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True polar wander: An analysis of Cenozoic and Mesozoic paleomagnetic poles
Abstract
It has been found that the geomagnetic poles deduced from worldwide paleomagnetic data deviate systematically from the present rotation axis. Studies conducted with the aim to provide explanations for this phenomenon, in most cases, did not take plate motions into account. This must be done if the analysis is to be extrapolated back in time beyond 7 Ma. If hotspots are assumed to be stationary in the mantle as well as with respect to each other, they offer a reference framework against which plate motions can be viewed. In the present investigation, a plate motion model incorporating such a fixed hotspot framework is employed to remove the effect of plate motion on the positions of Cenozoic and Mesozoic paleomagnetic poles. The paleopoles are grouped and averaged. The investigation indicates that the pole has moved 22 deg + or 10 deg in the past 180 m.y.
... The development of a reference frame based on the assumption that hotspots were fixed provided a basis for early tests. The inferred solid Earth motion was called "true polar wander" (TPW) (Duncan et al., 1972;Andrews, 1985), a term that has since become synonymous with polar wander, irrespective of the reference frame employed. ...
... For Mesozoic to recent times, some authors have proposed a longer term relatively slow TPW, at rates of <0.1 to 1.0 • /myr which, when viewed relative to today's spin axis, represent an offset of some 10-15 • over 120 million years . These efforts started with Andrews in 1985(Andrews, 1985 and were further advanced by Besse andCourtillot in 2002 (Besse andCourtillot, 2002). But a flaw of early studies was relying on a fixed hotspot frame of reference to detect TPW. ...
... For Mesozoic to recent times, some authors have proposed a longer term relatively slow TPW, at rates of <0.1 to 1.0 • /myr which, when viewed relative to today's spin axis, represent an offset of some 10-15 • over 120 million years . These efforts started with Andrews in 1985(Andrews, 1985 and were further advanced by Besse andCourtillot in 2002 (Besse andCourtillot, 2002). But a flaw of early studies was relying on a fixed hotspot frame of reference to detect TPW. ...
true polar wander apparent polar wander paleomagnetism mantle viscosity Knowledge of Earth's stability with respect to the spin axis sets boundary conditions for understanding the deep mantle and interpreting records of past climate. Recently it has been argued that the solid Earth rotated by 12 • , and then rotated back, 86 to 78 million years ago. Herein we reanalyze the paleomagnetic data and report new rock magnetic analyses, from the Scaglia Rossa limestone of Italy on which this true polar wander (TPW) was based. We find that these data record an unrecognized secondary magnetization carried by authigenic hematite. This overprint has a differential angular effect on the normal and reversed polarity primary magnetizations, creating biased directions. This bias, together with probable tectonic and sedimentary slide rotations, creates false polar wander signals. The recognition of these artifacts serves as a cautionary tale and indicates that the null hypothesis of no large TPW since the Late Cretaceous cannot be rejected. This stability is fundamentally different from that of other planetary bodies which have undergone large polar wander, and provides the framework for Earth's climate history. Because of uncertainties in paleolongitude and geomagnetic field morphology, the unambiguous definition of terrestrial TPW deeper in time remains elusive.
... True Polar Wander. Growing evidence suggests that the global set of hotspots has shifted relative to the earth's rotation axis (e.g., see Morgan, 1981;Jurdy, 1981;Gordon, 1983;Livermore and others, 1984;Andrews, 1985), a phenomenon sometimes described as true polar wander. The results obtained from the present analysis depend upon the global set of hotspots remaining at fixed distances from each other but not at fixed distances from the spin axis. ...
... The anomalies that bracket the quiet zone are more closely spaced to the north of the fracture zone (slower spreading velocity) than to the south (faster spreading velocity). The preferred model of Engebretson and others (1984a;1985), called here option NoM (North of Mendocino), uses the slower velocity determined from the north side, assuming that south of the Mendocino fracture zone the ridge jumped to the east, transferring the existing Farallon plate to the Pacific plate. An alternative option, SoM, (Engebretson and others, 1984a;1985) uses the faster velocity determined from the spacing of anomalies south of the Mendocino. ...
... The preferred model of Engebretson and others (1984a;1985), called here option NoM (North of Mendocino), uses the slower velocity determined from the north side, assuming that south of the Mendocino fracture zone the ridge jumped to the east, transferring the existing Farallon plate to the Pacific plate. An alternative option, SoM, (Engebretson and others, 1984a;1985) uses the faster velocity determined from the spacing of anomalies south of the Mendocino. Option SoM provides greater northward translation for terranes riding with the Farallon plate or driven by it along a continental margin during the interval 83 to 119 Ma. undergoing translation along the continental margin driven by oblique convergence of the Farallon or Kula plates, most of our modeling was based on the present continental margin as approximated (option CMA) by three small circles (Fig. 9). ...
In order to establish southern and western limits to possible points of origin of terranes, we calculated the routes or trajectories by which terranes were carried aross the Pacific basin and along the margin of North America. These trajectories were then tested for internal consistency with paleomagnetic results. Beginning with specific plate tectonic models that describe the motion of adjacent oceanic plates relative to North America, we found a set of terrane trajectories that show the position of terranes as a function of time as the terranes moved with the oceanic plates or were driven by them tangentially along the continental margin. Elements used to define a trajectory are (1) the stage poles describing the motion of the oceanic plates relative to the continent, (2) the sequence of plates carrying the terrane, (3) the time of docking of the terrane, and (4) the coordinates of the point of docking. Additional constraints are that terranes are not permitted to migrate across ridges and that the oceanic plate carrying a terrane cannot be younger than the terrane. The plate model of Engebretson and others (1985) and several of its variants were used in our analysis. For docking times of 30 Ma, trajectories are short because the Pacific-Farallon spreading system is close to the margin. For docking times of 60 Ma, terrane trajectories indicate northward transport by as much as 60° of latitude. For docking times of 120 and 90 Ma, trajectories indicate easterly transport across as much as 60° of longitude. For the Wrangellia, Central Salinia, and Point Arena terranes and the Laytonville Limestone, paleolatitudes were found as a function of time for different plate models, and these results were compared with paleomagnetically determined paleolatitudes from the terranes. The two sets of paleolatitudes were generally consistent only for plate models in which, when the Farallon plate rifted at 85 Ma to form a Kula plate in the north, leaving a smaller Farallon plate in the south, the newly formed Kula plate occupied a large region adjacent to North America, so that the Central Salinia and Point Arena terranes and the Laytonville Limestone were all located north of the Kula-Farallon rift. In addition, a rapid rate of spreading between the Farallon and Pacific plates during the Cretaceous normal superchron is required to model the paleolatitude of the Laytonville Limestone but not that of the other terranes. No terrane trajectory for Wrangellia was found that placed this terrane in the southern hemisphere during Jurassic time, as has been suggested on the basis of paleomagnetic data. The Wrangellia trajectories are consistent with the model of Irving and others (1985) in which Wrangellia arrived at the continental margin at the present latitude of Baja California at ∼100 Ma, was intruded at that latitude by the Coast Plutonic Complex, and then moved tangentially along the coast driven by oblique convergence of the Farallon and Kula plates.
... The first of these is drift, where a continent moves over the underlying mantle and with respect to the rotation axis/pole. The second APW phenomenon, true polar wander (TPW), occurs when the entire mantle and lithosphere (and, for that matter, most likely also the outer core) move relative to the axis of rotation (Harrison & Lindh, 1982; Livermore et al., 1984; Andrews, 1985; Courtillot & Besse, 1987; Anderson, 1989; Besse & Courtillot, 1991; Duncan & Richards, 1991; Spada et al., 1992; Ricard et al., 1993;). This motion, which embodies the repositioning of Earth on its rotation axis, is thus common to all lithospheric plates. ...
... Drift and TPW are both happening today. The latter, determined from astronomical data collected since 1900, is occurring at a rate of 70-110mm/yr (0.7-1.1°/Ma) towards eastern Canada (75°-80°W) (Munk & MacDonald, 1960; Yumi & Wako, 1968 Vicente & Yumi, 1969 Dickman, 1977 Dickman, , 1979 Dickman, , 1981 Andrews, 1985; Anderson, 1989). This current phase of TPW has been associated with the redistribution of water and ice and attendant isostatic readjustment following the end of the last ice age, approximately 10 ka ago (Sabadini & Peltier, 1981; Sabadini et al., 1982a-b; Anderson, 1989; Ricard et al., 1992). ...
... (TPW), as outlined above, can be defined broadly as global migration of Earth relative to its axis of rotation, although there has been considerable deliberation in the literature about what actually moves, in terms of the principal layers of Earth, when TPW occurs (McElhinny, 1973; Jurdy & van der Voo, 1975; Hargraves & Duncan, 1973; Jurdy, 1981 Jurdy, , 1983 Andrews, 1985). Andrews, noting that the three currently testable hypotheses embraced a relative shifting of the (entire) lithosphere only, the mantle only, or both the lithosphere and mantle together, proposed that the most likely cause of TPW involved a shifting of the lithosphere and mantle together, or whole Earth tumble, suggesting that it was difficult to envisage complete decoupling of the lithosphere from the mantle, and, if the core is coupled to the mantle, it would also be affected by TPW. ...
The Pangaean thermal anomaly (PTA) formed in the late Palaeozoic by heat buildup beneath the massive, mantle-insulating Pangaean supercontinent. Relics of the anomaly in Earth’s present-day geoidal surface are known to comprise the long-wavelength, African - eastern Atlantic residual geoid high. This geoidal feature appears to have controlled much of late Palaeozoic – early Mesozoic geological history, because it represented, until the Early Jurassic, a mass imbalance in Earth’s rotation. True polar wander (TPW) was set in motion, apparently in the latest Carboniferous (~300 Ma), so as to physically achieve correction of this rotational disorder. Considered primarily in terms of eastern Australia (eastern Gondwana), the limited evidence suggests that the thermal anomaly and the rotational adjustment were variously responsible for: widespread basin formation at ~300 Ma; the ensuing compressional tectonism of the Middle Permian - late Middle Triassic Hunter-Bowen Orogeny, as well as coeval global tectonism; first-order lowstand of global sea level at the Permian-Triassic boundary, with TPW-induced compressional uplift being superimposed on thermal uplift; the emergent nature of the Pangaean platform during the Triassic; and regional climate change during the late Palaeozoic and early Mesozoic, essentially caused by the changing palaeolatitude of the landmass and changes in base level/global sea level. This scenario seems to have controlled floral succession in eastern Australia, causing extinction of the Permian Glossopteris Flora, the development and subsequent termination of the largely Triassic Dicroidium Flora, and the following emergence of the Jurassic flora. The floral succession indicates increasing warmth of climate in the Australian region and thus reflects eastern Gondwana’s movement into lower latitudes from the end of the Palaeozoic, through the Triassic, and into the beginning of the Jurassic. Rotation-axis correction was then finally achieved through reorientation of the Pangaean landmass (and the Pangaean thermal and geoid anomaly) about a newly defined equator. Overall assessment of potential periods of significant Pangaea-related TPW not only points to the late Palaeozoic - early Mesozoic, but also to the Late Cretaceous - early Tertiary. The latter episode, however, was likely associated with accelerated breakup of the supercontinent, embracing mantle-mass reorganisation with the formation of new subduction zones. That TPW may have occurred during both of these intervals, from changes in Earth’s moment of inertia and in the location of its principal axis of inertia, is further drawn from published rotation-rate data obtained from tidally-controlled, skeletal-growth patterns preserved in fossil bivalves. Additionally, in terms of the apparent influence of TPW on the core and generation of the geomagnetic field, rapid axial change during the late Palaeozoic - early Mesozoic and the Late Cretaceous - early Tertiary may have terminated the Permo-Carboniferous Reversed Superchron at 265 Ma in the Middle Permian (Guadalupian) and the Cretaceous Normal Polarity Superchron at 83 Ma in the Late Cretaceous.
... Although Holocene and Pliocene-Pleistocene true polar wander rates are similar to or greater than average seafl oor spreading rates, ~40-100-k.y.-long glacialinterglacial excitation of polar wander appears to have driven <5° of secular true polar wander over the past 5 m.y. (Andrews, 1985;Schneider and Kent, 1986). ...
... Fixed-hotspot studies using different fi lters on the global paleomagnetic database have suggested mean Cenozoic-Mesozoic true polar wander rates of 1-5 cm/yr, and a total of 5-20° of secular true polar wander over the last ~200 m.y. (e.g., Livermore et al., 1984;Andrews, 1985;Besse and Courtillot, 1991;Prévot et al., 2000). Depending on how the global paleomagnetic database is fi ltered, some of the most recent analyses suggest episodic bursts of true polar wander, with almost zero true polar wander from 0 to 80 Ma and 150 to 200 Ma, but rapid true polar wander events from 80 to 150 Ma (Besse and Courtillot, 2002;Prévot et al., 2000). ...
We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5-6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in δ13C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis.
... TPW is occurring at present-day at ~10 cm a -1 , with about 40% of the driving perturbation arising from deglaciation (Adhikari et al., 2018;Vermeersen et al., 1997). Estimates of TPW back to the Cretaceous have been made through direct comparisons of paleomagnetic and hotspot-based reference frames (Andrews, 1985;Besse and Courtillot, 2002;Doubrovine et al., 2012), whereas estimates of TPW derived from integrated global plate/continental motions (Jurdy and Van der Voo, 1974) have been computed back into the Paleozoic (Steinberger and Torsvik, 2008;Torsvik et al., 2014). These estimates suggest that TPW has operated in a consistent way throughout the Phanerozoic, characterized by periodic oscillations of alternating sense about an equatorial axis, and occurring at rates on the order of 1° Ma -1 or less (Torsvik et al., 2014). ...
... Af tilgátum sem raeddar hafa verið í nýlegum greinum (en eru flestar gamlar), má nefna a) breytingar á útgeislun sólar (Wolfe 1978), eða á ryki í sólkerfinu b) breytingar á kolsýrumagni í andrúmsloftinu (Bemer o.fl. 1983;Barron 1985) c) brey tingar á möndulhalla j arðar miðað við j arðbrautina (Wolfe 1980) d) pólflutningar miðað við meginlöndin (Donn 1982, Andrews 1985 e) landrek og landbrýr (Strauch 1970) f) plöntur á fyrri tíð hafi átt betra en nú með að aðlagast árstíðasveiflum (Axelrod 1984; Barron 1984), g) áhrif halastjömuárekstra eða meiriháttar eldgosa á veðurfarið, og h) nýlega var það meira að segja haft eftir einhverri risatölvu (Ruddiman og Kutzbach 1989) að gamla kenningin hans Gardners (1878) um áhrif lóðréttra jarðskorpuhreyíingaá veðurfarið á Tertíertímabilinu komi mjög sterklega til álita. K-AR greiningar á íslandi og aldur surtarbrandsmyndana Fyrstu geislavirkni-aldursgreiningar frá íslandi birtust í tveim greinum 1966, og fjallaði önnur um myndanir frá ísöld á Jökuldal en hin um innskot. ...
... ~66-2 Mya). The NALB, which connected the northeast of North America with Europe, has been viewed as a principal migration route for thermophilic taxa during the Paleocene (Andrews, 1985;Brouillet & Whetstone, 2000). During the same period, migration via the BLB occurred between the northwest of North America and East Asia, but it was presumably more limited due to the higher latitude and cold temperatures (Tiffney & Manchester, 2001). ...
Artemisia frigida is a temperate grassland species that has the largest natural range among its genus, with occurrences across the temperate grassland biomes of Eurasia and North America. Despite its wide geographic range, we know little about the species’ distribution history. Hence, we conducted a phylogeographical study to test the hypothesis that the species’ distribution pattern is related to a potential historical migration over the ‘Bering land bridge’. We applied two molecular approaches: Genotyping‐by‐Sequencing (GBS) and Sanger sequencing of the plastid intergenic spacer region (rpl32 – trnL) to investigate genetic differentiation and relatedness among 21 populations from North America, Middle Asia, Central Asia and the Russian Far East. Furthermore, we identified the ploidy level of individuals based on GBS data. Our results indicate that A. frigida originated in Asia, spread northwards to the Far East and then to North America across the Bering Strait. We found a pronounced genetic structuring between Middle and Central Asian populations with mixed ploidy levels, tetraploids in the Far East, and nearly exclusively diploids in North America except for one individual. According to phylogenetic analysis, two populations of Kazakhstan (KZ2 and KZ3) represent the most likely ancestral diploids that constitute the basally branching lineages, and subsequent polyploidization has occurred on several occasions independently. Mantel tests revealed weak correlations between genetic distance and geographical distance and climatic conditions, which indicates that paleoclimatic fluctuations may have more profoundly influenced A. frigida’s spatial genetic structure and distribution than the current environment.
... The three regions: the Beringia area in the North Pacific, the North Atlantic, and the proto-Caribbean, are discussed, particularly in the context of timing of possible distribution patterns and paleoclimate. Comparisons of the paleolatitudes of the Beringia and the southern Greenland land connections from conventional paleopolar positions (A, based on Harrison & Lindh, 1982) and revised paleopolar positions (B, based on Andrews, 1985) that major events of the Caribbean basin are the latitudinal displacement of North and South America, and the in situ formation of most of the Greater Antilles (e.g., Anderson & Schmidt, 1983;Donnelly, 1985;Salvador & Green, 1980). The second hypothesis indicates that the evolution of the basin was primarily due to longitudinal displacement of various blocks, and the formation of the Greater Antilles between Central and South America and their subsequent northeastern movement to their current positions (e.g., Coney, 1982;Pindell & Dewey, 1982;Sykes et al., 1982). ...
... The first set of rotations (REFERENCE FRAME) moves North America relative to the reference frame. There are many possible reference frames, some based on paleomagnetics (McElhinney, 1973;Irving, 1979;Harrison and Lindh, 1982;Cox and Hart, 1986), others based on hotspots (Morgan, 1971(Morgan, , 1972(Morgan, , 1981(Morgan, , 1983; but see also Molnar and Atwater, 1973;Molnar and Stock, 1987;Lawver and Müller, 1994), others based on "true polar wander" (Livermore et al., 1984;Andrews, 1985;Courtillot and Besse, 1987), and others based on "absolute plate motions" (Kaula, 1975). showed that differences in latitude of as much as to 25° can be produced by different reference frames. ...
... Neither of these assumptions has been rigorously tested to date and could lead to systematic errors in the order of 10° (e.g. Andrews, 1985;Idnurm, 1985Idnurm, , 1986. Third, and probably of greatest importance, there remains a deficiency of reliable Palaeozoic palaeomagnetic data, in space and time, leading to a degradation of overall precision. ...
... This dichotomy in the hotspot trace-APWP relationship is usually explained as an effect of a true polar wander (TPW) on the paleomagnetic data. TPW corresponds to the differential movement of the mantle reference frame relative to the Earth's spin axes (Andrews, 1985;Gordon & Livermore, 1987 Richards et al., 1989;Courtillot et al., 1999); Iporá, Alto Paranaíba and Serra do Mar provinces (TR; Gibson et al., 1995;Thompson et al., 1998); and the northeastern alkaline provinces (FN; Fodor et al. 1998) where magmatism younger than 50 m.yr. can be found. ...
... (1) Hot spots are not generally fixed with respect to each other (Molnar & Stock 1987; DiVenere & Kent 1999). (2) Hot spots are moving with respect to the spin axis (e.g. Gordon & Cape 1981; Andrews 1985; Tarduno & Cottrell 1997). (3) Mantle plumes might be subject to distortion by advection in a large-scale mantle flow, resulting in the migration of the hot spots with respect to the deep mantle (Steinberger & O'Connell 1998; Christensen 1998). ...
... [3] When two true polar wander (TPW) paths, developed from worldwide paleomagnetic studies [Livermore, Vine, and Smith, 1984;Andrews, 1985], were plotted with this hotspot pole path, the combination appeared to collocate the dissimilar pole data into closely related spherical spirals (see Figures below). The paleomagnetic TPW poles approximately follow a loxodromic spherical spiral that crosses all meridians at an angle of 66.5°. ...
... This migration from north-west to south-east, passing the equatorial line, should have had also other detectable geodynamic consequences, namely in the data of TPW and PW. Also in this case I made a simplified assumption to try, at least roughly, to reproduce the already known paths of the true polar wander (Andrews, 1985; Courtillot, 1991, 2002). I assume: 1) The modern observed polar motion is the actual modern expression of the true polar wander, and the actual rate of drift (little more then 10 cm/yr, equivalent to nearly one geographical degree every million years) can be applied, with corrective factors, to the geological past. ...
The possibility of a link between asymmetrical Earth's expansion and Polar Motions (PM) is investigated, searching for possible mechanisms of inner and/or surface material displacement, which can lead to the observed Chandler Wobble (CW; Eulerian free oscillation) and its secular drift. While the main result of this work is the identification of the possible cause of PM in the diffuse emplacement of new mass in and under the triple point zones, the problem of the continuous excitation of the CW is still not resolved. Only a qualitative argument in favour of a cause of the CW's excitation by the not uniformity in time (from short, tens of years, to long time scale), low frequency noise or pulsations, of the diapirical flow of mantle material is proposed. The inversion of paleogeography towards geodynamics is performed, and the result is found that the PM's current parameters can be prolonged in the current geological past up to 100 Myr, reproducing with satisfactory approximation the more reliable true polar wander path and its slowing down around 50 Myr, which was detected by Besse and Courtillot (1991, 2002). The very long pulsation of the expansion rate - a fundamental new acquisition supporting a pulsating tectonic activity of the Earth (see the «Half Spreading Map of the Ocean Floors»; fig. 5.19 in McElhinny and McFadden, 2000, based on Müller et al., 1997) - could be the cause of the modern undetectability of the effects of the Earth's expansion on LOD and on geodesy. This is because of the minimum of the half spreading rate and tectonic activity occurring today - and the consequent minimum variation of radius and inertial moment -, and because the possible compensating effects due to differentiation of the mantle and core, with consequent accretion of the inner core still underway today.
... Inclination errors due to compaction can be excluded (basalts) or are not expected (ashes) following the Site 757 results, and seismic profiles show no measurable dip. This consistent discrepancy between observed and expected inclinations possibly reflects inaccuracies in India-Africa relative motion data prior to Chron 29 (Patriat and Achache, 1984;Patriat and Ségoufin, 1988;Royer et al., 1988;Molnar et al., 1988;Royer and Sandwell, 1989) offset between the geomagnetic and the hot-spot reference frame observed by some (Harrison and Lindh, 1982;Andrews, 1985;Sager and Bleil, 1987) but not by others (Livermore et al., 1983(Livermore et al., , 1984Schneider andKent, 1990a, 1990b). ...
Details the Late Cretaceous and Tertiary northward movement of the Indian plate. Breaks in India's northward movement rate are identified, dated, and correlated with the evolution of the India-Asia convergence. Paleolatitudinal constraints on the origin of Ninetyeast Ridge are discussed, and limited magnetostratigraphic detail is provided. -from Authors
... Also in this case I made simplified assumptions in order to try to reproduce, at least roughly, the already known paths of the True Polar Wander (Andrews, 1985;Besse and Courtillot, 1991). I assume: ...
A short review of the more relevant modern arguments in favour
of the conception of the Earth in expansion is provided. The
advantage of the expanding planet idea is a common explanation
of several outstanding problems coming from palaeontology,
palaeomagnetism, geology and climatology. All these problems
should be regarded as be a sort of distortion effects, which arise if
we try to reconstruct the situation of old geologic times adopting
the modern Earth’s radius – the distortions become larger and
larger as we came back in time. As a consequence the expanding
Earth could be considered a natural generalization of the plate tectonic.
A strong support to this generalization came from the simple
and united explanation that can be found in the expanding
Earth of the classical geodynamic phenomena of the polar motion
and the true polar wander by an inversion of the paleogeographic
position of the triple points. The conviction is expressed that basic
information about this old global tectonic conception should be
provided in secondary school and university courses.
... Also in this case I made simplified assumption in order to try, at least roughly, to reproduce the already known paths of the True Polar Wander (Andrews, 1985;Besse and Courtillot, 1991). I assume: ...
The possibility of a link between asymmetrical Earth's expansion and Polar Motions is investigated, searching for possible mechanisms of inner and/or surface material displacement, which can lead to the observed Chandler Wobble (CW; eulerian free oscillation) and its secular drift. While a main result of this work is the individuation of the possible cause of PM in the diffuse emplacement of new mass in and under the triple point zones, the problem of the continuous excitation of the CW is still not resolved, and only a qualitative argument is proposed. The result is found that the PM's actual parameters can be prolonged in the geological past, reproducing with satisfactory approximation the more reliable True Polar Wander path up to 100 Ma, and its slowing down and inversion of walk around 50 Ma. RELAZIONI TRA UNA TERRA IN ESPANSIONE, IL TPW E IL MOTO DEL POLO Riassunto. Si è investigata la possibilità di un legame tra espansione asimmetrica della Terra ed i moti del polo (PM e TPW), cercando possibili meccanismi di spostamenti di massa interna o superficiale che producano la Chandle Wobble osservata (CW, o oscillazione libera euleriana) e la sua deriva secolare. Il risultato principale di questo lavoro è la individuazione della possibile causa del PM nella messa in posto diffusa di nuova massa nelle zone di punto triplo e sotto di loro, ma il problema della continua eccitazione della CW è rimasto quantitativamente insoluto, sebbene un argomento qualitativo possa essere proposto. Si è anche trovato che gli attuali tassi di PM possono essere prolungati nel passato geologico, riproducendo con approssimazione sufficiente la più affidabile curva di TPW disponibile per i passati 100 Ma, e il suo rallentamento e inversione di rotta intorno a 50 Ma.
... The hypothesis of true polar wander (TPW) questions the basic assumption of the fixed hotspot model (Hargraves and Duncan, 1973). A version of this hypothesis supposes that the entire solid shell of the Earth (the crust, mantle, and the hotspots contained within) sometimes undergoes a finite rotation, or "tumbling," with respect to the Earth's spin axis and its paleoequator (Andrews, 1985). Perhaps this is due to a change in the Earth's angular momentum resulting from a change in plate motion or mantle convection patterns. ...
Studies of remanent magnetic inclinations and Paleoecology of planktonic communities in sediments recovered during Ocean Drilling Program Leg 129 are combined with the skewness of M-sequence magnetic anomalies, paleomagnetism of seamounts, and Pacific plate motion models based on fixed hotspots to derive the paleolatitude and plate tectonic histories of the oldest portion of the Pacific plate. It originated as a small plate, perhaps of microplate dimensions, at equatorial southern paleolatitudes during the Middle Jurassic. The Pacific plate drifted south from the Kimmeridgian throughout the remaining Late Jurassic and Early Cretaceous, carrying Site 801 to a maximum southern paleolatitude of 22°S, which is 40° south of its present-day location. This southward drift was accompanied by about 30° of clockwise rotation. The southward motion tapered off, and was replaced by northward drift sometime between the Hauterivian and the Aptian, perhaps near the Aptian/Barremian boundary. This was probably in response to changing mantle dynamics associated with the mid-Cretaceous superplume episode. The Pacific plate drifted northward throughout the Late Cretaceous, and Site 801 crossed the paleoequator from south to north during the late Campanian (75-80 Ma). Northward motion continued until the late Eocene (43 Ma) when true polar wander, possibly induced by the collision of India and Africa/Arabia against Eurasia, caused 8° of retrograde (southern) motion of the Pacific plate relative to the spin axis and paleoequator. The Pacific plate has drifted monotonically to the northwest since the Eocene, and that motion continues today.
... The presence of Pachymerina implies mild winters (Figs. S5 and S6) across this cooler montane setting from northern Washington into midlatitude Canadian localities, all a few degrees more northerly in the Eocene (42). This strengthens the supposition of equable Eocene extratropical climates, with the consequent implications for a globally nonmodern climatic context impacting the emergence of modern ecosystems in the late greenhouse world. ...
Significance
Elevated CO 2 combined with globally warm temperatures in the Eocene make its climate ideal for understanding modern global warming and its biotic consequences. Globally low temperature seasonality—the relationship between winter and mean annual temperatures—has been proposed as key to differential Eocene biodiversity and community patterns. Palms are important winter temperature indicators by their sensitivity to frost; however, their presence in paleocommunities may be masked by taphonomic constraints and identification difficulties. We used fossil obligate palm-feeding beetles to establish the presence of palms in a cool upland in midlatitude western North America. In this way, we provide a more precise characterization of climate during an important interval of the emergence of modern ecosystems.
... The three regions: the Beringia area in the North Pacific, the North Atlantic, and the proto-Caribbean, are discussed, particularly in the context of timing of possible distribution patterns and paleoclimate. Comparisons of the paleolatitudes of the Beringia and the southern Greenland land connections from conventional paleopolar positions (A, based on Harrison & Lindh, 1982) and revised paleopolar positions (B, based on Andrews, 1985) that major events of the Caribbean basin are the latitudinal displacement of North and South America, and the in situ formation of most of the Greater Antilles (e.g., Anderson & Schmidt, 1983;Donnelly, 1985;Salvador & Green, 1980). The second hypothesis indicates that the evolution of the basin was primarily due to longitudinal displacement of various blocks, and the formation of the Greater Antilles between Central and South America and their subsequent northeastern movement to their current positions (e.g., Coney, 1982;Pindell & Dewey, 1982;Sykes et al., 1982). ...
The biogeographic affinities of the Cretaceous and early Tertiary angiosperm floras of the North American area (which includes Meso-America, and the Greater Antilles) have been the subject of considerable interest. Although recent treatments of isolated taxa have shown affinities between North American, European, east Asian and Neotropic floras, the relationships have not been quantified. This study compiles the records of fossils whose familial relationships seem secure. This provides a carefully culled, and uniformly presented review of the Cretaceous and Paleogene record from 1950 to 1989 and supplements LaMotte (1950). A subset of these records, which showed compelling evidence of subfamilial relationships, was analyzed to quantify the relationships of the Cretaceous, Paleocene, Eocene and Oligocene floras to other regions. The analysis suggests that for the entire period 24% of the fossil species had affinities with extant taxa from the Northern Hemisphere; 10% with taxa from the Northern Hemisphere that have a few species in South America; 17% with taxa from Eurasia; 3% with taxa with a disjunct Eurasian-South American pattern; 19% with taxa from South America and/or Africa; 8% with taxa from South America and/or Africa that have an important sister group in southeast Asia; 5% with taxa from the Old World; and 13% with taxa having other distribution patterns. Those fossils with affinities to Laurasian taxa are mostly found in the northern and western portions of the North American area. The fossils with affinities to South American and/or African taxa are found in the southern portions of North America, Meso-America, and the Greater Antilles. The taxa with disjunct distributions show both patterns. These patterns suggest that during this time there were wide-spread temperate elements, found throughout Laurasia; Boreotropical flora elements, distributed in North America, Europe and along the Tethys seaway to southeast Asia; and West Gondwana elements which show dispersion from South America across the proto-Caribbean. The paleobotanical data are compatible with current geological, paleontological and biogeographical studies.
... (2) Hot spots are moving with respect to the spin axis (e.g. Gordon & Cape 1981;Andrews 1985;Tarduno & Cottrell 1997). ...
2005.03 Synthetic apparent polar wander (APW) paths for North America, South America, Eurasia, India, Central Africa, Australia and Antarctica for the last 200 Myr are proposed. Computation of these APW paths is based upon the latest version (4.5a) of the Global Paleomagnetic Database (GPMDB), a revised global plate tectonic model since the Early Jurassic, and a new technique for generating smoothed APW paths. The smoothing technique includes the following steps: (1) pre-selection of palaeopoles, including pre-filtering parameters (number of sites, number of samples per site, 95 per cent confidence circle about mean direction, cleaning procedure, and time uncertainty); (2) generation of palaeolatitude and declination plots for a reference site on each continent that combines palaeopoles via a global plate tectonic circuit; (3) independent spline regression analyses of the palaeolatitude and declination plots; (4) removal of palaeolatitude or declination data that deviate by more than 10° from the regression curves (post-filtering process); (5) generation of synthetic APW paths from the resulting palaeolatitude and declination plots. These synthetic APW paths are then rotated into African coordinates to determine the best-fit APW path and a global palaeomagnetic reference frame. Four representative plate tectonic reconstructions and global plate velocity fields are presented for the three time intervals that correspond to globally synchronous changes in plate motion.
... This would place the early Tertiary bridge in an area of extended winter darkness and raise the question of the ability of ever-green angiosperms to cross ( fig. 2). Note that some reconstructions (Andrews 1985) have placed the bridge as low as 70Њ in the early Tertiary. ...
Phylogeography posits that the sequence of speciation events within a clade should parallel the geographic migration and isolation of members of the clade through time. The primary historical features that govern migration and allopatry in land plants are changes in physical geography (e.g., oceans, mountains, and deserts) and in climate (e.g., moisture, temperature, and day length), features that are often interrelated. If we assume that living genera retain physiological stability through time, much as they retain the morphological features that allow their identification, then these environmental features of the geologic past may be used to test phylogeographic hypotheses of living genera based on phylogenetic analysis. The history of the climatic and geographic features of the Tertiary of the Northern Hemisphere agrees with many phylogenetically based phylogeographic hypotheses of living angiosperm genera but indicates that some hypotheses require reanalysis. While the parallel comparison of phylogenetic hypotheses and historical biogeographic evidence is in its infancy, the reciprocal illumination of the two approaches shows great promise for future application.
The factors which most critically affect paleogeographic maps are the spatial resolution of the data base; the map projection; the initial fits and rotations of the major continental blocks; the number of plates rotated independently for the reconstruction; the number of terranes shown; palinspastic reconstruction of continental margins and interiors, and palinspastic reconstruction of terranes; and the reference frame. Text accompanying the reconstructions should describe the factors listed above. The basis for paleogeographic features, such as the shoreline, topography and paleobathymetry should also be explicitly stated. The sea level position used to plot the shoreline, whether at maximum transgression, some intermediate or average position, or at regression, should be clearly stated. -from Authors
The data derived from a study of geological maps, relief maps and remote sensing have enabled the author to locate and chart over one hundred circular features in the U.S. and Canada. A Diameter versus Cumulative Distribution graph of these features strongly resembles the similarly plotted curves of Lunar and Martian impact craters of the same size range, indicating that these large Earth features may have a similar origin and history. The one hundred-plus features mapped are made up of the composite-edges of relief features, rivers, and land-water boundaries, and span terranes of all ages, an indication that they are manifestations imposed by deep basement structure. One fifth of the catalogued circular features were found to possess “twins” of identical diameter whose azimuths (of lines between centers) change slightly in a systematic way across the continent. The patterns presented by the “twins” show such conformity that horizontal movement of the upper crust over wide areas may be indicated. Imposition and reimposition of structural controls at depth are inferred.
There is substantial support of these interpretations from other geologic research: recent work with hot spot migrations demonstrate quite close agreement with the azimuths of lines between centers of the circular feature “twins.” Interpretation of deep seismic programs also support the existence of wide horizontal upper crust movements. In addition, the spacings between major parallel lineaments in the U.S. closely match the distances between centers of “twin” features in the same region.
This paper traces the history of ideas regarding the geological "age of Iceland', as determined primarily from plant fossils exposed in the older lava sequences of the island. The first statement on this age to become generally known among geologists, was that of the great Swiss paleontologist Oswald Heer (1809-1883). Heer's opinion was of a Lower Miocene age for fossil outcrops in the Northwest peninsula of Iceland. Heer also studied a number of other fossil floras from the Arctic and from Britain, finding almost all of them to be either Miocene or Mesozoic. A dispute developed in 1878-79 between Heer and the British geologist J. Starkie Gardner (1844-1930) who was studying fossil localities in South England. He claimed that the British and Arctic floras were much more likely to be Eocene than Miocene. Gardner's conclusions on the age of South England floras were generally accepted very soon, and in the following decades new fossil discoveries supported the view of an Eocene age for the Northern British and Arctic areas. New paleontological and palynological work on Icelandic material by Askelsson (1946) and Pflug (1956, 1959) appeared to confirm an Eocene or even older age. This view hence prevailed through the critical early years of continental drift revival, and was not refuted until mid-Miocene K-Ar dates on the older lava series in Iceland appeared in 1967-68. Along with certain other persistent erroneous conceptions of the geology of Iceland, the view of an Eocene age may have delayed the general acceptance of sea floor spreading by local scientists. -from English summary
Although most of the paleomagnetic data available for arctic Alaska show evidence of a magnetic overprint, which commonly obscures the original magnetization, there are a few localities which appear to have recorded an original magnetic field. However, any conclusions drawn on the basis of these apparently reliable paleomagnetic data have to be considered tentative, since they have not all had rigorous stability tests applied. With this caveat in mind, it is found that the northernmost localities indicate a post Early Cretaceous counterclockwise rotation commensurate with a rotation of arctic Alaska away from the region of the Canadian arctic islands, followed by a smaller clockwise rotation recorded in sites further to the west, but still on the Arctic Slope. South of the Brooks Range, sediments of Cretaceous age from the Yukon-Koyukuk Province on-lap the Brooks Range and are thus considered part of Arctic Alaska. These sediments give paleomagnetic directions that indicate variable amounts of clockwise rotation. This rotation is most easily interpreted in terms of ball-bearing style motions due to dextral strike-slip motion along the southern margin of Arctic Alaska. These clockwise rotations are in contrast to the counterclockwise motion seen throughout southwest Alaska.
This paper is devoted to the historical development of paleomagnetology, the state of the art of studies on the fine structure and long-period variations in the geomagnetic field, and their comparison with surface processes such as variations in the climate, organic world, and movements of the continental plates.
The motion of hotspots in large-scale mantle flow is discussed. The
concept of mantle plumes and the experiments and observations on which
it is based are reviewed. Results t h a t support hotspot motion (from
experiments, plate reconstructions and paleolatitude data) are
contrasted with observations on which t h e concept of hotspot fixity is
based (mainly geometry and age progression of hotspot tracks). A
numerical model of hotspot motion in large-scale mantle flow is
introduced. It is shown how hotspot motion may approximately represent
flow at mid-mantle depth, particularly plate return flow. Such a model
can explain some aspects of t h e observed hotspot distribution, yields
predictions on the origin and shape of plume conduits and gives
constraints on mantle viscosity structure. Some results for the motion
of individual hotspots are shown, including coherent motion toward the
southeast for Hawaii and Louisville, westward motion for Easter, and
southward motion for Kerguelen. For Hawaii and Louisville, the
calculated motion fits well the observed geometry and age progression of
hotspot tracks. Furthermore, calculations can explain observed relative
motions of Pacific and African hotspots back to 43 Ma without invoking
any additional plate boundary in the Pacific-Antarctic region.
Paleomagnetic and modelling results of polar motion are reviewed and
combined with models of hotspot motion; resulting predictions of
paleolatitudes are compared with observations for t h e Pacific plate.
Calculated hotspot motion in combination with polar motion can explain
any latitudinal shift of Pacific hotspots back to about 70-80 Ma.
Paleopoles derived from seamounts have been used to reconstruct the tectonic history of ocean basins; however, the interpretation of seamount magnetization models and the validity of seamount paleopoles may be affected by inhomogeneous magnetization. Multibeam bathymetric data, sea surface and deep-tow magnetic field data, and paleomagnetic analyses of dredged samples were used to examine the origin of nonuniform magnetization within Jasper Seamount (30°27'N, 122°44'W). Models indicate that the seamount is predominantly reversely magnetized with local zones of normal polarity as corroborated by deep-two measurements. Lithologies likely to be volumetrically important in a seamount eddifice show highly variable magnetic properties. Basalts have high intensities (0.5-27.0 A/m), high Koenigsberger ratios (Q) and low viscous remanence (VRM) acquisition. Low Q ratios and high VRM acquisition coefficients of coarse-grained material and volcaniclastics suggest that they may have substantial viscous and induced components. Models for Jasper are characterized by low uniform intensities and far-sided paleopoles. The shallow model inclinations may be attributed to nondipolar components in the time-averaged geomagnetic field. The low intensities of the uniform models and the large nonuniform component in the seminorm solutions imply a complex distribution of magnetization sources within Jasper. This nonuniformity may result from either lithological variability or construction of the seamount spanning two or more polarity intervals.
The apparent polar wander curves for major continents constitute powerful tools for testing paleogeographic reconstructions, evaluation of global tectonics, and analyzing the evolution of deformed belts. On the other hand, the mid-Cretaceous is an interesting interval in the annals of global change, characterized by the occurrence of major geodynamic events such as fast sea-floor spreading, increased mantle plume activity, and an about 37 m.y. interval of uniform geomagnetic polarity. During the mid-Cretaceous occurred the final dismembering of Gondwana leading to consolidation of present-day continents and oceanic basins. This contribution presents new paleomagnetic data from mid-Cretaceous (92 Ma from averaging available 40Ar/39Ar ages) sedimentary rocks in the San Bernardo foldbelt, Patagonia. Paleomagnetic routines allowed observing high unblocking temperature, high-coercivity magnetizations of normal polarity carried by magnetite. A positive tilt-test constraints the age of the remanences to be older than folding (middle Miocene). This, together with the presence of normal and reversed polarity zones in conformably overlying Paleogene units suggest that the observed remanences were acquired during the Cretaceous Normal Superchron, and then close to the time of deposition. Samples from paleosols show scattered magnetic foliations, suggesting that the last important modification to their primary sedimentary fabric is related to bioturbation associated with pedogenesis instead to post-paleosol compaction linked to burial, which in turn argues for negligible post-paleosol inclination flattening. The obtained paleomagnetic pole is indistinguishable from those deriving from 125 to 100 Ma magmatic rocks in the Brazilian platform, indicating 1) integrity of Patagonia and stable South America since the mid-Cretaceous, and 2) that the continent was essentially motionless with respect to the paleomagnetic axis during the 125-95 Ma time span. This behavior resembles the coeval polar standstill previously recognized for North America, Eurasia, and possibly also the Pacific.
The apparent polar wander path for a plate is determined from paleomagnetic data by plotting a time sequence of paleomagnetic poles, each representing the location of the earth's spin axis as seen from the plate. Apparent polar wander paths consist of long, gently curved segments termed tracks linked by short segments with sharp curvature termed cusps. The tracks correspond to time intervals when the direction of plate motion was constant, and the cusps correspond to time intervals when the direction of plate motion was changing. Apparent polar wander tracks, like hot spot tracks, tend to lie along small circles. The center of a circle is called a hot spot Euler pole in the case of hot spot tracks and a paleomagnetic Euler pole in the case of paleomagnetic apparent polar wander paths. Both types of tracks mark the motion of a plate with respect to a point, a rising mantle plume in the case of hot spot tracks and the earth's paleomagnetic axis in the case of apparent polar wander paths. Unlike approaches uced in previous studies, paleomagnetic Euler pole analysis yields all three components of motion—including the east‐west motion—of a plate with respect to the paleomagnetic axis. A new method for analyzing paleomagnetic poles along a track by using a maximum likelihood criterion gives the best fit paleomagnetic Euler pole and an ellipsoid of 95% confidence about the paleomagnetic Euler pole. In analyzing synthetic and real data, we found that the ellipsoids are elongate, the long axes being aligned with a great circle drawn from the paleomagnetic Euler pole to the center of the apparent polar wander track. This elongation is caused by the azimuths of circular tracks being better defined than their radii of curvature. A Jurassic‐Cretaceous paleomagnetic Euler pole for North America was determined from 13 paleomagnetic poles. This track begins with the Wingate and Kayenta formations (about 200 Ma) and ends with the Niobrara Formation (about 87 Ma). Morgan's hot spot Euler pole for 200–90 Ma lies only 15° outside the 95% confidence ellipsoid of the paleomagnetic Euler pole. The good but not perfect agreement reflects displacement between the hot spot and paleomagnetic reference frames at an average rate that is smaller by an order of magnitude than the rate at which the faster plates are moving. The angular velocity of North America about the Jurassic‐Cretaceous paleomagnetic Euler pole was determined by plotting the angular positions of paleomagnetic poles along the track as a function of age. For the Cretaceous the angular velocity was too small to measure. During the Jurassic the angular velocity was high, corresponding to a root‐mean‐square velocity of 70 km/m.y. for the North American plate. A short time interval of even more rapid movement during the Middle and Late Jurassic, possibly corresponding to the beginning of rapid displacement between North America and Africa, is suggested by the data. The direction of absolute motion of North America during the Jurassic was toward the northwest. A Carboniferous‐Permian‐Triassic paleomagnetic Euler pole was determined from 26 paleomagnetic poles. The progression of poles along this track is consistent with known ages and stratigraphy, except for some systematic differences between poles from Triassic rocks on the Colorado Plateau and poles from Triassic rocks off the Colorado Plateau. These differences could be due to a small clockwise rotation of the Colorado Plateau with respect to cratonal North America, or to miscorrelations between Triassic rocks on the Colorado Plateau and off the Colorado Plateau, or to large lag times between the deposition and magnetization of some rock units, or to some combination of these possibilities. Despite these ambiguities in interpreting paleomagnetic data from Triassic rocks, the general pattern of apparent polar wander and plate motion during the Carboniferous through Triassic is clear: The root‐mean‐square velocity of North America was slow (about 20 km/m.y.) during the Carboniferous, probably slow (about 20 km/m.y.) during the Permian, but rapid (60–100 km/m.y.) during the Triassic. Paleomagnetic Euler pole analysis establishes that the present slow (less than 30 km/m.y.) velocity of large continental plates like North America is not an intrinsic property of the plates. Occasionally these plates have, for intervals of 50 ± 20 m.y., moved as rapidly as the oceanic plates are moving today. In our interpretation, during times of rapid motion the continents were attached along a passive margin to oceanic lithosphere that was being subducted at some distance from the continent. Rapid motion stopped when the oceanic lithosphere had been consumed by subduction. If North America, Greenland, and Eurasia were joined as a single land mass during the Jurassic, then a likely location for the subducting oceanic plate attached to this landmass is along the southern margin of the cratonal core of Asia with the oceanic plate extending into Tethys. At the cusp between the Carboniferous‐Permian‐Triassic track and the Jurassic‐Cretaceous track, the trend of the path changes by 160°. The western point of the cusp, which is delineated by paleomagnetic poles from the Chinle, Wingate, and Kayenta formations, is 13° farther west in our analysis than it is in commonly accepted apparent polar wander paths for North America. An implication for terrane analysis is that northward displacements found by using our Late Triassic and Early Jurassic poles are up to 2000 km smaller than are those found by using previously published Late Triassic and Early Jurassic cratonal poles.
The mapping of magnetic anomalies generated by ocean crustal spreading and the sampling of this crust by deep drilling have produced a detailed kinematic model for growth, destruction and relative movements of the outer quasi-rigid shell of the Earth (the Lithosphere) during the last part of geological times. The refinement of this model, and its extension into more ancient epoch is limited by the inexorable consumption of the ancient and dense oceanic lithosphere back into the asthenosphere at the subduction zones. Earth scientists are now actively extending the concepts of global mobility derived from the surface layer of the globe to the interior comprising the silicate mantle and iron-nickel core. Much interest centres on the nature of the mantle layer adjoining the core at the core-mantle boundary (CMB), the topography of this zone, and the implications of these parameters for the processes of heat release from the Earth’s interior. The key observations come from seismic tomography, material properties at high temperatures and pressures, and satellite geodesy. Collectively, this evidence has important consequences for our understanding of mantle convection and the mobility of the Earth’s interior, but it is limited by lack of the time dimension. The latter is provided by the study of the magnetic field incorporating geomagnetism (over historical times) and palaeomagnetism (over geological times) with which this review is largely concerned.
Zusammenfassung Zwischen Orogenese, Magmatismus und zugehörigen Sedimenten existiert ein enger Zusammenhang, der meist wenig verstanden ist und dringend eingehender Studien bedarf. Gewisse Sediment-Typen sind meist an einen ganz spezifischen, magmatischen Kontext gebunden. Dies ist insbesondere der Fall für die ozeanische Kruste, könnte aber auch für den bimodalen, kontinentalen magmatischen Zyklus mit der Bildung von Granit zutreffen. Die gro\räumigen Regressionen und die damit verbundene Entstehung von kontinentalen, detritischen Ablagerungen könnte die Antwort auf vertikale Mantel-Konvektion oder Plume-Aktivität sein.
Describes a number of outrageous causal hypotheses for terrestrial changes. Discusses ideas of internal pole shifts and astronomical pole shifts, and the effects, both primary and secondary, of these events. Bombardment by extraterrestrial objects has occurred and can produce catastrophic changes. -K.Clayton
Two types of rifting may have characterized the two-stage breakup of Pangea---passive rifting during Early Jurassic rapid supercontinental motion and active rifting during Cretaceous supercontinental stillstand. This suggestion is based primarily on the observation that global continental angular momentum in the hotspot reference frame was very high from 180 to 150 Ma and very low from 140 to 90 Ma. From 180 to 150 Ma, motion of the continents was probably too high to allow active rifting. Instead, rifting may have been due to stresses associated with the motion. The pole of rotation for the continents was very close to that for opening of the central Atlantic. Africa lagged behind the other continents in its progress over the mantle, perhaps because of the anchoring effect of a mantle slab under the Alleghenian suture. From 140 to 90 Ma, the low continental momentum is consistent with Anderson's model for continental breakup, in which stillstand of a supercontinent leads to mantle up-welling and continental dispersal through active rifting and sea-floor spreading. Major breakup of the hemispherical supercontinent occurred during the low-momentum period, and the geoid anomaly coincides with the supercontinent's position at that time. The two types of rifting have different geological and paleomagnetic signatures that may allow recognition of them for earlier supercontinents.
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.
Using the geometry and ages from 12 Pacific seamount chains, we have determined two new absolute plate motion models that now extend our self-consistent and high-resolution models with covariance estimates back to 145 Ma. The WK08-A model maps the full uncertainty in the age progressions into uncertainties in rotation opening angles, yielding a relatively smooth plate motion model. The WK08-G model relaxes the mapping of age uncertainties in order to better isolate secondary geometry changes seen along many coregistered chains. Both models have been used to assess the viability of the fixed hot spot hypothesis in the Pacific. In determining the models, we found that only a small group of age samples had to be discarded on the grounds that they were discordant with the dominant trends. We were able to connect plate motions for pre- and post-Emperor age intervals by including the Ratak-Gilbert-Ellice and Musicians trails in our analysis. However, as no active hot spot locations exist for the older chains, their inclusion adds additional model parameters. Both age and geometry misfits increase with age, reflecting the observed increase in age uncertainties and the general widening of trails. Secondary (and short-lived) changes in absolute plate motion mapped in WK08-G appear to correlate with the timing and sense of motion of known Pacific Rim tectonic events. Analysis of interchain distances between coeval samples from the Hawaii and Louisville chains suggests possible discrepancies during the older Emperor stage that are compatible with predictions of hot spot drift. We computed a new apparent polar wander path for the Pacific and found a high degree of correspondence with paleomagnetically derived paths, as long as solutions allowing for anomalous skewness were included in the latter. Our polar wander path suggests that there might have been some true polar wander during the Emperor stage, complemented by a smaller amount of hot spot drift than otherwise required. We show that chain geometries and ages, combined with future paleolatitude determinations from additional sites and chains could enable an observation-based description of both hot spot and plate motions without relying on predictions of hot spot drift derived from mantle flow calculations.
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. Five topics of current research on Earth's mobilistic surface are reviewed and their future directions discussed. It is concluded 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. -from Author
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 agrees with previously published mid-Cretaceous poles for North America, which together give a mid-Cretaceous standstill reference pole at 71.2°N, 194.1°E (A95=3.7°, N=5 studies). It is argued that this mean pole represents the North American mid-Cretaceous reference field for nominally 36 m.y. (124 to 88 Ma). During the same mid-Cretaceous interval, the New England hotspot track 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 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'. -from Authors
We have reviewed paleomagnetic data available for the Eurasian, African, North American and Indian plates over the last 200 Ma. We propose and describe revised APW paths, but more importantly, we next use relative motion models to transfer all data in a common reference frame. Transferred data from all plates are averaged in 20-Ma windows to generate synthetic APW paths for all plates studied. The synthetic paths display interesting features, such as a previously ill-recognized APW loop for Eurasia. Paleomagnetic and hotspot APW are next compared, and a determination of true polar wander (TPW) is derived. -from Authors
New paleomagnetic results from 101-89 Ma Laytonville Limestone of the Franciscan Central Belt melange of northern California confirm a southern hemisphere origin as first proposed by Alvarez et al (1980). Discovery and study of new Laytonville Limestone outcrops extend the paleomagnetic record and provide a definitive determination of age, stratigraphic polarity, and hemisphere of deposition. An "instantaneous' poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m.y. section. Conservative estimates place this minimum velocity at 15 cm/yr. The consistency between the values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization. -from Authors
In prior studies paleomagnetic data have been used to determine ancient minimum velocities of the continents. The greatest minimum velocities, ~50km/m.y., exceed the present-day absolute velocities of the major continent. This result is important because the observation that present-day oceanic plates move faster than present-day continental plates can be treated as a major constraint on plate dynamics. However, these prior studies are limited because they ignore known errors in the paleomagnetic data. In this paper we present a technique for incorporating these errors and using them to estimate 95% confidence limits on the minimum velocities. This new technique was applied to Pennsylvania to Cretaceous paleomagnetic data from North America and to the ancient geometry og Laurasia. When errors are considered, minimum velocities are 10 to 50km/m.y. lower than when errors are ignored. Additional uncertainties owing to hypothesized clockwise rotation of the Colorado Plateau, a source of many of the data, are considered and found to alter the minimum velocities only negligibly. Allowing for uncertainties in the pole positions, uncertainties in ages, and the uncertainty due to possible rotation of the Colorado plateau, we estimate that Laurasia moved at least 48 km/m.y. from Early Jurassic to Early Cretaceous time, and at least 43 km/m.y. from Pennsylvanian through Late Triassic time. There is only a 5% risk that the true minimum velocities are lower than these lower bounds. These velociites are less than the peak velocities over short intervals found in prior analyses, but exceed the average velocities previously found for Triassic and Jurassic time. These newly estimated velocities are still much higher than the present velocities of the continents and, as in the prior studies, imply that the slow motion characteristic of major continents at present is not a fundamental attribute of plate motions.
Cenozoic global plate motions relative to the hot spots are investigated
and compared to plate motions in a mean-lithosphere reference frame.
Plate motions were analyzed over six time intervals divided by ages (10,
25, 43, 48, and 56 Ma) chosen, as much as possible, to coincide with key
plate reorganizations. Alternative motion circuits and rotational
parameters were considered and evaluated with paleomagnetic data from
the Pacific and North American plates. The circuit found to be in best
agreement with the paleomagnetic data is one in which the hot spots in
the Atlantic region are assumed to be fixed relative to the hot spots in
the Pacific region. Throughout the Cenozoic, the hot spot and
mean-lithosphere reference frames have been in continual, slow relative
motion. The rate of motion is nonuniform, however, most of the motion
having occurred during the middle Cenozoic. The net Cenozoic rotation of
the lithosphere relative to the hot spots is described by a right-handed
rotation of 7° about a Euler pole at 46°S, 87°E, which
yields a 5° displacement of the north poles of the two reference
frames. This motion is small enough that inferences drawn about plate
speeds in one reference frame should be valid in the other. Analysis of
the global motions resulting from our preferred model showed that many
characteristics of current plate motions have persisted throughout the
Cenozoic. Plate speeds correlate with latitude, plates moving faster
near the equator than near the poles throughout the Cenozoic. As at
present, continental plates (except for the Indian plate) moved slower
than oceanic plates throughout the Cenozoic. Even the structure of the
velocity fields as revealed in a contour of root-mean-square velocities
in equatorial bands persists throughout the Cenozoic. The migration of
the paleomagnetic axis over time is also compared to the hot spot and
mean-lithosphere reference frames. The paleomagnetic axis has shifted
5°-10° relative to the hot spot frame, and a lesser amount
relative to the mean-lithosphere frame.
mated for the past 180 m.y. by combining previously published plate reconstructions. These velocities agree well with velocities relative to the spin axis inferred from paleomagnetic data. From-169 m.y. B.P. to -144 m.y.B.P., the hot spot data indicate that Laurasia moved rapidly, '70 mm/yr, but the reconstructions permit a veloc{ty as low as 50 mm/yr. We also estimate that from -63 m.y.B.P. to '48 m.y.B.P. India moved rapidly ('140 +30 mm/yr), faster than the present velocity of any plate , - 20 and more than twice as fast as the present velocities of several subducting oceanic plates (those attached to downgoing slabs along a substantial fraction of their margin). The data require no other continent to have exceeded a velocity of 45 mm/yr during the last 180 m.y. In agreement with previous studies based on paleomagnetic data and in agreement with previous studies based on no-nettorque analyses, we conclude that the continents have moved faster, occasionally much faster, than at present. From this we infer that continental lithosphere is unlikely to have much greater resistance to plate motion than oceanic lithosphere. Instead, oceanic plates tend to move more rapidly than continental plates because the subduction and rapid motion of a plate that includes no continents tends to persist longer than the subduction and rapid motion of a plate that includes one or more continents. On the basis of the very rapid early Tertiary motion of India, we conclude that the velocities bf subducting plates can vary by a factor of 2. Therefore models of the driving forces of the plates that predict a uniform velocity of subducting slabs are invalid.
A paleomagnetic study in Upper Pliensbachian–Lower Toarcian rocks of Extra-Andinian Patagonia was carried out. Characteristic magnetizations isolated from three sections pass the tilt test. The resulting Early Jurassic paleomagnetic pole (PP) (Long.=129.4°E, Lat.=75.5°S, A95=6.8°, N=13, K=38.7, R=12.7), together with other reliable Jurassic PPs of cratonal South America defines a hitherto undocumented Jurassic track of apparent polar wander path (APWP). This track suggests recurrent movement of South America with high velocity during the Lower Jurassic. A comparison among the Jurassic PPs of Gondwana continents suggests that APWP can account for a great part of the differences observed between northwestern and southern African Jurassic paleopoles.
Hot spots are located on the Earth's surface and are caused by hot plumes of upwelling mantle material. They manifest themselves by volcanic activity which changes its location in response to the movement of the lithosphere over the mantle plume. It has been suggested1–3 that hot spots are relatively stationary with respect to each other, and also that they represent a frame of reference with which to measure the absolute movement of the lithospheric plates over the surface of the Earth. Another frame of reference can be partially defined by the position of the mean magnetic dipole field of the Earth. As the dipolar field is believed to be closely related to the spin axis, this frame of reference automatically gives information about the palaeolatitude and orientation of the lithospheric plates. But since the position of the ancient dipole field of the Earth does not give any information about palaeolongitude, and since in many cases the definition of the field is relatively poor due to less than adequate rock collections during the time of interest, hot spot reference frames have been favoured by many people. We now report a comparison of the two frames of reference and show that over the past 200 Myr there has been relative movement between the two frames of reference of about 20°, and that this motion has not been constant with time.
An instantaneous plate-motion model, Relative Motion 2 (RM2), is obtained by inverting a data set comprising 110 spreading rates, 78 transform fault azimuths, and 142 earthquake slip vectors. RM2 is compared with angular velocity vectors which best fit the data along individual plate boundaries and, while the model performs close to optimally in most regions, attention is directed to those regions which are not suitably described by the model. Reasons for the discrepancies between RM2 and observations for the India-Antarctica plate boundary, the Pacific-India plate boundary, and the east-west trending transform fault azimuths observed in the French-American Mid-Ocean Undersea Study area are discussed.
The motions of the lithospheric plates have been reconstructed for three time intervals back to the Early Cretaceous. These displacements were analyzed to determine the best-fitting rigid rotation, which could then be ascribed to true polar wander. The true polar wander so obtained is no larger than a few degrees and is within the magnitude of the uncertainties involved.
Disaccordance among the coordinates of the pole which are derived from several kinds of combination of the ILS stations suggests a possibility of secular changes in mean latitudes of the cooperative stations on + 39°8′ irrespective of the effect by the secular drift of the mean pole. Local drifts of — 0″.00156/year for Mizusawa and of + 0″.00105/year for Ukiah were derived from the analyses of the notable increases in the residual latitudes of the five ILS stations during the period 1900–65. Subtraction of the apparent motion of the mean pole due to the local drifts of the stations from the total motion makes the secular motion of the mean pole as 0″.00220/year in the direction 77°7 and this seems to be the real one.
Assuming lithospheric plates to be rigid, we systematically invert 68 spreading rates, 62 fracture zones trends and 10^6 earthquake slip vectors simultaneously to obtain a self-consistent model of instantaneous relative motions for eleven major plates. The inverse problem is linearized and solved iteratively by a maximum likelihood procedure. Because the uncertainties in the data are small, Gaussian statistics are shown to be adequate. The use of a linear theory permits (1) the calculation of the uncertainties in the various angular velocity vectors caused by uncertainties in the data, and (2) quantitative examination of the distribution of information within the data set.
The existence of a self-consistent model satisfying all the data is strong justification of the rigid plate assumption. Slow movement between North and South America is shown to be resolvable.
We then invert the trends of 20 linear island chains and aseismic ridges under the assumptions that they represent the directions of plate motions over a set of hot spots fixed with respect to each other. We conclude that these hot spots have had no significant relative motions in the last 10 My.
Integration of marine and terrestrial geophysical, paleontologic, paleomagnetic, and other historical geologic data and application of new quantitative techniques important in stratigraphic exploration necessitates an increasingly refined numeric geologic time scale. This paper is an attempt to combine a Cretaceous numeric time scale with biostratigraphic schemes and the geomagnetic-reversal scale. Basement ages at some Deep Sea Drilling Project sites are evaluated and related to the geomagnetic-reversal scale. Refs.
Application of geologic ages expressed in numeric time (millions of years) is a recent but rapidly expanding factor in petroleum exploration (rates of tectonic movements; basin models; geohistory analysis; prediction of reservoir quality, source potential, and HC maturation; correlation of geophysics with geologic events, etc. ). This paper proposes an emendation of published versions of the Jurassic time scale. The standard geochronologic subdivision is combined with a linear-numeric scale, with a biostratigraphic scheme (ammonites, calcareous nannofossils), and with the geomagnetic-reversal time scale. To this purpose, basement ages at some Deep Sea Drilling Project sites are evaluated and related to the geomagnetic-reversal scale. The numeric scale certainly cannot be regarded as precisely and firmly determined throughout the Jurassic, but it seems adequate as a broad framework. Refs.
A scheme of deep mantle convection is proposed in which narrow plumes of deep material rise and then spread out radially in the asthenosphere. These vertical plumes spreading outward in the asthenosphere produce stresses on the bottoms of the lithospheric plates, causing them to move and thus providing the driving mechanism for continental drift. One such plume is beneath Iceland, and the outpouring of unusual lava at this spot produced the submarine ridge between Greenland and Great Britain as the Atlantic opened up. It is concluded that all the aseismic ridges, for example, the Walvis Ridge, the Ninetyeast Ridge, the Tuamotu Archipelago, and so on, were produced in this manner, and thus their strikes show the direction the plates were moving as they were formed. Another plume is beneath Hawaii (perhaps of lesser strength, as it has not torn the Pacific plate apart), and the Hawaiian Islands and Emperor Seamount Chain were formed as the Pacific plate passed over this "hot spot." Three studies are presented to support the above conclusion. (1) The Hawaiian- Emperor, Tuamotu-Line, and Austral-Gilbert-Marshall island chains show a remarkable parallelism and all three can be generated by the same motion of the Pacific plate over three fixed hot spots. The accuracy of the fit shows that the hot spots have remained practically fixed relative to one another in this 100 m.y. period, thus implying a deep source below the asthenosphere. (2) The above motion of the Pacific plate agrees with the paleo-reconstruction based on magnetic studies of Pacific seamounts. The paleomotion of the African plate was deduced from the Walvis Ridge and trends from Bouvet, Reunion, and Ascension Islands. This motion did not agree well with the paleomagnetic studies of the orientation of Africa since the Cretaceous; however, better agreement with the paleomagnetic studies of Africa and of seamounts in the Pacific can be made if some polar wandering is permitted in addition to the motion of the plates. (3) A system of absolute plate motions was found which agrees with the present day relative plate motions (deduced from fault strikes and spreading rates) and with the present trends of island chains-aseismic ridges away from hot-spots. This shows that the hot spots form a fixed reference frame and that, within allowable errors, the hot spots do not move about in this frame.
Coordinates of the pole from the past ILS for the period 1900.0–1962.0 were reduced to the Conventional International Origin and published by us in 1969. Among them, data for 1941.0–1962.0 were revised owing to the revisions in their original values which were made and published recently by Nicolini and Cecchini.
Paleomagnetic data from the Pacific plate show that the average azimuth of the Pacific plate decreased by ~12° between 90 Ma and 81 Ma, whereas trends of Late Cretaceous seamount chains suggest that the Pacific plate moved northwestward relative to the hotspots with little change in azimuth during the same time interval. This difference is used to infer a 13° +/-4° shift of the spin axis relative to the hotspots in the Pacific Ocean basin. This shift occurred within 5 m.y. of the occurrence of anomalous behavior of the paleomagnetic field recorded in anomalies 33 and 34. this coincidence in timing suggests there may be a causal link between these three phenomena.
The study of the geomagnetic secular variation in prearcheological times is based upon investigations of the angular dispersion of paleomagnetic results and its variation with latitude. The westward drift of the geomagnetic field observed in historic times and apparently confirmed by archeomagnetic data back 1000 years or more suggests the way in which the present geomagnetic field may be analyzed for comparison with paleomagnetic results. Generalized models of paleosecular variation suppose that the angular dispersion arises from two contributions, one due to variations in the dipole field (dipole wobble) and one due to variations in the nondipole field. Attempts at distinguishing between these two contributions are inherently nonunique. To overcome this nonuniqueness, it has been proposed previously that the present low contribution made by the nondipole field to the total field in the Pacific region has persisted for the past 0.7 m.y. The low angular dispersions measured on Hawaii are then a direct measure of dipole wobble because of the so-called Pacific dipole window. However, recent evidence suggests that the data from lava flows for any single locality may be insufficient to measure paleosecular variation adequately. We have analyzed data for 2372 flows distributed over the earth's surface to see if even gross features in the secular variation can be resolved. Between-site angular dispersions, measured with respect to the geographic axis, have been averaged over 15° latitude strips for rocks with ages in the Holocene, Brunhes, and last 5 m.y. The too few data for the Holocene suggest that secular variation during this time has been lower than it has during most of the last 5 m.y. Although data for the Brunhes-aged lavas appear to be inadequate to give a completely reliable estimate of the latitudinal effects of secular variation, data from lava flows formed during the last 5 m.y. do appear sufficient. To overcome the nonuniqueness problem, we have proposed a model for paleosecular variation (model M) that fits both the paleomagnetic data and the variation expected from analysis of the present field and that is also compatible with current theories of the origin of secular variation. This results in an average dipole wobble of 9° over the past 5 m.y. The best fit to the nondipole dispersion then arises from a combination of two mechanisms for the origin of secular variation. The major contribution arises from a nondipole field that originates from some sort of interaction with the main poloidal field, probably akin to Hide's magnetohydrodynamic wave mechanism. The secondary contribution, about two-thirds the magnitude, arises from a fixed field whose intensity is independent of latitude compatible with Bullard's mechanism of fluid eddies interacting with the toroidal field near the core-mantle boundary. The two types of nondipole field are compatible with Yukutake's subdivision into drifting and standing parts, respectively, and provide a possible physical basis for his analysis. During a polarity transition the standing part of the nondipole field should predominate.
Comparison of plume traces with palaeomagnetic data from lithospheric plates for the past 50 m.y. suggests that the mantle has rolled about its own independent axis within an outer lithospheric shell which as a whole is fixed in respect to the Earth's spin axis.
Hot spots and subduction zones may control the position of the earth's
rotation axis. Paleomagnetic reconstructions in the hot spot framework
require a relative motion of about 10° between the hot spot
reference frame and the earth's spin axis since the early Tertiary. If
the viscosity of the mantle permits, a changing distribution of mass
anomalies can control the position of the earth's rotation axis as it
tracks the maximum principal axis of the perturbations. The present
plate geometry should give the best indication of what features are
associated with the controlling density anomalies. For the present,
geoid anomalies indicate that subducting slabs are major mass
contributions; removing these effects, a simpler `residual' geoid
remains with highs strongly correlated with hot spot locations. Although
neither the hot spots alone nor subducting slabs alone have maximum
principal axes near the present pole, adding the contributions of the
two gives a combined axis within a degree of the pole, suggesting that
these two effects control the location of the spin axis. To establish
whether changes in plate geometry could account for the observed polar
motion, the locations of subduction zones are reconstructed in the hot
spot framework for the early Tertiary, a time of significantly different
plate motions. For this reconstruction, the combined maximum principal
axis of the subductionhot spot systems is not at the present pole but
about 10° away in a location similar to the pole position required
by paleomagnetic studies. This suggests that changes in subduction
geometry may cause a shift in the position of the rotation pole and,
furthermore, that inertia tensor effects may be the link between plate
velocities and the spin axis.
Standard time series analysis tech- niques have been applied to the homogeneous polar motion data recently published by the ILS-IPMS (Yumi and Yokoyama, 1980) in order to study some of the more controversial features apparently possessed by the older ILS data. The magnitude and direction of the secular trend unbiased by the presence of harmonics in the data were deter- mined, yielding a rate of polar wander 43.52x10 -3 arc sec/yr (which extrapolates to 40.98 ø/m.y.) in direction 80.1øW longitude. The long-period Markowitz wobble, which dominates the retrograde power spectrum of the data, has a signal to noise ratio in that spectrum of 21:1; its period is well-determined as 31 years. Variations with time of the annual wobble and Chandler wobble were investigated using complex demodulation; the annual wobble was found to undergo relatively in- significant variations in amplitude and phase, in contrast to some analyses of the older ILS data, while the amplitude modulation and 19'25-1940 phase change of the Chandler wobble were re- confirmed.
All available paleomagnetic data for the last 130 m.y. meeting certain selection criteria are drawn from standard compilations and returned to precontinental drift site locations and orientations using spreading poles based on sea floor magnetic anomalies. The resulting data set is divided into six time periods, and global patterns of inclination and declination anomalies are used to identify important nondipole components. Spherical harmonic analysis is then performed to evaluate the corresponding coefficients. An axially symmetry quadrupole component (g20) is found to be important for at least the last 100 m.y., and an axially symmetric octupole component (g30) for the last 50 m.y. Possible correlations are noted between the variation with time of the reversal frequency of the geomagnetic field, compiled by A. Cox, the polarity bias published by Irving and Pullaiah, and our nondipole field magnetude. Both current explanations for the nondipole components seen in paleomagnetic data, the offset dipole model and Cox's zonal nondipole model, are incompatible with the large values of g30 found in this study. However, as a possible alternative, Cox's model can be generalized to accomodate the new information.
The internal consistency of early Tertiary reconstructions of the Pacific, Antarctic, and Indian plates has been tested by combining seafloor spreading data and paleomagnetic data. These tests show that substantial motion has taken place during the Tertiary across a fossil plate boundary located within one of these plates. We consider two alternative models to reconcile the available paleomagnetic and plate tectonic information. In the first model, East and West Antarctica were separate plates, the early Tertiary position of West Antarctica being one in which the Antarctic Pennisula is adjacent to the southwest border of South America. This model is consistent with the paleomagnetic data from Australia and Antarctica. In the second model the northern and southern parts of the Pacific plate have undergone left-lateral strike slip motion across a fossil plate boundary located northest of Chatham Rise. The second model is supported by paleomagnetic data from the northern and sourthern segments of the Pacific plate. The joint assumptions (1) that the hot spots are fixed and (2) that during the early Tertiary the present Pacific plate was two separate plates (the Chatham Rise plate and the north Pacific plate) are consistent with all of the paleomagnetic data and all of the geometrical constraints of early Tertiary plate reconstructions. The joint assumtions (1) that the hot spots are fixed and (2) that the present Antarctic plate was two early Tertiary plates produce unacceptable overlap and are not consistent with Pacific plate paleomagnetic data unless the Chatham Rise has moved independently of adjacent seafloor.
The proposal that the time-averaged paleomagnetic field is not strictly that of a geocentric axial dipole but that of an axial dipole displaced slightly northward from the geocenter is examined in terms of spherical harmonic expansions. Standard procedures for spherical harmonic expansions such as are applied to the present instantaneous geomagnetic field are not necessarily applicable to paleomagnetic data. A technique is proposed that enhances the time-averaging process that is a necessary part of determining the paleomagnetic field. This involves analyzing the inclination anomaly DELTA I around latitude strips to determine the zonal harmonics and the declination anomaly DELTA D around longitude sectors to determine the first nonzonal harmonics. The technique is applied to a carefully selected data set covering the last 5 m. y. All data are divided into normal or reversed sets providing 266 land-based points (171 normal, 95 reversed) and 100 deep-sea core results with inclination data only (50 normal, 50 reversed). These data demonstrate clearly that the time-averaged field is not simply that of a geocentric axial dipole and also that it is different for the normal and reversed fields. With respect to the present field the zonal harmonics of the time-averaged field are reduced less than the nonzonal harmonics, the indication being that westward and/or eastward drift of the nondipole field is dominant over 5 m. y. The magnitude of the second zonal harmonic suggests that on the average, paleomagnetic poles could be in error by about 3 degree when calculated by the usual geocentric axial dipole assumption. The data show clear asymmetries between the northern and southern hemispheres and possibly between the oceanic (Pacific) and continental hemispheres.
The average frequency of geomagnetic reversals seen through a sliding window 10 m.y. long typically remains constant or nearly so for intervals of the order of 5 × 107 years and then increases or decreases to another level. Frequency shifts of this type are known from paleomagnetic studies to have occurred at times 107, 86, and 45 m.y. B.P. and at many other times earlier in the earth's history. The most likely cause of the frequency shifts is a change in the topography of the core-mantle interface or changes in some physical property such as temperature along the interface. If such lateral variations at the base of the mantle exist, if they have changed with time, if they are large enough to create eddies or Taylor columns in the fluid motions of the core, and if these fluid motions affect the operating characteristics of the dynamo, then a simple model is at hand to account for the observed changes in reversal frequency. The generation of fluid eddies in the outer core by irregularities at the core-mantle interface would also probably induce variations in the nondipole field. These variations should be observable if they include departures of the nondipole field from the zonal symmetry expected solely from rotational effects. Among the candidates for such asymmetrical components of the field are the standing waves that are currently being reported by geomagneticians analyzing the historic field and the apparent northward shift of the main dipole currents of the core dynamo that is suggested by time-averaged global paleomagnetic data. On the basis of a reexamination of these data the conclusion is here drawn that the apparent northward displacement of the earth's main dipole moment is not the result of a standing zonal field but is rather the result of time averaging individual small excursions of the field caused by the westward drift of major features of the nondipole field. Paleomagnetic results indicate that these features of the nondipole field are not randomly distributed with respect to latitude, as is usually accepted. Instead, it appears that during times of normal polarity, flux usually emerges from the core in localized regions near the equator and reenters the core in localized regions located about 55° from either pole. During times of reversed polarity this entire pattern is reversed, suggesting that when the dipole field reverses, the toroidal field reverses also. The latitudes at which the large features of the nondipole filed are generated by convective cells or other types of fluid motion thus appear to be linked directly to the polarity of the dynamo. These findings are supportive of the reversal mechanism suggested by Parker and Levy.
Any topological framework requires the development of a theory of errors of characteristic and appropriate mathematical form. The paper develops a form of theory which appears to be appropriate to measurements of position on a sphere. The primary problems of estimation as applied to the true direction, and the precision of observations, are discussed in the subcases which arise. The simultaneous distribution of the amplitude and direction of the vector sum of a number of random unit vectors of given precision, is demonstrated. From this is derived the test of significance appropriate to a worker whose knowledge of precision lies entirely in the internal evidence of the sample. This is the analogue of 'Student's' test in the Gaussian theory of errors. The general formulae obtained are illustrated using measurements of the direction of remanent magnetization in the directly and inversely magnetized lava flows obtained in Iceland by Mr J. Hospers.
It is pointed out that at least 3 times in the last billion years the planet earth, for some unknown reasons, has repeatedly entered and left long regimes of repeated continental glaciation. An investigation is conducted concerning the problem of a temporal evolution of polar motion as a result of active glaciation and deglaciation. Analytic solutions are developed for a three-tier model which includes an elastic lithosphere, a viscoelastic mantle, and an inviscid core. The important role played by the elastic lithosphere in preserving a final state, which is not compensated isostatically, is demonstrated. The obtained results suggest the importance of including the forcing from the Antarctic ice sheet in order to obtain a better overall fit to the rotational data and possibly to remove the existing nonuniqueness in deriving mantle viscosity from rotational dynamics.
The Pacific plate's late Maastrichtian (approx 69Ma) palaeomagnetic pole, which constrains the N motion of the Pacific plate during the Cainozoic and latest Cretaceous, was studied. A recently proposed method for obtaining oceanic plate palaeomagnetic poles by combining dissimilar data was extended to accept, as input, the relative amplitudes of magnetic lineations with different azimuths or widely separated sites or both. Combining late Maastrichtian palaeomagnetic data - the relative amplitudes and skewness of magnetic lineations, palaeolatitudes from a palaeomagnetic study of basalt and sediment in vertical cores, a pole from the inversion of the magnetic anomaly over a seamount, and present locations of equatorial sediment facies - yielded a best fit pole of 71oN, 9oE and a 95% confidence ellipse with the major semiaxis of 6o striking 91o clockwise from N and the minor semiaxis of 2o striking 1o clockwise from N. This best fit pole, when compared to the pole expected if the hotspots have been fixed with respect to the spin axis, demonstrates that the hotspots in the Pacific Ocean have shifted approx 10o S with respect to the spin axis during the Cainozoic. This best fit pole, when compared to the best fit Campanian pole of the Pacific plate, demonstrates that the pole wandered rapidly, 1.1oMa-1, with respect to the Pacific plate during the latest Cretaceous.-Author.
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DUNCAN et al.
1 have analysed the igneous activity in the Central Volcanic province of Europe and the Thulean province of Iceland, the Faeroes and the British Isles on the assumption that these provinces represent plume traces. Assuming that plumes are fixed to the mantle and that plume traces are due to lithospheric plate motion only, Duncan et al. showed that the motion of the European plate over the past 50 m.y. is not in accord with palaeomagnetic data unless motion of the mantle with respect to the rotation axis (polar wandering) has occurred. Polar wandering of 23° along the 123° meridian was required since the early Tertiary.
We have tested the hypothesis that the apparent increase in the northward offset of the axial dipole (i.e. the ratio g⁰2/g⁰1) with age for the last 25 Ma (Wilson & McElhinny) is due to the failure to correct for plate motions. Spherical harmonic analyses were performed on two types of data, palaeomagnetic poles from continents, islands and seamounts, and magnetic inclinations from deep sea cores, after returning the sampling sites to their predrift locations in fixed hotspots coordinates. The results show that the quadrupole term g⁰2 maintained a value of about 0.05 g⁰1 throughout the last 35 Ma, and that the axial octupole had a small value of about 0.02 g⁰1 for the last 5 Ma. Sea core inclinations analysed separately gave essentially the same results as continental palaeopoles for the last few million years. Independent data sets for the past 2 Ma and for the period 2–6 Ma gave nearly identical solutions, showing that the persistence of the axial terms is not a phenomenon of the more densely sampled Quaternary field alone. Nor is it an artifice of predominantly northward motion of the plates: returning all 0–5 Ma data to a 5 Ma reconstruction failed to eliminate the quadrupole. The non-zonal coefficients for the 0–5 Ma field are typically an order of magnitude less than the zonal terms, with the exception of 4¹2, and are probably insignificant, suggesting that longitudinal drift is generally effective in averaging out these components.
The present distribution of data is inadequate to determine coefficients of third and higher degree prior to 5 Ma, but there is evidence that g⁰2 may have changed little during the last 35 Ma. In addition, there has been very little relative motion between the palaeomagnetic dipole axis, as defined by the first degree terms, and the axis of the hotspots reference frame.
Analysis of palaeomagnetic data from the USSR, which Khramov and Sholpo have separated into normal and reversed mean data, reveals that there is considerable and obvious dissimilarity between normal and reversed regimes of the geomagnetic field source.
These new data also strengthen the case for a world-wide eastward declination of the geomagnetic field during normal regimes, and westward (west of south) declination during reversed regimes. The question of how such fields could be maintained is discussed.
Investigation of collected palaeomagnetic results from continental igneous rocks and from oceanic sediment-cores, shows a persistent off-centre displacement of the effective dipole source of the main field over Quaternary and Recent times (about the past two million years). This dipole displacement is 191±38 km northward along the rotational axis. Further evidence suggests a similar enduring displacement during all of Upper Tertiary time. The effect of this result on palaeomagnetic interpretations is discussed. The evidence also suggests that we might search for significant differences between the time-average normal and reversed field configurations, and for other internal manifestations of the north-south asymmetry.
A consistently world-wide eastward declination of 3.3±1.0° has been found for the time-average Upper Tertiary and later field, but no satisfactory explanation has been proposed.
A re-evaluation of the existence of true polar wander (TPW) since the Late Cretaceous and a comparison among the various approaches are made using updated paleomagnetic, hotspot and relative motion datasets. Previous attempts to determine the existence of TPW had resulted in different conclusions: comparison of hotspot locations and paleomagnetic poles required significant pole motion, although lithospheric plate displacement analysis yielded insignificant motion. However, these earlier determinations cannot be directly compared to find the reason for the discrepancies, because each used different datasets. For this study the different approaches are applied to a single updated model with three alternative relative motions of East and West Antarctica. Although the results are model-dependent, in general there was not significant motion of the pole relative to the lithosphere (1–5°) since the early Tertiary, but a large motion (10–12°) relative to the hotspot framework. It is unlikely that errors in the determinations could account for this disagreement: the A95 of the plate reconstruction is about 3°, the uncertainty in Antarctica motion is estimated to no larger than 3°, and cumulative errors in the relative plate motions may also amount to 3°. Only if all these errors are present in the maximum estimated amount, and in the same direction, could they account for the 10–12° gap between the two approaches. This conclusion of pole motion relative to the hotspots, but not the lithosphere, may indicate an independent shift of the mesosphere relative to the lithosphere (or “mantle roll” of Hargraves and Duncan).
Pacific plate equatorial sediment facies provide estimates of the northward motion of the Pacific plate that are independent of paleomagnetic data and hotspot tracks. Analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the Hawaiian-Emperor chain. The discrepancy is largest 60–70 Ma B.P.; the 60- to 70-Ma equatorial sediment facies data agree with recent paleomagnetic results from deep-sea drilling on Suiko seamount [1] and from a northern Pacific piston core [2]. Equatorial sediment facies data and paleomagnetic data, combined with K-Ar age dates along the Emperor chain [3], indicate a position of the spin axis at 65 Ma B.P. of 82°N, 205°E in the reference frame in which the Pacific Ocean hotspots are fixed. This pole agrees well with the position of the spin axis in the reference frame in which the Atlantic Ocean hotspots and the Indian Ocean hotspots are fixed [4,5], supporting the joint hypotheses that (1) the Pacific Ocean hotspots are fixed with respect to the hotspots in other oceans, (2) the hotspots have shifted coherently with respect to the spin axis, and (3) the time average of the earth's magnetic field 65 Ma B.P. was an axial geocentric dipole. Global Neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the Neogene in the same direction as the shift between 65 Ma B.P. and the present. All data are consistent with a model in which the hotspots (and by inference the mantle) have shifted with respect to the spin axis about a fixed Euler pole at a constant rate of rotation for the last 65 Ma.
The geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time; that is, motion of an order of magnitude less than the relative motion between plate pairs. In most cases it is concluded that inter-hotspot movement cannot be discerned for the period 100 m.y. to Present and that widely distributed hotspots in the Atlantic and Indian Oceans provide a frame of reference for plate motions following the disintegration of Gondwanaland, which is independent of paleomagnetism. This frame of reference is “absolute” in that it gives the motion of the lithosphere with respect to the mantle (= hotspots). The absolute motion model indicates that Africa and Antarctica are now moving only very slowly, that there has been significant relative movement between East and West Antarctica since the Cretaceous, and prescribes the relative motion between the Somali and African plates.
Many hotspot tracks appear to become the locus of later rifting, as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines. Examples are the west coast of Greenland-east coast of Labrador (Madeira hotspot), the south coast of Mexico-north coast of Honduras (Guyana hotspot), and the south coast of West Africa-north coast of Brazil (St. Helena hotspot). A modern day analog of a possible future rift is the Snake River Plain, where the North American continent is being “pre-weakened” by the Yellowstone hotspot track.This conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years. The relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots. However, once the motion of this one plate is chosen, the motions of all the other plates are prescribed by the relative motion constraints.In addition to the correlation between the predicted tracks and sites of later continental breakup, exposed continental shields correlate with the tracks. Their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover.
Assuming lithospheric plates to be rigid, we systematically invert 68 spreading rates, 62 fracture zones trends and 10^6 earthquake slip vectors simultaneously to obtain a self-consistent model of instantaneous relative motions for eleven major plates. The inverse problem is linearized and solved iteratively by a maximum likelihood procedure. Because the uncertainties in the data are small, Gaussian statistics are shown to be adequate. The use of a linear theory permits (1) the calculation of the uncertainties in the various angular velocity vectors caused by uncertainties in the data, and (2) quantitative examination of the distribution of information within the data set.
The existence of a self-consistent model satisfying all the data is strong justification of the rigid plate assumption. Slow movement between North and South America is shown to be resolvable.
We then invert the trends of 20 linear island chains and aseismic ridges under the assumptions that they represent the directions of plate motions over a set of hot spots fixed with respect to each other. We conclude that these hot spots have had no significant relative motions in the last 10 My.
Earth polar wanderings attributed to rotation axis angular displacements generated by density redistribution on geologic time scale
Referential Data for the U
- A N Khramov
- Paleomagnetic Directions
- Khramov
Palaeomagnetic indications of a permanent aspect of the non-dipole field
- R L Wilson
- J M Palaeogeophysics
- Wilson