Synthetic Apparent Polar Wander Paths Research Articles

Paleomagnetism and rock magnetism of early Cretaceous Rajmahal basalts, NE India: Implications for paleogeography of the Indian subcontinent and migration of the Kerguelen hotspot

Abstract We present here new paleomagnetic and rock magnetic results from Rajmahal Traps (RT), India. The results are comprising of 350 standard specimens collected from 18 sites (90 samples). The detailed rock magnetic studies indicate that a Single Domain (SD) magnetite [Fe3O4] is the dominant magnetic carrier with ilmenite, pyrrhotite, and maghemite in the accessory form. The calculated Characteristic Remanent Magnetization (ChRM) directions yield a mean declination of 317.51°; mean inclination of −61.92° (α95 = 3.6°; k = 93.39). The obtained Virtual Geomagnetic Pole (VGP) position of the RT at ~117 Ma is 11.37°N, 297.58°E (62.42°W) with A95 = 3.5°, k = 99.99, dp = 4.33, dm = 5.58, and the calculated paleolatitude is at 43.1°S indistinguishable from that of earlier works. This paleomagnetic pole of RT basalts has minimal effect of secular variation and comply with the Geocentric Axial Dipole (GAD) model. The VGP is plotted on synthetic Apparent Polar Wander Path (APWP) for India, constrained better, and agrees well with the previously reported poles. The postulation of migration of mantle plume and the global mantle circulations are tested, and the present study results are in accord with this, suggesting the southward migration of the Kerguelen hotspot (mantle plume) by at least 6°.

Assessing the intersection/remagnetization puzzle with synthetic apparent polar wander paths

Palaeomagnetic data are of variable quality. To assist in a systematic assessment of data, a set of seven quality criteria (VQ1–VQ7) were introduced by Van der Voo. The last of those criteria ‘VQ7’ concerns the possibility of remagnetization when a particular palaeomagnetic pole resembles a younger palaeopole from the same stable region. While remagnetizations are often the culprit, the mere resemblance of an older pole to a younger pole does not a priori require that the rocks under investigation are remagnetized. Given that the Earth has a finite surface area, that apparent polar wander paths (APWPs) are represented as wide swathes rather than points, and that continental motion has taken place over several billion years, we ask the question ‘How likely is it for an APWP to loop back on itself?’ To answer this question, we constructed synthetic APWPs in an effort to evaluate the likelihood of self-intersection. We find that given 500 Myr of apparent polar wander, ∼60 per cent of the synthetic APWPs show self-intersection. Given 1000 Myr of apparent polar wander, ∼95 per cent of the synthetic APWPs show self-intersection. These results show that resemblance to younger palaeopoles, over the long term, may be governed by simple probability rather than only remagnetization. We recognize that remagnetization does occur, sometimes pervasively, and must be reckoned with in the assessment of palaeomagnetic data. Perhaps VQ7 should be amended to the first sentence in the original discussion, and focus on satisfying ‘No suspicion of remagnetization’ via other means rather than solely a resemblance to younger poles.

New paleomagnetic and paleointensity data from Pliocene lava flows from the Lesser Caucasus

A paleomagnetic, rock-magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two Pliocene (K–Ar age between 3.09±0.10Ma and 4.00±0.15Ma) sequences (Apnia and Korxi) from the eastern Djhavakheti Highland in southern Georgia (Caucasus).Measurement of strong-field magnetisation versus temperature curves yielded three types of thermomagnetic curves: (i) Reversible curves with magnetite as only remanence carrier (type H); (ii) irreversible curves with magnetite as only carrier of remanence (type H−) and (iii) irreversible curves showing a low Curie-temperature phase and magnetite (type L). Analysis of hysteresis curves showed that samples were characterised by a mixture of single-domain and multi-domain grains.Paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities (6 flows), reversed polarities (7 flows) and intermediate polarities (1 flow) were observed.. Paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities. The paleomagnetic pole obtained from flows of both combined sequences (latitude λ=77.9°N, longitude ϕ=152.1°E, n=13, A95=11.8°, k=13.4) showed a good agreement with the 5Ma window of the European synthetic apparent polar wander path of Besse and Courtillot (2002). The paleomagnetic direction of the combined Apnia-Korxi flows agrees well with the expected one, showing no significant tectonic rotation. The latter cannot be however, completely excluded in the Korxi section. In that section, analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected.Paleointensity experiments using the Coe method were performed on 31 specimens from 10 flows. After application of specific selection criteria, 19 samples from 8 flows were observed to provide successful determinations, with mean flow values showing a wide scatter. If only flows with more than one successful paleointensity determination are taken into account, virtual dipole moments (VDMs) vary between 3.5×1022Am2 and 8.3×1022Am2. In intermediate polarity site AP2 no weak transitional paleostrength values were observed.

Palaeomagnetic and AMS study of the Tarfaya coastal basin, Morocco: an early Turonian palaeopole for the African plate

Abstract An early Turonian ( c. 93 Ma) anoxic, cyclic marine deposition is registered in the unfolded outcrops from the Tarfaya coastal basin, where very high sedimentation rates enable the investigation of past geomagnetic field record at high temporal resolution. One hundred and fourteen samples have been sampled along a 10.5 m vertical profile ( c. 200–500 ka) of orbital-scale forced sedimentation. Rock magnetic investigations reveal mineralogy principally controlled by diamagnetic and paramagnetic behaviour, along with very low concentrations of low-coercivity ferromagnetic material which is probably magnetite. A well-defined magnetic fabric can be seen with the minimum susceptibility axis perpendicular to the foliation plane, and magnetic lineation compatible with NW African palaeostress since sedimentation times and/or the palaeocurrent associated with upwelling system deposition. Magnetic signature has the potential for performing reliability checks of reversed tiny wiggles, which were found in four samples not considered for the tectonic analysis. Alternating field demagnetization shows a single, stable, low-coercivity directional component. The new palaeopole ( N =88; PLat=64.3°, PLon=256.3°, A 95 =2.5°; K =38.7), obtained after moderate ( f =0.8) inclination flattening correction, is the first early Turonian palaeopole for the NW African Craton. It can contribute to the 90 Ma-centred sliding window of the different proposed synthetic Apparent Polar Wander Paths.

Linking the Late Cretaceous to Paleogene Pacific plate and the Atlantic bordering continents using plate circuits and paleomagnetic data

Late Cretaceous to Paleogene paleomagnetic data from the Pacific plate (the Emperor Seamounts) can be compared with data from the Atlantic bordering continents through the use of plate circuit reconstructions. Here we summarize the uncertainties in all data sets and present formal tests. We report agreement between Late Cretaceous Pacific paleomagnetic data and predictions based on the estimates of non‐Pacific pole positions from synthetic apparent polar wander paths. This congruency points to the veracity of the plate circuits and the accuracy of the paleomagnetic estimates. In contrast to the agreement seen for the Late Cretaceous, small discrepancies are observed in the comparisons of the Pacific Paleogene data and predictions from synthetic apparent polar wander paths. Such a disparity in a younger time interval is unexpected, given the agreement of the Late Cretaceous data. The possibility that minor, temporally variable nondipole field components contribute to the discrepancy cannot be completely discounted. However, an alternative and more straightforward explanation is suggested by further comparisons of the mean non‐Pacific paleomagnetic data and the highest‐quality poles that contribute to the means. In particular, we note that (1) the Pacific Paleogene data are in full agreement with coeval poles from North America meeting strict reliability criteria and (2) the non‐Pacific Paleogene poles of synthetic apparent polar wander paths are dominated by results from the North Atlantic Igneous Province (NAIP), but taken as a whole, the NAIP data fail a paleomagnetic reversal test. Hence, minor discrepancies between Paleocene paleomagnetic data from the Pacific and Atlantic hemispheres may point to limitations of the latter, which incorporate a relatively large number of older, lower‐quality data. These findings call for renewed data collections utilizing comprehensive rock magnetic and paleomagnetic (demagnetization) procedures to improve resolution of Paleocene non‐Pacific data. The uncertainties of the Pacific paleolatitude data and the non‐Pacific reference poles are larger than the differences related to the use of the two alternative plate circuits (through East to West Antarctica and through Australia to the Lord Howe Rise) that link the Atlantic and Pacific hemispheres. While no preference can thus be given to either plate circuit, their overall consistency with paleomagnetic data suggests that they can be used to investigate long‐term motion of the Pacific plate since Late Cretaceous times.

Testing corrections for paleomagnetic inclination error in sedimentary rocks: A comparative approach

Paleomagnetic inclinations in sedimentary formations are frequently suspected of being too shallow. Recognition and correction of shallow bias is therefore critical for paleogeographical reconstructions. This paper tests the reliability of the elongation/inclination ( E/ I) correction method in several ways. First we consider the E/ I trends predicted by various PSV models. We explored the role of sample size on the reliability of the E/ I estimates and found that for data sets smaller than ∼ 100–150, the results were less reliable. The Giant Gaussian Process-type paleosecular variation models were all constrained by paleomagnetic data from lava flows of the last five million years. Therefore, to test whether the method can be used in more ancient times, we compare model predictions of E/ I trends with observations from five Large Igneous Provinces since the early Cretaceous (Yemen, Kerguelen, Faroe Islands, Deccan and Paraná basalts). All data are consistent at the 95% level of confidence with the E/ I trends predicted by the paleosecular variation models. The Paraná data set also illustrated the effect of unrecognized tilting and combining data over a large latitudinal spread on the E/ I estimates underscoring the necessity of adhering to the two principle assumptions of the method. Then we discuss the geological implications of various applications of the E/ I method. In general the E/ I corrected data are more consistent with data from contemporaneous lavas, with predictions from the well constrained synthetic apparent polar wander paths, and other geological constraints. Finally, we compare the E/ I corrections with corrections from an entirely different method of inclination correction: the anisotropy of remanence method of Jackson et al. [Jackson, M.J., Banerjee, S.K., Marvin, J.A., Lu, R., Gruber, W., 1991. Detrital remanence, inclination errors and anhysteretic remanence anisotropy: quantitative model and experimental results. Geophys. J. Int. 104, 95–103] which relies on measurement of remanence and particle anisotropies of the sediments. In the two cases where a direct comparison can be made, the two methods give corrections that are consistent within error. In summary, it appears that the E/ I method for recognizing and corrected the effects of sedimentary flattening is reasonably robust for at least the Mesozoic and Cenozoic when the source of scatter is geomagnetic and sedimentary flattening in origin.

Oligo‐Miocene magnetostratigraphy and rock magnetism of the Xishuigou section, Subei (Gansu Province, western China) and implications for shallow inclinations in central Asia

Magnetostratigraphy of 222 remanent directions together with late Oligocene to early Miocene mammal and charophyte paleontology suggest that 2179 m of the Xishuigou section (Subei, Gansu Province, China) were deposited from ∼26 to ∼19 Ma. Stratigraphic patterns of bulk susceptibility, anisotropy of magnetic susceptibility parameters, and natural and anhysteretic remanent magnetization intensities demonstrate that (1) faulting does not significantly affect the record, (2) sediment deposition was relatively continuous, (3) sediment source changed around 23 Ma, and (4) rapid uplift near Subei occurred at 21 Ma. Subei rotated 27°±5° counterclockwise with respect to the 20 Ma pole from the Eurasian synthetic apparent polar wander path. Folding and rotation of the section took place after 19 Ma. The paleolatitude of Subei is 14° less than at present and 19°±3° less than predicted from the reference pole. Both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing, a phenomenon which has likely affected other paleomagnetic data from central Asia.

Palæomagnetic results and palæointensity of Late Cretaceous Madagascan basalt

For a palæomagnetic study of the Late Cretaceous flood basalts of Madagascar (age ∼ 88 Ma), 144 oriented drill cores from 15 sites located in the northwestern part of the island were sampled. Despite the very common occurrence of secondary isothermal remanent magnetisation related to lightning, the direction of the primary characteristic remanent magnetisation could, in many cases, be isolated using alternating field demagnetisation. Combining the palæodirectional results from this study with previously published results, the authors obtain a well-defined palæomagnetic pole for Madagascar with coordinates 67.8°S, 48.5°E (A95 = 3.7°, K = 34, N = 44). This pole is in good agreement with the synthetic apparent polar wander path (APWP) for Africa. Furthermore, Bingham statistics provided a long axis lying along the APWP indicating that the duration of volcanic activity was sufficiently long to record apparent polar wander. Palæosecular variation, S = 13.9 (+2.4/−1.8), is slightly smaller than expected for this time interval and latitudinal band.The absolute palæointensity, using the Thellier technique, could be obtained only from a single cooling unit, yielding a palæofield of 61.5 ± 5.5 μT. The corresponding virtual dipole moment of 10.7 × 1022 A m2 is somewhat higher than previously published results from the Cretaceous normal superchron; but, obviously, secular variation is not averaged.

Mid‐Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: A test of post‐100 Ma relative motion between East and West Antarctica

As part of the tripartite, United States ‐ United Kingdom ‐ New Zealand, 1990–1991 South Pacific Rim International Tectonics Expedition, oriented samples were collected for paleomagnetic analysis from mid‐Cretaceous (circa 100 Ma) intrusive rocks at sampling localities across 350 km of the Ruppert and Hobbs Coast area of Marie Byrd Land, West Antarctica. Paleomagnetic results are presented along with several lines of evidence, including a positive tilt test based on the attitude of circa 117 Ma volcanic rocks that the circa 100 Ma rocks intrude, which argue that these results are a representative estimate of the mid‐Cretaceous magnetic field in Marie Byrd Land (MBL). The new circa 100 Ma mean south pole (224.1°E/75.7°S, A95 = 3.8°, N = 19 site means) is concordant with other West Antarctic results of similar age implying that at least Marie Byrd Land, Thurston Island and the Antarctic Peninsula have not experienced any paleomagnetically resolvable relative motion since the mid‐Cretaceous. However, the poles from these Pacific‐bordering blocks of West Antarctica are significantly offset from a synthetic apparent polar wander path that was produced for the East Antarctic craton, implying relative movement between East Antarctica and Pacific West Antarctica since about 100 Ma. Though the paleomagnetic estimate for east‐west Antarctic relative motion may be reconciled with geologic estimates for extension in the Ross Sea at the extremes of the error envelope, the best paleomagnetic estimate of relative motion suggests a larger amount of total extension between East and West Antarctica (MBL) than previously suspected. Both estimates call for several hundreds of kilometers of post‐100 Ma displacement between East Antarctica and the Pacific‐bordering blocks of West Antarctica.

The configuration of Asia prior to the collision of India: Cretaceous paleomagnetic constraints

Paleomagnetic data from Central Asia show that 1700±610 km of shortening of southern Asia since Cretaceous time have been absorbed by distributed deformation between southern Tibet and the Siberia craton. This result is based on a compilation of Cretaceous poles from the Junggar, Tarim, Tibet, Indochina, South China, North China, and Mongolia blocks, complementing the recent compilation of Enkin et al. (1992a). We propose a paleogeographic reconstruction of Asia in the Cretaceous, in which the position of Siberia is derived from the synthetic apparent polar wander path of Besse and Courtillot (1991). The resulting map, which likely represents Asia as it remained throughout the Cretaceous until the collision with India began, features an “unbent” Tibet, with an east‐west trending Andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from Sichuan to Tarim through Tibet. The map allows one to estimate continental shortening and rotations between the blocks, which are attributed to the collision. Despite large uncertainties, these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of Avouac (1991).Appendix is available with entire paper on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, DC 20009. Document B93‐007; $2.50. Payment must accompany order.

Revised and synthetic apparent polar wander paths of the African, Eurasian, North American and Indian Plates, and true polar wander since 200 Ma

We have reviewed paleomagnetic data available for the Eurasian, African, North American and Indian plates over the last 200 Ma. Selection criteria are those generally accepted, with an emphasis on evidence for lack of remagnetization, accurate dating and proper structural analysis. This results in 23, 35, 51 and 2 poles for Eurasia, Africa, North America and India, respectively. We believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander (APW) paths than averaging of larger data sets involving less stringent selection criteria. 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. We find good agreement between transferred data, when they are averaged separately for each plate in independent 20‐Ma windows. This is a check on consistency of paleomagnetic data, kinematic models, and of the geocentric dipole hypothesis. Transferred data from all plates are averaged in 20‐Ma windows to generate synthetic APW paths for all plates studied. These synthetic paths are in agreement with the original (revised) APW paths that use only data from a single plate. Moreover, both geographic and time resolution are improved (spatial confidence intervals are of the order of 5°). 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. We find significant TPW, amounting to over 20° in the last 200 Ma. TPW appears to be episodic, with a standstill between 180 and 110 Ma. There is general agreement with a previous study of Livermore et al. (1984). The TPW standstill appears to correlate with a time of decreasing reversal frequency, ending with the Cretaceous Long Normal Superchron. Other periods of fast TPW would seem to correspond to increasing reversal frequency. However, it is suggested that the major TPW hairpin at 50 Ma might correspond to the collision of India with Eurasia. Tentative correlations with core or mantle indicators can be understood as a result of couplings between the core, mantle and lithosphere, we believe primarily related to episodic evolution of the D″ layer.

Latitudinal evolution of the Réunion hotspot deduced from paleomagnetic results of Leg 115

During ODP Leg 115 in the Indian Ocean, four holes penetrated the basement. Paleomagnetic measurements on basement cores yield good quality results, but their interpretation is limited by possible but often undetectable subsequent tectonic tilting, and in one case by insufficient averaging of secular variation. Two Sites (64 and 55–60 Ma) yield apparently reliable results. Results from sediments of another Site (36 Ma) studied by Schneider and Kent also appear to be reliable. The three corresponding paleolatitudes are roughly in agreement with values predicted from the synthetic apparent polar wander paths of the African and Indian plates. Together with results from the Deccan (67 Ma), they indicate smooth and rather slow (8mm/yr) northward drift of the Réunion hotspot since its inception at the Deccan. This motion is consistent with the hotspot models of Morgan and Duncan.