The palaeomagnetism of Jersey volcanics and dykes, and the Lower Palaeozoic apparent polar wander path for Europe

Summary. Stable natural remanent magnetization (NRM) in the Jersey Volcanics and in a single rhyolite dyke was probably acquired during the Cambrian before folding of the volcanics in the Cadomian Orogeny. After dip correction, the volcanics yield a palaeomagnetic pole at 323° E, 52° N ( dp = 33°, dm = 35°). In Jersey dolerite dykes three groups of stable NRM directions are recognized, with palaeomagnetic poles at 248° E, 26° N ( dp = 10°, dm = 20°), 339° E, 1° S ( dp = 9°, dm = 12°), and 336° E, 31° S ( dp = 5°, dm = 9°). Comparison with the European apparent polar wander path implies that stable NRM in these groups was acquired respectively during Late Precambrian or early Cambrian, Siluro-Devonian and middle Carboniferous time. The stable NRM of the Jersey lamprophyre dykes yields a palaeomagnetic pole at 322° E, 16° N ( dp = 31°, dm = 38°) and is probably of Silurian or Devonian age.
These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole.     

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.     

Palaeomagnetism of the Cretaceous Pirgua Subgroup (Argentina) and the age of the opening of the South Atlantic

Palaeomagnetic data for the Cretaceous Pirgua Subgroup from 14 different time units of basalts and red beds exposed in the north-western part of Argentina (25° 45' S 65° 50' W) are given.
After cleaning all the units show normally polarized magnetic remanence and yield a palaeomagnetic pole at 222° E 85° S ( d Φ= 7°, d χ= 10°).
The palaeomagnetic poles for the Pirgua Subgroup (Early to Late Cretaceous, 114–77 Myr), for the Vulcanitas Cerro Rumipalla Formation (Early Cretaceous,<118 Myr, Valencio & Vilas) and for the Poços de Caldas Alkaline Complex (Late Cretaceous, 75 Myr, Opdyke & McDonald) form a 'time-group' reflecting a quasi-static interval (mean pole position, 220° E 85° S, α₉₅= 6°) and define a westward polar wander in Early Cretaceous time for South America.
Comparison of the positions of the Cretaceous palaeomagnetic poles for South America with those for Africa suggests that the separation of South America and Africa occurred in late Early Cretaceous time, after the effusion of the Serra Geral basalts.
The K-Ar ages of basalts of the Pirgua Subgroup (114 ± 5; 98 ± 1 and 77 ± 1 Myr) fix points of reference for three periods of normal polarity within the Cretaceous palaeomagnetic polarity column.     

Late Mesozoic and Cenozoic palaeomagnetism of Australia–I. A redetermined apparent polar wander path

Summary. Palaeomagnetic measurements have been carried out on one Cretaceous and five Cenozoic sedimentary sequences of Australia; of those, one is in the Carnarvon Basin of north-western Australia, and the others are in the Otway Basin of south-eastern Australia. The new results are used together with those published previously to define a revised late Mesozoic–Cenozoic apparent polar wander path for Australia. This path differs from earlier, basalt-derived paths by the absence of a large westward excursion and zig-zag irregularities. It is characterized instead by a substantially straight Cenozoic trajectory, a sharp bend in the Late Cretaceous, and a non-uniform rate of apparent polar wander.
The early Tertiary segment of the new path lies north of the original paths, thus eliminating a discrepancy that has been noted previously between the Indian and Australian palaeomagnetic data (Luyendyk & Rennick, Peirce and Klootwyk & Peirce). A re-examination of the original data suggests that this discrepancy, as well as two others in the Australian results, may have been caused principally by incomplete time-averaging of remanence directions, because the volcanics on which the results were based had been extruded episodically.     

西南极菲尔德斯半岛白垩纪晚期和第三纪早期岩石的古地磁学及其大地构造意义

采自菲尔德斯半岛白垩纪晚期和第三纪早期五个岩层单位的12个采点109块定向岩芯标本进行了系统的古地磁学测定,从中得知,55～45Ma时期,该区极位置与澳大利亚的同时期结果不同,它经历了大约20°～30°的南向水平移动与70°～80°的西向旋转,逐渐地构成了今日彼此相对位置的格架。文章绘制出南极洲的视极移曲线。     

PALEOMAGNETISM OF THE LATE CRETACEOUS AND EARLY TERTIARY ROCKS FROM FILDES PENINSULA,WEST ANTARCTICA, AND ITS GEOTECTONIC SIGNIFICANCE

The paleomagnetis m of 109 oriented samples collected from drill cores through 5 rock units of Late Cretaceous and Early Tertiary on Fildes Peninsula were systematically studied. According to the study, the paleomagnetic pole position of this area is different from the position of Australia during the 55-45 Ma period. This means that when the break-up of the Gondwanaland at 55-45 Ma ago, Australia was separated from the Gondwanaland, drifting southward 20°-30°Lat. and rotating 70°-80°westward and then gradually arriving at recent position. The paleolatitudinal data indicate that it is not impossible that the area studied was covered with land glacier at that time. The apparent polar wander path of Antarctica through the geological time are also roughly worked out.     

Palaeomagnetism of some Late Cretaceous and Miocene igneous rocks on Jamaica

Summary. In order to contribute to the resolution of the problem of the plate tectonic character of the Caribbean, a palaeomagnetic study has been carried out on some Jamaican igneous rocks. Sixteen Late Cretaceous intrusives and lavas and one Late Miocene lava sampled in five sites have been investigated. Because of widespread maghaemitization of the predominantly large-grained deuteric class 1 titanomagnetites, some difficulty was experienced in identifying stable directions of magnetization in the Cretaceous rock units. Using thermal demagnetization technique, two distinct directions of magnetization were obtained, significantly different from those observed in contemporaneous North American rocks. Nine units yield a palaeomagnetic pole at 143.8°W, 44.1°N, referred to as 'normal', while seven units yield'equatorial'poles situated both east and west of Jamaica. It is not possible to decide which of the two directions of magnetization reflects the Late Cretaceous geomagnetic field in Jamaica, but from other evidence they appear to merit palaeotectonic interpretation. They are consistent with the plate tectonic behaviour of the Caribbean since Late Cretaceous and its motion from the southwest relative to the present day frame of reference, with a simultaneous large anticlockwise horizontal rotation of Jamaica. Late Miocene lavas, containing high-coercivity magnetic material, yield closely grouped directions giving a palaeopole at 152.4°W, 73.3°N, supporting the deductions made from the Cretaceous data.     

A weighted least-squares fit of the Australian apparent polar wander path for the last 100 Myr

Summary. Recent versions of the Australian apparent polar wander path (APWP) for the late Mesozoic and Tertiary show considerable variation. Re-examination of the Australian igneous data suggests that they are more reliable than assumed by some recent authors. The trajectory of the Australian APWP is defined by fitting the position of a set of poles including both igneous and laterite/overprint data. This allows the dated igneous poles to be used to determine age as a function of distance along the trajectory. Both the trajectory and the age are fitted by means of weighted least-squares regression, and are given approximate confidence limits.
Age is best fitted in the Australian case as a linear function of distance along the APWP. This result contrasts with that of Idnurm, who suggested a variable rate of polar wander during the Tertiary. The new APWP is in better agreement with hot-spot data. Dating of New Caledonian laterites by the new APWP gives a result consistent with geological evidence, while dating by reference to Idnurm's path does not. Large non-dipole components or significant true polar wander are not needed to explain the Australian Tertiary APWP.     

The significance of the pdaeomagnetism of Jurassic—Cretaceous rocks from South America: predrift movements, hairpins and magnetostratigraphy

Summary. In this paper we show that: (1) The positions of the Cretaceous palaeomagnetic poles (PP) for South America and Africa exhibit elongated distributions that are due to rapid movement of these continents from the south pole.
(2) The positions of the Middle—late Jurassic virtual geomagnetic poles for South America exhibit an elongated distribution along the meridians 20–200° E; it is suggested that this is due to a rapid shift of South America in Middle—late Jurassic time.
(3) The late early—early late Cretaceous sections of the apparent polar wandering paths for South America and Africa are consistent with South Atlantic seafloor spreading data.
On the basis of the comparison of the reliable late Palaeozoic—late Cretaceous PPs for South America and Africa, taking into account the restrictions established by geological, palaeontological and seafloor spreading data, it is suggested that minor movements could have occurred within Western Gondwana in middle—late Jurassic time along a narrow zone which later became the South Atlantic divergent boundary.
Four 'hairpins' are defined in the late Palaeozoic—late Cretaceous section of the apparent polar wandering path for South America; the two youngest of these can be correlated with the origin of the South Atlantic Ocean basin and the onset of the Andean Orogeny, respectively.
The magnetostratigraphy for the Serra Geral lava flow sequence suggests that some of these flows were poured out rapidly without significant interruption.     

A sea-level test for inertial interchange true polar wander events

The possibility of inertial interchange true polar wander (IITPW) events, in which the rotation pole moves 90° with respect to the solid Earth in a matter of ∼10  Myr, has been discussed in the geophysical literature for more than three decades. Recent evidence for an IITPW event in Early Cambrian time has renewed interest in the issue; however, the veracity of supporting palaeomagnetic evidence remains a matter of significant debate. We propose that sea-level variations driven by polar wander provide an important independent test for the occurrence of IITPW events. Our numerical simulations of the response of a viscoelastic planet to an IITPW-induced forcing predict sea-level changes of up to 200  m, depending on the details of the earth model, the location of the site relative to the rotation path and the elapsed time for the reorientation of the pole. A preliminary comparison of our predictions to Early–Middle Cambrian sea-level records for Australia, Laurentia and Baltica shows qualitative agreement. This comparison suggests that a definitive test for the Cambrian IITPW hypothesis is possible given a sufficiently accurate, and globally distributed, database of sea-level histories.     

A palaeomagnetic study of fracture fills in the Holy Cross Mountains of Central Poland and its application in dating tectonic processes

Calcite and sedimentary fills in fractures cutting the Upper Devonian carbonates in the Holy Cross Mountains (HCM) were dated palaeomagnetically by comparison with the apparent polar wander path (APWP). Haematite-bearing calcite possessed well-defined components of natural remanent magnetization (NRM), which were preserved under thermal demagnetization to temperatures of approximately 500 °C, when specimens disintegrated. Although not completely demagnetized, some specimens revealed a stable NRM component before destruction, thus making a component analysis possible. Five components were determined using density point distribution and cluster analysis. One has a mean that is similar to the present-day local geomagnetic vector. The remaining four components yielded palaeomagnetic poles located at: A (70.3°S, 5.5°E), B (71.3°S, 31.2°E), C (48.7°S, 351.0°E, virtual geomagnetic pole), and D (11.6°S, 312.3°E). Antipodal polarities found in the fracture fills, together with dissimilarities in magnetization found in calcite and hosting carbonates, indicate the lack of simultaneous remagnetization, and different times of remanence acquisition for the rocks under comparison. Taking both palaeomagnetically inferred palaeolatitudes and regional tectonics into consideration, a Mesozoic (Cretaceous?) age is estimated for palaeopoles A and B, a Permian age for pole C, and a Carboniferous age for pole D. These age determinations are in line with the calcite ages estimated from isotopic studies. A comparative palaeomagnetic study performed on a well-dated Upper Devonian neptunian dyke of limestone and a Lower Triassic clastic vein yielded virtual geomagnetic poles (VGPs) close to the APWP for Baltica. Generally, the remanence from fracture fills may be useful for dating related tectonics, karst phenomena and mineralization processes.     

Palaeomagnetism of granitic intrusives from the Precambrian basement under eastern Kansas: orienting drill cores using secondary magnetization components

Summary. The Precambrian basement under east-central Kansas was drilled at two circular aeromagnetic positives, one at Osawattamie and one at Big Springs. The core retrieved from these sites is a coarse to medium grained granite which has been dated by U-Pb to be 1350 Ma old. The palaeomagnetism of these azimuthally unoriented cores was studied to see if a technique which uses low-coercivity, low-temperature magnetization components to orient the cores would allow an independent confirmation of the core's mid-Proterozoic age. Orthogonal projection plots of the alternating field (af) and thermal demagnetization data show that the magnetization of these cores is relatively simple, having only two components: a low-temperature, low-coercivity magnetization with steep positive inclinations and a shallow, negative inclination characteristic magnetization for the Osawattamie core or a positive, moderate inclination characteristic magnetization for the Big Springs core. If the declination of the low-temperature, low-coercivity component is aligned parallel to the present field declination, the characteristic directions may be azimuthally oriented. This allows the calculation of palaeomagnetic poles for the Big Springs core (lat. = 4.5°S, long. = 29.9°E) and the Osawattamie core (lat.= 20.2°N, long. = 39.3°E) which are consistent with Irving's apparent polar wander path for Laurentia at about 1300–1400 Ma. Comparison of anhysteretic remanent magnetization (ARM), viscous remanent magnetization (VRM), and isothermal remanent magnetization af demagnetization curves with a natural remanent magnetization (NRM) demagnetization curve suggests that the Osawattamie core probably acquired a piezoremanent magnetization (PRM) parallel to the core axis during drilling.     

Late Mesozoic and Cenozoic palaeomagnetism of Australia – III. Bias-corrected pole paths for Australia, Antarctica and India

Summary. Palaeomagnetic results from Part I of this study and their analysis in Part II are combined to eliminate bias from the Cenozoic apparent polar wander path for Australia – a bias due to non-dipole components in past geomagnetic fields or, for poles calculated from hot-spot data, due to the motion of hot spots relative to the Earth's rotational axis. This path is extended in approximately bias-free form to the late Mesozoic, and indicates a significant change in the drift direction of the continent between 26 and about 60 Ma.
The bias-corrected Australian path is used, first, with seafloor spreading data for the Southern Ocean to derive a corresponding late Mesozoic–Cenozoic pole path for Antarctica. The latter shows that the Antarctic drift direction reversed in the early Tertiary. It is suggested that the early Tertiary directional changes of both Australia and Antarctica are part of a global reorganization of plates during the Eocene, postulated by Rona & Richardson, Cande & Mutter and Patriat & Achache.
Next, the Australian path is compared with hot-spot data from the African and Australian plates, indicating a movement of the hot spots relative the Earth's rotational axis during the Cenozoic. The direction of this movement is found to be consistent with previous results from other parts of the world.
Finally, the Australian path is used together with non-dipole components in the geomagnetic field to explain a prominent westward displacement of the mid- and late Cenozoic poles of India relative to those of Australia.
Because of uncertainties in the original poles and in the analysis, the present results are likely to contain appreciable errors. Nevertheless, their consistency with independent findings supports the dipole-quadrupole model of Part II for mid- and late Cenozoic geomagnetic fields.     

Palaeomagnetism of limestones from the Gargano Peninsula (Italy), and the implication of these data

The platform limestones of Apulia are usually too weakly magnetic for precise measurement. The East Gargano basin is an autochthonous extension of Apulia and incorporates deeper water limestones which, though weakly magnetic ( J NRM ≃ 50 nG), can be reliably measured using a cryogenic magnetometer. The magnetization is attributed to the presence of detrital magnetite and the pelagic limestones yield a mean magnetic direction for the Late Cretaceous (Dec. = 335°, Inc. = 38°, α₉₅= 6.5°). The circle of confidence associated with this direction overlaps with those associated with Late Cretaceous magnetic directions from Iblei (Sicily) and from the Vicentinian (Southern) Alps. Palaeomagnetic pole positions for Iblei, Gargano/Apulia and the Southern Alps indicate that these three autochthons need not have rotated significantly relative to each other since the Late Cretaceous. An inferred Late Cretaceous pole position for Africa coincides with the pole positions obtained from these Italian data.     

Palaeomagnetism and Rb-Sr ages of the Ratagan and Comrie intrusions

Summary. Study of the palaeomagnetism of two complexes from the Newer Granite Suite in Scotland, at Ratagan (NW Highlands) and Comrie (central Highlands), reveals the variable nature of the natural remanence encountered in granodioritic intrusions and the surrounding metamorphic country rock. Forty-eight specimens from Ratagan, dated at 415 ± 5 Ma, gave a mean direction: D = 8°, I =−32°, and a palaeomagnetic south pole: 15°S, 346°E (δ p = 5°, δ m = 9°). Twenty-eight specimens from Comrie, dated at 408±5 Ma, gave a mean direction: D = 75°, I =−30°, and a palaeomagnetic south pole: 6°S, 287°E (δ p = 4°, δ m = 7°). These results have been compared with the established apparent polar wander path (APWP) for Britain. The Ratagan pole improves the reliability of the APWP but doubt remains as to whether the primary magnetization from Comrie represents a true late Silurian direction or whether it has been affected by post-cooling rotation, possibly associated with the nearby Highland Boundary Fault.     

Palaeomagnetic and K—Ar age studies of a 6 km-thick Cretaceous section from the Chilean Andes

Summary. Thirty-six palaeomagnetic sampling sites distributed within 6000 m of dominantly andesitic flows and tuffs of Cretaceous age from the La Serena area, Chile confirm the normal polarity bias of the Cretaceous period. Af, thermal and limited chemical demagnetization techniques have been used in testing the stability of the remanent magnetization isolated in samples from these sites. A positive fold test in the Quebrada Marquesa Formation, the second lowest in the stratigraphic pile, confirms that the magnetization isolated is pre-Tertiary in age. Ages calculated by the K–Ar whole rock method however, appear to have been variably up-dated probably due to argon loss caused by Cretaceous–Tertiary intrusives. Thermal and hydrothermal effects of these intrusions have probably reset the magnetization in the youngest formation of the volcanic pile. A composite palaeomagnetic pole calculated from the 30 site poles of the three lower formations (209° E, 81° S, A₉₅= 4½°), is in good agreement with mid to Late Cretaceous poles derived from rock units of the stable platform of South America. The use of Andean–Caribbean palaeomagnetic data however, to resolve small time-dependent polar shifts within the Cretaceous and thus to estimate the time of opening of the south Atlantic is questioned. Many of the Andean–Caribbean Cretaceous poles appear to have been affected by local tectonic rotation.     

Palaeomagnetism of middle Proterozoic (c. 1.25 Ga) dykes from central North Greenland

From a nunatak in central North Greenland (81.5°N, 44.7°W) nine sites of Middle Proterozoic basic dykes, cutting Archaean basement, were palaeomagnetically investigated. After AF and thermal cleaning the nine dyke sites and three adjacently baked gneiss sites give a stable characteristic remanent mean direction of D = 265°, I = 21.5° ( N = 12, α ₉₅= 5.6°), the direction being confirmed by a detailed and positive baked contact test.
The polarity of the dykes in the nunatak area is opposite to that of the Zig-Zag Dal Basalts and the Midsommersø Dolerites in eastern North Greenland some 200–300 km away, the volcanics of which are assumed to be of similar age (about 1.25 Ga). The remanent directions of the two sets of data are antiparallel within the 95 per cent significance level of confidence.
When rotating Greenland 18° clockwise back to North America by the 'Bullard fit', the pole of the central North Greenland dolerites (NDL) falls at (14.3°N, 144.3°W). The reversed pole (14.3°S, 35.7°E) fits well on to the loop between 1.2 and 1.4 Ma on the apparent polar wander swath of Berger & York for cratonic North America.
The palaeomagnetic results from the Middle Proterozoic basic dykes from central North Greenland thus strengthen previous palaeomagnetic results from the Midsommersø Dolerites and Zig-Zag Dal Basalts from the Peary Land Region in eastern North Greenland, suggesting that Greenland was part of the North American craton at least for the period between c . 1.3 and 1 Ma (and probably up to the end of Cretaceous time). The major geographical meridian of Greenland was orientated approximately E–W, and the palaeo-latitude of Greenland was about 10°–15°.     

Palaeomagnetism and the age of the Cracow volcanic rocks (S Poland)

Permian rhyodacites, melaphyres and tuffs from the Cracow area (South Poland) were sampled for the palaeomagnetic and isotope studies. Single-grain U-Pb dating of most zircon grains separated from the rhyodacites gave mean age of magma emplacement of 294.2 ± 2.1 Ma. Some zircons, however, displayed younger ages (268.7 ± 3.4 Ma), probably related to the metasomatic alterations of these rocks. Two Permian components of magnetizations related to these processes were isolated and together with previously defined Late Carboniferous–Permian palaeomagnetic poles from South Poland were used for construction of the regional apparent polar wander path (APWP). The Early Permian segment of this APWP shows a certain departure from the coeval part of the Fennoscandian APWP due to anticlockwise rotations of studied rocks most probably caused by mid-Permian sinistral tectonic movements along reactivated prominent Variscan faults of Central Europe. This sense of tectonic mobility does not support the hypothesis about transformation from Pangea 'B' to Pangea 'A' along an intra-Pangea dextral megashear during the Permian. Older than previously assumed ages of the post-Variscan igneous rocks of Central Europe reduce overlap of Gondwana's and Laurussia's parts of the Early Permian Pangea 'A'.     

Palaeomagnetism of Lower Cambrian sediments from the Olenek River section (northern Siberia): palaeopoles and the problem of magnetic polarity in the Early Cambrian

New palaeomagnetic data from the Lower and Middle Cambrian sedimentary rocks of northern Siberia are presented. During stepwise thermal demagnetization the stable characteristic remanence (ChRM) directions have been isolated for three Cambrian formations. Both polarities have been observed, and mean ChRM directions (for normal polarity) are: Kessyusa Formation (Lower Cambrian) D = 145°, I = -40°, N = 12, α₉₅= 12.8°; pole position: φ= 38°S, A = 165°E; Erkeket Formation (Lower Cambrian, stratigraphically highly) D = 152°, I = - 47°, N = 23, α₉₅= 6.8°; pole position: φ= 45°S, A = 159°E; Yunkyulyabit-Yuryakh Formation (Middle Cambrian) D = 166°, I = - 33°, N = 38, α₉₅= 4.6°; pole position: φ= 36°S, L = 140°E. These poles are in good agreement with the apparent polar wander path based on the bulk of existing Cambrian palaeomagnetic data from the Siberian platform. In Cambrian times, the Siberian platform probably occupied southerly latitudes stretching from about 35° to 0°, and was oriented 'reversely' with respect to its present position. Siberia moved northwards during the Cambrian by about 10° of latitude. This movement was accompanied by anticlockwise rotation of about 30°. The magnetostratigraphic results show the predominance of reversed polarity in the Early Cambrian and an approximately equal occurrence of both polarities in the part of the Middle Cambrian studied. These results are in good agreement with the palaeomagnetic polarity timescale for the Cambrian of the Siberian platform constructed previously by Khramov et al. (1987).     

Effect of the viscoelastic lithosphere on polar wander speed caused by the Late Pleistocene glacial cycles

Previous studies of the wander of the rotation pole associated with the Late Pleistocene glacial cycles indicate that the predicted polar wander speed is sensitive to the density jump at the 670 km discontinuity, the thickness of the elastic lithosphere, and the lower mantle viscosity. In particular, the M1 mode related to the density jump at 670 km depth has been shown to contribute a dominant portion of predicted polar wander speed for sufficiently small lower mantle viscosities. In this study, we examine the sensitivity of polar wander to variations in the viscosity of the viscoelastic lithosphere using simplified compressible Maxwell viscoelastic earth models. Model calculations for earth models with a viscoelastic lithosphere of finite viscosity indicate that the contribution of the M1 mode is similar to those associated with the density discontinuity at the core–mantle boundary (C0 mode) and the lithosphere (L0 mode). We speculate that this is due to the interaction between the M1 mode and the transient mode associated with the viscoelastic lithosphere, which reduces the magnitude of polar wander rates. Therefore, the M1 mode does not contribute a dominant portion of the predicted polar wander speed for earth models with a viscoelastic lithosphere of finite viscosity. In this case, predictions of polar wander speed as a function of lower mantle viscosity exhibit the qualitative form of an 'inverted parabola', as predicted for the J ˙₂ curve. We caution, however, that these results are obtained for simplified earth models, and the results for seismological earth models such as PREM may be complicated by the interaction between the M1 mode and the large set of transient modes.