approach yields amphibole melt trace element partition coefficients that reproduce measured values from the literature to within 40 45 this degree of reproducibility is considered reasonable given that model parameters are derived from partitioning relations involving iron and potassium free amphibole

magnetostratigraphy nannofossil stratigraphy and apparent polar wander for adria africa in the jurassic cretaceous boundary interval article i nfo existing correlations of early cretaceous nannofossil events to polarity zones in italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages and hence calibration of the early cretaceous geomagnetic polarity timescale here we present correlations of nannofossil events to polarity chrons for the late jurassic and jurassic cretaceous boundary interval from the southern alps making the case for revisions of current geologic timescales in this interval the jurassic cretaceous and kimmeridgian tithonian boundaries can be usefully defined at the onsets of polarity chrons cm18r and cm22r respectively the oxfordian kimmeridgian boundary can be assigned to cm25 an apparent polar wander path apwp for the southern alps derived from and time calibrated by bio magnetostratigraphic data is similar to and provides a proxy for the african apwp in the absence of adequate african data of this age the southern alpine apwp is characterized by a cusp hairpin at 30 n 270 e at a time close to the oxfordian kimmeridgian boundary polarity chron cm25 154 ma rapid apwp from high latitude n60 n in the preceding middle jurassic bajocian oxfordian interval coincident with the onset of central atlantic ligurian oceanic rifting and a return to high latitudes during the cretaceous early jurassic poles at high latitudes n60 n are evident for sinemurian to toarcian aalenian time 175 195 ma an apwp for this time interval with this level of temporal calibration cannot be constructed using data from africa itself or from other atlantic bordering continents by rotation to african coordinates

theoretical constraints on true polar wander 1 for the present geologic epoch true polar wander tpw is relatively small but simple theoretical considerations suggest that it could have been larger in other epochs in this work we use scaling arguments to assess the qualitative behavior of tpw and a simple maxwell model to analytically describe how changes in mass anomalies translate into tpw unlike previous work we derive simple analytical estimates of tpw based on the characteristic amplitudes and timescales for changes in the moment of inertia we find estimates for both the amplitude and speed of tpw as a function of earth properties the following four main factors influence how large the maximum tpw can be the geological timescale over which the desired tpw occurs tpw the viscosity structure of the mantle which yields a weighted average viscosity the characteristic amplitude of the nonhydrostatic changes in the moment of inertia c and the added moment of inertia due to the equatorial bulge c a for the nominal values 3 1022 pa s and c c a 0 003 the maximum tpw is 61 over 100 myr and 8 over 10 myr the maximum tpw speed is only sensitive to c and c a and is 2 4 myr 1 for the nominal values tpw is shown to act as a low pass filter rapid changes in moment of inertia produce smaller and delayed tpw a consequence of this is that inertial interchange tpw does not have a different character than tpw tpw can have an important contribution to plate motions over relatively long timescales but not over shorter timescales our simple approach allows us to assess whether multiple tpw events are possible but the major uncertainty continues to be the mantle viscosity structure

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

ice age earth rotation modern predictions of the rotational stability of an ice age earth reflect a convergence of two classic problems in geophysical analysis the modeling of the glacial isostatic adjustment gia process and the rotational stability of terrestrial planets recent theoretical advances in this area have been motivated not by conventional applications such as the inference of earth s deep mantle viscosity but rather by efforts to address vexing problems in global climate change research these advances have demonstrated that traditional calculations of the ongoing motion of the rotation pole relative to the surface geography or true polar wander tpw driven by ice age loading have systematically overestimated this motion by up to a factor of 4 by underestimating by 1 the background flattening of earth s oblate form the physics of this sensitivity is related to concepts that appear in canonical mid twentieth century discussions of earth rotation and avoiding the associated inaccuracy resolves numerous p

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

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

comment on high latitude paleomagnetic poles from middle jurassic plutons and moat volcanics in new england and the controversy regarding jurassic apparent polar wander for north america by mickey c van fossen and dennis v kent

a guide to phanerozoic cold polar climates from high latitude ice rafting in the cretaceous the high latitude extent of warm climate indicators at certain times in earth history has been considered as evidence that the globe was ice free for long intervals despite theoretical considerations and results from numerical modelling experiments1 2 indicating that this was unlikely one of the warmest periods the cretaceous displays faunal and floral evidence for cool temperate to sub tropical conditions very near to the poles however our studies of lower cretaceous mudstones of central australia that contain outsized exotic blocks have led to the conclusion that the blocks were emplaced by ice rafting implying that high latitude ice was present at sea level strata of a similar origin of mid jurassic to mid cretaceous age occur on other continents that were positioned between 65 and 78 palaeolatitude indeed there is a record of high latitude ice rafting throughout the phanerozoic suggesting that ice was present on earth for much of its history and that ice free conditions could have been at most only episodic over the past 600 myr

a paleomagnetic pole position of late jurassic early cretaceous dolerites from hinlopenstretet svalbard and its tectonic implications abstract paleomagnetic examination of late jurassic early cretaceous dolerites hinlopenstretet reveal a mean paleofield direction d 181 i 71 95 4 2 k 117 9 17 sites 96 samples which is argued to represent a deuteric magnetization formed at the time of cooling the resultant paleopole 66 n 200 e dp 7 dm 8 accords with paleomagnetic poles of similar age for north america after taking into account post jurassic continental separation the result from the present study is interpreted to represent a late jurassic early cretaceous paleopole for eurasia the similarity of the paleopoles from cratonic north america and eurasia in its pre drift configuration at about 145 ma makes it reasonable to conclude that relative displacement due to extension between eurasia and north america prior to the onset of sea floor spreading must have taken place after 145 ma and before the start of sea floor creation in the cretaceous

astrometric and space geodetic observations of polar wander optical astrometric measurements of star positions taken during the past century have been recently re reduced using the final hipparcos star catalog in order to determine variations in the mean location of the earth s rotation pole with respect to the earth s crust this newly available polar motion series which is the longest homogeneous polar motion series currently available allows the drift in the pole path to be newly estimated during the 1900 0 to 1992 0 span of the smoothed hipparcos polar motion series the earth s rotation pole is observed to drift at a mean linear rate of 3 51 0 01 milliarcseconds year mas yr towards 79 2 0 2 w longitude this new estimate for the observed trend in the pole path which can be considered to be the present day expression of true polar wander is nearly the same as that estimated in previous studies using the homogeneous ils polar motion series

a palaeomagnetic study of 143 ma kimberlite dikes in central new york state summary a palaeomagnetic study of 143 ma serpentinized kimberlite dikes near ithaca new york yields a dual polarity high unblocking temperature and high coercivity magnetization which passes the reversal test and two baked contact tests the mean pole position 58 n 203 e a95 3 8 n 7 differs from published late jurassicearly cretaceous north american poles currently used to define the apparent polar wander path the angular dispersion in mean directions 3 5 is low but the presence of reversals argues that the ithaca kimberlites magnetization should represent sufficient time for averaging of palaeosecular variation similar findings apparently typify palaeomagnetic studies of other serpentinized kimberlite supporting the suggestion that thermo chemical remanent magnetization in this lithology prolongs the duration of magnetization acquisition sufficiently to average secular variation per dike a consistent but weak foliation in anisotropy of arm parallels the n s and vertical orientation of the ithaca dikes but is apparently unrelated to the northwest down or southeast up remanence the ithaca kimberlites pole may therefore record a previously undocumented sharp bend or cusp at 143ma and the initiation of a cretaceous and cenozoic interval of apparent polar wander that generally follows a great circle along the 200 e meridian to geographic north a coeval 145 ma kimberlite pole from southern africa transferred to north america agrees with the ithaca kimberlites pole position whereas reported poles from the berriasian stratotype southern france and 144ma svalbard dolerites provide less diagnostic tests of the ithaca kimberlites pole due in part to uncertainties in europe north america reconstructions

paleomagnetism of mesozoic kimberlites in southern africa and the cretaceous apparent polar wander curve for africa including those previously published good paleomagnetic data have been obtained from 48 of 51 sites in 28 separate kimberlites radiometric dates by one or more methods are available for 17 of these bodies and range from 81 to 145 ma with maxima around 90 and 120 ma these ages correlate with geochemical contrasts distinguishing what have been designated group i younger and ii older kimberlites smith et al 1985b stable magnetization appears to be associated with magnetite developed deuterically after emplacement but in most bodies secular variation has not been completely averaged in the remanence acquired the distribution of vgps calculated is distinctly but not sequentially streaked yet correlates broadly with the older and younger petrochemical types mean north poles calculated on this basis yield kp1 n 14 81 100 ma 64 1 n 226 1 e a95 5 2 and kp2 n 10 113 145 ma 47 6 n 269 9 e a95 9 7 with the exception of one undated pipe meltonwold which was not included in the averages and the melilititic male fissure at swartruggens in group ii average all the kimberlites sampled are normally magnetized including these new data 10 of 17 published african cretaceous poles meet acceptability criteria specified and define an apparent polar wander apw curve from 130 to 75 ma the curve appears to consist of two segments trending latitudinally before 100 ma and meridionally thereafter the latter trend is evident even in the distribution of the individual kp1 poles and may be related to the true polar wander speculated by gordon and livermore 1987 to have occurred in that interval

indian and african plate motions driven by the push force of the reunion plume head mantle plumes are thought to play an important part in the earth s tectonics yet it has been difficult to isolate the effect that plumes have on plate motions here we analyse the plate motions involved in two apparently disparate events the unusually rapid motion of india between 67 and 52 million years ago and a contemporaneous transitory slowing of africa s motion and show that the events are coupled with the common element being the position of the indian and african plates relative to the location of the reunion plume head the synchroneity of these events suggests that they were both driven by the force of the reunion plume head the recognition of this plume force has substantial tectonic implications the speed up and slowdown of india the possible cessation of convergence between africa and eurasia in the palaeocene epoch and the enigmatic bends of the fracture zones on the southwest indian ridge can all be attributed to the reunion plume an analysis of the plate motions involved in two apparently disparate tectonic events the unusually rapid motion of india between 67 million and 52 million years ago and a transitory slowing of africa s motion at around the same time has uncovered a possible common cause for the two the reunion volcanic hotspot beneath the indian ocean has been active for about 67 million years and the close coupling of the motion of india and africa strongly suggests that it is the waxing and waning of the reunion plume head that is driving these motions

style of density stratification in the mantle and true polar wander induced by ice loading 1 the present day velocity of true polar wander tpw and the displacement of the axis of rotation of the earth in response to ice ages resulting from stratified viscoelastic earth models are sensitive to the nonadiabatic density gradient in the mantle previous studies based on a fully nonadiabatic or chemically stratified mantle overestimated the present day tpw for lower mantle viscosities in the range 1021 1022 pa s for a density profile in agreement with the reference seismological model where nonadiabaticity is confined to the transition zone between 420 and 670 km the predicted present day tpw for viscosities on the order of 1021 pa s is 0 65 0 9 deg myr substantially lower than the 3 0 deg myr obtained for the chemical mantle this decrease is due to the lack of an isostatic restoring force in the adiabatic case or to a global reduction of the buoyancy which favors the attainment of rotational equilibrium the correctness of this physical interpretation is demonstrated by the behavior of a fully adiabatic phase change that can be satisfactorily modeled by deleting the buoyancy restoring modes due to chemical density contrasts this finding provides quantitative support to the procedure used in previous studies to simulate the rotational behavior of an adiabatic transition zone by simply deleting the buoyancy modes triggered by the chemical density contrasts at 670 km as done by mitrovica and milne 1998 for present day tpw and by ricard and sabadini 1990 for long term tpw driven by mantle density anomalies the reduction of present day tpw induced by the pleistocenic deglaciation for an adiabatic mantle with nonadiabatic density gradients due to phase changes localized in the transition zone impacts the inversion of the lower mantle viscosity predictions based on such a model are characterized by two best fit values in proximity of 1021 and 1022 pa s resembling the behavior of the time derivative of the second degree component of the gravity field the reduction of predicted present day tpw due to pleistocene deglaciation suggests that other mechanisms such as present day ice mass instability in antarctica and greenland are presently contributing to the drift of 0 9 deg myr of the axis of rotation toward newfoundland the secular drift of the adiabatic mantle model during the continuous occurrence of ice ages is increased by 50 with respect to the chemically stratified one showing a longer decay time after termination of the ice cycles this mantle model confirms that ice cycles cannot be the major source of tpw for timescales of 106 107 years