Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the Famatina Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana
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Palaeomagnetic results from Palaeozoic volcanic and sedimentary units of the Famatina Ranges, in NW Argentina (28.7°S, 67.8°W) are reported. A late Early to late Middle Ordovician palaeomagnetic pole was obtained from a pre-tectonic remanence carried by magnetite and isolated in volcanics of the Molles Formation and the Cerro Morado Group (MCM1, 16.7°S, 357.2°E, A95 = 6.5°, K = 38.5, N = 14 sites). This pole position is rotated 39° clockwise respect to the coeval reference pole for Gondwana but it is consistent with previous Early Ordovician poles from the Famatina belt and the Faja Eruptiva Oriental in the Puna region of NW Argentina. The sedimentary layers of the Molles Formation, however, present a secondary magnetization carried by hematite, which is interpreted of Permian age and yields a pole position (MCM2) at 78.7°S, 330.8°E (A95 = 7.2°, K = 16.1, n = 27 samples). Two additional independent palaeomagnetic poles were obtained from the Permian De La Cuesta Formation, exposed at two different localities in the same area. While one consisted in a exclusively reverse polarity magnetization and a pole position (LC1, 76.9°S, 345.2°E, A95 = 6.0°, K = 21.1, n = 29 samples) compatible with the late Early to early Late Permian palaeomagnetic poles from South America, the other presented only normal polarities and a pole position (LC2, 74.5°N, 275.4°E, A95 = 2.0°, K = 258.3, n = 21 samples) suggestive of a Cretaceous remagnetization. These new palaeomagnetic results confirm on a much more robust database previous proposals that the Ordovician rocks of the Famatina belt have undergone a large clockwise rotation. They also constrain the rotation to pre-Permian times. Different tectonic models involving the Late Ordovician docking of a large para-authochthonous terrane or a pattern of systematic large-scale rotations in the Early Palaeozoic continental margin of Western Gondwana are discussed.Keywords:
Clockwise
Polar wander
Geomagnetic pole
Apparent polar wander
Red beds
Apparent polar wander
Polar wander
Geomagnetic pole
Natural remanent magnetization
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Apparent polar wander
Red beds
Polar wander
Magnetostratigraphy
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Geomagnetic pole
Apparent polar wander
Geomagnetic secular variation
Polar wander
Secular Variation
Magnetic dip
Hotspot (geology)
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A new paleomagnetic study in the South Sichuan basin (South China Block) has been carried out on sediments of Early Triassic to Middle Jurassic age. Detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components. A low‐temperature component (component A) falls close to the local present Earth field direction. A middle‐temperature component (component B) is found both in limestone and red beds of Early Triassic age, in limestones of Middle Triassic age, and finally in red beds of Jurassic age. The component B directions failed the fold test but show a significant increase of the ks / kg ratio at 60% when a progressive stepwise unfolding is applied. The directions obtained at 60% unfolding are grouped best and yield a pole position at 76.5°N, 174.1°E, A 95 = 3.7° that is close to the Late Cretaceous pole of the South China Block (SCB) or the Early Tertiary pole of the Eurasian plate. Characteristic directions are finally obtained (component C) for Early Triassic and Early to Middle Jurassic sediments only, both with reversal and fold tests. They yield pole positions at 44.1°N and 217.2°E ( A 95 = 4.9°) and 79°N and 227°E ( A 95 = 4.5°), respectively. The Early Triassic result well fits other data of the same age from the SCB. The revised Late Permian to Cretaceous paleopole data provide a new Mesozoic apparent polar wander path for south China. The successive pole positions from Late Permian to Early Triassic indicate motion of the South China Block as a whole rather than local rotations linked to important strain within the block. The Early to Middle Jurassic pole is statistically indistinguishable from the compiled North China Block (NCB) poles and indicates that final suturation between the two blocks occurred by the end of the Triassic, which is coherent with the age of ultra high‐pressure metamorphism in the Dabie‐Sulu areas of eastern China. The Asian plate including the Mongol‐Central Asian Block, NCB, and SCB, however, was probably separated from Siberia by the Mongol‐Okhotsk ocean during Jurassic time.
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Pacific apparent polar wander was delineated for mid‐Cretaceous to early Tertiary time using 22 seamount paleomagnetic poles, 11 paleocolatitudes from azimuthally unoriented cores, five effective inclinations from two‐dimensional magnetic anomaly inversions, and one magnetic lineation relative amplitude datum. Three of the seamount paleomagnetic poles were new data calculated in this study. The apparent polar wander path was defined with six mean poles having ages of 39, 66, 76, 82, 85, and 94 Ma. An alternate path was constructed combining data from two pairs of mean poles (66 and 76 Ma, 85 and 94 Ma) that were statistically indistinguishable. The apparent polar wander path is hook‐shaped with a nearly right angle bend at 82 Ma. Its post‐82 Ma section trends north–south, the pre‐82 Ma part, east–west. The path indicates uneven polar wander. Unresolvable polar motion occurred between 66–76 Ma and 85–94 Ma, suggesting Stillstands. However, the large distances between the 85 and 82 Ma poles (15.2°±3.8°) and 82 and 76 Ma poles (11.3°±2.8°) imply rapid polar wander at rates of 5.1°±1.3°/m.y. and 1.9°±0.5°/m.y., respectively. Both the sharp bend and rapid polar wander occurred at the end of the Cretaceous Normal Polarity Superchron. The mid‐Cretaceous paleomagnetic data show a trend suggestive of southward motion of the Pacific plate that may be the end of a period of southward drift postulated by others to have begun in the Jurassic. The evolution of various features of the apparent polar wander path are discussed in terms of plate motion, true polar wander, and time‐varying non‐dipole geomagnetic fields. We argue that the overall shape of the polar wander path resulted from the change of Pacific plate motion from southward to northward, perhaps brought about by the beginning of the subduction of the plate beneath Asia.
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Paleomagnetic and rockmagnetic data are reported for the Floresta Formation (Santa Fé Group) of the Sanfranciscana Basin, central Brazil. This formation represents the Permo‐Carboniferous glacial record of the basin and comprises the Brocotó (diamictites and flow diamictites), Brejo do Arroz (red sandstones and shales with dropstones and invertebrate trails), and Lavado (red sandstones) members, which crop out near the cities of Santa Fé de Minas and Canabrava, Minas Gerais State. Both Brejo do Arroz and Lavado members were sampled in the vicinities of the two localities. Alternating field and thermal demagnetizations of 268 samples from 76 sites revealed reversed components of magnetization in all samples in accordance with the Permo‐Carboniferous Reversed Superchron. The magnetic carriers are magnetite and hematite with both minerals exhibiting the same magnetization component, suggesting a primary origin for the remanence. We use the high‐quality paleomagnetic pole for the Santa Fé Group (330.9°E 65.7°S; N = 60; α 95 = 4.1°; k = 21) in a revised late Carboniferous to early Triassic apparent polar wander path for South America. On the basis of this result it is shown that an early Permian Pangea A‐type fit is possible if better determined paleomagnetic poles become available.
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Natural remanent magnetization
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The decade from 1951 to 1961 witnessed the birth of a new geophysical subdicipline, paleomagnetism. Early studies in Europe, North America, and Australia led to the following conclusions: (1) rocks could preserve directions of magnetiziation for hundreds of millions of years in red beds, (2) late Cenozoic lavas had directions of magnetiziation that led to the conclusion that the mean geomagnetic field was a geocentric dipole aligned along the axis of rotation, (3) rocks of Triassic age and older yield directions which depart widely from the present axis of rotation, (4) if these directions are used to calculate pole positions, then poles for older and older rocks fall farther and farther from the present pole of rotation, (5) these data may be used to construct polar wander curves, (6) polar wander curves from different continents do not coincide with one another, (7) they may be reconciled if the continents move with respect to each other, and (8) the distribution of climatic indicators show that the pole of rotation of Earth and the paleomagnetic pole for the same periods coincide for Phanerozoic time. These observations changed the perspectives of many Earth scientists and paved the way for seafloor spreading and plate tectonics.
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We report new palaeomagnetic data for red beds from the Ediacaran Brachina and Wonoka formations in the Adelaide Geosyncline, South Australia, and discuss their place with previously determined poles in the Ediacaran apparent polar wander path for Australia. Both formations behave similarly on thermal demagnetization, displaying high-temperature components that decay to the origin at 680 °C, consistent with haematite being the only magnetic mineral present. Restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence, indicating that acquisition of magnetization occurred before the early Palaeozoic Delamerian Orogeny. For the Brachina Formation (N= 91 specimens) the mean direction after unfolding is declination D= 178.2°, inclination I=–22.6° (α95= 4.4°), indicating a palaeolatitude λ= 11.8 ± 2.5° and a pole position at latitude λp= 46.0°S, longitude ϕp= 315.4°E, with confidence semi-axes dp= 2.4° and dm= 4.6°. The mean direction for the Wonoka Formation after unfolding (N= 70) is D= 255.9°, I=–23.7° (α95= 6.4°), indicating λ= 12.3 +3.8/–3.4° and a pole position at latitude λp= 5.2°S, longitude ϕp= 30.5°E (dp= 3.6° and dm= 6.8°). The mean directions for these units and other Ediacaran units in the Adelaide Geosyncline are significantly different from each other, which excludes blanket remagnetization of the units before Delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition. The late Cryogenian–Ediacaran–Cambrian apparent polar wander path for South Australia spans 150 Myr from ∼635 to 490 Ma and places Australia in low palaeolatitudes throughout the interval studied. The poles differ significantly from each other, suggesting Australia underwent continual drift during that time. Whereas the directional difference between the late Cryogenian Elatina Formation and early Ediacaran Nuccaleena Formation is mainly in inclination, for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences, indicating Australia was rotating about a nearby Euler pole in low palaeolatitudes. The large and perhaps rapid polar shifts at 615–590 and 575–565 Ma in the Laurentian apparent polar wander path are not evident in the Ediacaran apparent polar wander path for Australia. Because true polar wander should be recorded globally, in the absence of evidence for any major stratigraphic break in the South Australian succession we conclude that large true polar wander did not occur during the Ediacaran.
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Baltica
Laurentia
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