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 Permian Volcanic Rocks of Moissey (French Jura): Implications For the Palaeofield and Tectonic Evolution

Rocks from the Massif de la Serre in the French Jura (latitude: 47.3°N longitude: 5.6°E) belonging to an ignimbritic assemblage dominated by vitrophyric rhyolites, and whose age of formation is probably Permian (Autunian to Saxonian) have been studied by applying thermal and alternating field demagnetization. the characteristic magnetization has a mean direction derived from 89 samples of D= 170°, I = - 16°, k = 26.2°, α₉₅= 3° and a corresponding north palaeopole at 41°N, 172°E, A ₉₅= 5°. the pole, which is very close to the Permian European poles, can thus be considered as a new contribution. Some samples are found to carry a unique normal polarity magnetization, others carry both normal and reverse polarities. It therefore seems that, similar to Permian series in the USSR, these west European rocks have registered a normal event in the Kiaman interval. From a structural point of view, we may conclude that during the Alpine tectonic phases the Massif de la Serre has not been subjected to substantial rotation.     

The palaeomagnetism of Cambro-Ordovician redbeds, the Erquy Spilite Series and the Trtégastel-Ploumanac'h granite complex, Armorican Massif (France and the Channel Islands)

Summary. In addition to a component (A) of recent origin, two NRM components are distinguished in the Cambro-Ordovician redbeds of the Armorican Massif. In most sites other than those from northern Brittany the oldest (C) is probably Silurian or early Devonian, and is mainly carried by specularite with high blocking temperatures. This component was variably overprinted by a Devonian or early Carboniferous component (B3) which was probably acquired as a viscous PTRM on uplift after burial, and is carried by hematite pigment with intermediate to high blocking temperatures. In the red succession of Plourivo-Bréhec (northern Brittany) declination scatter of two intermediate to high blocking temperature components (B1 and B2) is consistent with clockwise rotation of the bulk of Europe during the late Carboniferous, implied independently by published European Carboniferous palaeomagnetic data.
Stable NRM in the Erquy Spilite Series yields a palaeomagnetic pole at 344° E, 35° N ( dp = 21°, dm = 22°), and was probably acquired during remagnetization following Late Precambrian or early Cambrian folding. This is consistent with a middle to late Cambrian age of remagnetization estimated by comparison with other poles of known age.
A palaeomagnetic pole position at 332° E, 34° S ( dp = 4°, dm = 7°) determined for the Hercynian Trégastel-Ploumanac'h complex is consistent with other middle to late Carboniferous poles from elsewhere in Europe.     

Palaeomagnetic study of Palaeoproterozoic granitoids from the Voronezh Massif, Russia

A palaeomagnetic investigation has been carried out of rocks from the eastern part of the Voronezh Massif, which constitutes, together with the Ukrainian Shield, the Sarmatian segment in the southern part of the East European Craton. The samples were collected in a quarry close to the town of Pavlovsk (50.4°N, 40.1°E), where a syenitic-granitic body intrudes Archaean units. U–Pb (zircon) dating has yielded an age of 2080  Ma for the intrusion.
  Two characteristic magnetic components, A and B, were isolated by thermal and alternating-field demagnetization. Component A was obtained from granites and quartz syenites (11 samples) and has a mean direction of D = 229°, I = 28°, and a pole position at 12°N, 172°E. This pole is close to a contemporary mean pole (9°N, 187°E) for the Ukrainian Shield, which implies that the Voronezh Massif and the Shield constituted a single entity at 2.06  Ga. These poles differ from contemporaneous poles of the Fennoscandian Shield, indicating that the relative positions of the two shields were different from their present configuration about 2100  Myr ago.
  A component B, isolated only in quartz monzonites (five samples), has a mean direction D = 144°, I = 49°, and a pole position at 4°N, 251°E, which is close to late Sveconorwegian (approximately 900  Ma) poles for Baltica. This suggests that the East European Craton was consolidated some time between 2080 and 900  Ma. Comparison with other palaeomagnetic data permit us to narrow this time span to 1770–1340  Ma.     

Palaeomagnetism of Upper Cretaceous—Lower Tertiary igneous rocks from Central Argentina

Summary. From nine Upper Cretaceous—Lower Tertiary (85 ± 5–66 ± 5 Ma) volcanic hills in Central Argentina (33°S, 65°W), 26 hand samples were collected yielding a palaeomagnetic pole at 45°E 70°s ( A ₉₅ = 12.1°; k = 13.6; N = 12) after AC cleaning. Three sites show normal and nine reversed polarity. This pole is close to the pole for the late Cretaceous (69 Ma) Andacolo Series.     

Palaeomagnetic results from two basic volcanic formations in the western Himalayas and a Phanerozoic polar wandering curve for India

Summary. We present palaeomagnetic results from the Durgapipal and Rudraprayag formations, which are basic volcanic formations in the Lesser Himalayas of Uttar Pradesh State. NRM measurements and AF demagnetization stability tests were made on specimens cored from oriented block samples collected at representative sites. Mean stable remanent magnetic directions were used for calculating the Virtual Geomagnetic Pole (VGP) positions; where necessary tectonic corrections were applied.
The virtual geomagnetic north poles were found to be located at:

• (a). 

  Durgapipal (Permian): λ _p = 10° S, L_p = 42° W;
• (b). 

  Rudraprayag (Silurian-Devonian): λ _p = 30° S, L_p = 12° W.

A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature.     

New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian

A palaeomagnetic study of Vendian red sediments from the Lena River section on the western margin of Lake Baikal in the region of Cisbaikalia (54°N, 108°E) has isolated a stable remanence with direction D = 296.3°, I = −27.7° (high-temperature component) and a corresponding pole of 2.7°S, 168.2°E. The primary nature of this remanence is confirmed from a positive fold test, dual polarities and the presence of detrital haematite. This result, together with all late Precambrian–Early Cambrian palaeomagnetic data from Siberia, indicates that Siberia occupied low latitudes during that time. It has been proposed on the basis of palaeomagnetic data that Laurentia occupied high latitudes during the Vendian, so it would appear that there cannot have been any Laurentia–Siberia connection at that time. A review of Vendian to Cambrian Laurentian palaeomagnetic data shows that such an interpretation is ambiguous. An alternative interpretation places Laurentia in low latitudes and confirms the Laurentia–Siberia fit of Hoffman (1991 ) and Pelechaty (1996 ). However, the lack of Late Vendian palaeomagnetic data for Siberia still allows the possibility that it could have occupied high latitudes during that time.     

Palaeomagnetic evidence for unification of the North and West Australian cratons by ca.1.7 Ga: new results from the Kimberley Basin of northwestern Australia

A palaeomagnetic study of the Elgee Formation red siltstones and shales in the Palaeoproterozoic Kimberley Basin of northwestern Australia has been carried out. All seven sampling sites revealed an extremely stable magnetic remanence carried by haematite. The age of the formation is confined by precise SHRIMP U–Pb ages of early diagenetic xenotime from rocks both above and below it to be 1704 + 7/−14 Ma, but this may represent a minimum age. The youngest detrital zircon grains in the underlying formation provide a maximum age of 1786 ± 14 Ma for the formation. The extreme stability of the remanence, the dissimilarity of the remanent direction from expected younger palaeomagnetic directions, and the lack of regional overprint in the 1790 ± 4 Ma Hart Dolerite just north of the study region support a primary origin for the remanence. A marginally positive fold test also supports a primary origin. The mean direction of D = 92.2°, I = 14.9°, α ₉₅ = 6.4° gives a palaeopole at 4.4°S, 210.0°E with dp = 3.3°, dm = 6.5°. This pole, a previously reported palaeopole from the Hart Dolerite and ca. 1700 Ma overprint poles from the Pilbara Craton all agree with palaeopoles of similar ages from the McArthur Basin of northern Australia. Palaeomagnetic results thus suggest that the North and West Australian cratons were possibly joined together by approximately 1.7 Ga.     

Palaeomagnetism of the Lower Cretaceous Sierra De Los Condores Group Cordoba Province Argentina

Measurements are described of the directions of remanent magnetization of 89 samples from nine lava flows and red beds. Stable remanent magnetization was isolated after AC demagnetizing. All the units have normal remanent magnetization, except one lava flow which yields a direction toward the north with positive inclination. From the mean direction of stable remanence, referred to the bedding, of each unit a virtual geomagnetic pole is computed; the mean of eight of these poles is 90·6 °E, 84·2° South, α₉₅= 4·7° and represents the position of the palaeomagnetic pole for the exposures of the Sierra de Los Condores group from El Estrecho-Cerro Libertad. The position of this pole is reasonably close to the positions of the South American Lower Cretaceous palaeomagnetic poles for the Serra Geral and Vulcanitas Cerro Colorado formations and the trachybasaltic dykes from Rio Los Molinos. This supports the interpretations that the South Atlantic Ocean was formed in Lower Cretaceous times and that the Earth's magnetic field was on average similar to that of a geocentric dipole in South America in the Lower Cretaceous, and suggests that there has not been substantial relative movements between Central Argentina and Southern Brazil.     

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).     

Palaeomagnetic results from the Cretaceous Bagh Group in the Narmada Basin, central India: evidence of pervasive Deccan remagnetization and its implications for Deccan volcanism

A palaeomagnetic study of 115 samples (328 specimens) from 22 sites of the Mid- to Upper Cretaceous Bagh Group underlying the Deccan Traps in the Man valley (22°  20'N, 75°  5'E) of the Narmada Basin is reported. A characteristic magnetization of dominantly reverse polarity has been isolated from the entire rock succession, whose depositional age is constrained within the Cretaceous Normal Superchron. Only a few samples in the uppermost strata have yielded either normal or mixed polarity directions. The overall mean of reverse magnetization is D _m=144°, I _m=47° ( α ₉₅=2.8°, k =152, N =18 sites) with the corresponding S-pole position 28.7°S, 111.2°E ( A ₉₅=3.1°) and a palaeolatitude of 28°S±3°. The characteristic remanence is carried dominantly by magnetite. Similar magnetizations of reverse polarity are also exhibited by Deccan basalt samples and a mafic dyke in the study area. This pole position falls near the Late Cretaceous segment of the Indian APWP and is concordant with poles reported from the Deccan basalt flows and dated DSDP cores (75–65  Ma) of the Indian Ocean. It is therefore concluded that the Bagh Group in the eastern part of the Narmada Basin has been pervasively remagnetized by the igneous activity of Deccan basalt effusion. This overprinted palaeomagnetic signature in the Bagh Group indicates a counter-clockwise rotation by 13°±3° and a latitudinal drift northwards by 3°±3° of the Indian subcontinent during Deccan volcanism.     

Cambro-Ordovician palaeomagnetic and geochronologic data from southern Victoria Land, Antarctica: revision of the Gondwana apparent polar wander path

We present new palaeomagnetic and isotopic data from the southern Victoria Land region of the Transantarctic Mountains in East Antarctica that constrain the palaeogeographic position of this region during the Late Cambrian and Early Ordovician. A new pole has been determined from a dioritic intrusion at Killer Ridge (⁴⁰Ar/³⁹Ar biotite age of 499 ± 3 Ma) and hornblende diorite dykes at Mt. Loke (21°E, 7°S, A 95 = 8°, N = 6 VGPs). The new Killer Ridge/Mt. Loke pole is indistinguishable from Gondwana Late Cambrian and Early Ordovician poles. Previously reported palaeomagnetic poles from southern Victoria Land have new isotopic age constraints that place them in the Late Cambrian rather than the Early Ordovician. Based upon the new palaeomagnetic and isotopic data, new Gondwana Late Cambrian and Early Ordovician mean poles have been calculated.     

Palaeomagnetism and K—Ar age of the Upper Ordovician Alcaparrosa Formation, Argentina

Summary. Palaeomagnetic data from 71 hand samples of igneous rocks of Late Ordovician age exposed in western Argentina (31.3°S, 69.4°W, Alcaparrosa Formation) are given. Stable remanent magnetization was isolated in the majority of samples; they yield a palaeomagnetic pole at 56°S 33°E ( N = 8, α₉₅= 16°). Whole rock K-Ar age determinations yield an age of 416 ± 10 Myr for a pillow lava of the Alcaparrosa Formation.
Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent.     

Palaeomagnetism of a sequence of red beds of the Middle and Upper Sections of Pagnazo Group (Argentina) and the correlation of Upper Palaeozoic-Lower Mesozoic rocks

Palaeomagnetic data from 182 hand samples collected in a rock sequence of about 620-m of red beds of Late Palaeozoic to Early Triassic age exposed in north-western Argentina (30.3° S 67.7° W), are given.
After cleaning, the majority of the Upper Palaeozoic samples (Middle Section of Paganzo Group) show reversed polarity and yield a palaeomagnetic pole at 78° S 249° E (α₉₅= 3°). They also record a polarity transition which we have correlated with the Middle Permian Quebrada del Pimiento Normal Event. The position of the palaeomagnetic pole and the K-Ar age of a basalatic sill at the base of the sequence support this correlation.
Stable remanent magnetization has been isolated in the majority of samples from the Upper Section of the Paganzo Group; it is predominantly reversed and reveals three normal events and also three geomagnetic excursions suggesting an Illawarra Zone age (post Kiaman, Late Tatarian-Early Scythian). The palaeomagnetic pole of the reversely magnetized samples is located at 75° S 285° E(α₉₅= 13°).
The red beds involved in this study are correlated with red beds from the Corumbataí Formation (State of Paraná, Brazil) and with igneous rocks from the Quebrada del Pimiento Formation (Province of Mendoza, Argentina).
The South American Middle and Upper Permian, Upper Permian—Lower Triassic, Lower, Middle and Upper Triassic and Middle Jurassic palaeomagnetic poles reflect a quasistatic period with mean pole at 82° S 244° E, (α₉₅= 4°) which followed the South American Late Palaeozoic polar shift.     

Comments on'The palaeomagnetism of the Central Zone of the Lewisian foreland, north-west Scotland'by J. D. A. Piper

Summary. Piper suggested that the Lewisian has rotated 30° anticlockwise since magnetization, whereas the opposite appears more likely. The main magnetization in the Lewisian recognized by Piper and Beckmann was imposed upon cooling after the Laxfordian metamorphism at about 1750 (± 50) Ma. The palaeomagnetic pole corresponding to this magnetization is at 37.6° N, 273.2° E ( dp = 3.7°, dm = 5.2°).
In Greenland, palaeomagnetic poles similar to each other, with a mean pole at 21.6° N, 280.1° E ( K = 52, A ₉₅= 9.4°), have been determined from five widely separated regions in central West Greenland and from Angmags-salik in East Greenland. The magnetization observed in all these regions was established upon cooling after the Nagssugtoqidian metamorphism, again at about 1750 (± 50) Ma.
The Laxfordian and Nagssugtoqidian metamorphisms were equivalent. It is therefore assumed that the two palaeomagnetic poles quoted above were originally identical. Their present difference can be explained by clockwise rotation of north-west Scotland about a local rotation pole since the Lewisian became magnetized, in addition to opening of the Atlantic assuming conventional reconstructions:
(1) assuming the reconstruction of Bullard, Everett & Smith, the local rotation proposed is 39.5° (± 18.1°) about a pole of rotation at 60.3° N, 354.5° E, or
(2) assuming the reconstruction of Le Pichon, Sibuet & Francheteau, the local rotation is 28.0° (±17.7°) about a pole of rotation at 54.1° N, 354.6° E.
These proposals of local clockwise rotation of north-west Scotland accord with that of Storetvedt based on palaeomagnetic results from Devonian rocks on the north-west side of the Great Glen Fault.     

A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China's largest dyke swarm, which trends for about 1000 km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U–Pb dating of primary zircon, is 1769.1 ± 2.5 Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D  = 36°, I  = − 5°, k  = 63, α ₉₅ = 4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp  = 2°, dm  = 4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800 Ma     

Early Triassic palaeomagnetic results from the Ryeongnam Block, Korean Peninsula: the eastern extension of the North China Block

Greenish sandstones in the Early Triassic Nogam Formation of the Ryeongnam Block, Korean Peninsula were collected at 23 sites for palaeomagnetic study. A high-temperature magnetization component with unblocking temperatures of 670–690 °C was isolated from seven sites and yielded a positive fold test at the 95 per cent confidence level. The high-temperature component is interpreted to be of primary origin because the folding age is Middle Triassic. The Early Triassic palaeomagnetic direction for the Ryeongnam Block after tilt correction is D =347.1°, I =23.8° ( α ₉₅=5.5°). The palaeomagnetic pole (62.5°N, 336.8°E, A ₉₅ = 4.7°) shows good agreement with the coeval pole for the North China Block, suggesting that the Ryeongnam Block has been part of the North China Block at least since Early Triassic times. A tectonic history of the Korean Peninsula includes obduction of the eastern part of the South China Block onto the central part of the Korean Peninsula in the Permian, with the Ryeongnam Block geographically isolated from the main part of the North China Block. Collision of the North and South China blocks commenced initially at the Korean Peninsula, and suturing of the two blocks progressed westwards.     

New palaeomagnetic results from the Kerguelen Islands

New palaeomagnetic analyses have been carried out in the Kerguelen Islands on 32 lava flows of well-established age (20–22 Ma). Combined with previous studies, they yield a reliable pole for the Lower Miocene for the Antarctic plate: N = 59 flows. 349.9°E, 83.5°S, A⁹⁵= 6.1°. A reversal sequence reversed-normal has been identified in the Port Jeanne d'Arc section.     

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.     

The late Maastrichtian palaeomagnetic pole of the Pacific Plate

Summary. The Pacific plate's late Maastrichtian (∼ 69 Ma) palaeomagnetic pole, which constrains the northward 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 71°N, 9°E and a 95 per cent confidence ellipse with the major semiaxis of 6° striking 91° clockwise from north and the minor semiaxis of 2° striking 1° clockwise from north. 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 ∼ 10° south 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, 1.1° Ma^-1, with respect to the Pacific plate during the latest Cretaceous.