264 research outputs found
Paleomagnetism and Tectonics
No full textNational audienceRock magnetism was one of the essential building blocks in the construction of plate tectonics, the major revolution in Earth science. In fact, continental drift was quantified by apparent polar wander path of virtual geomagnetic poles, and records of the reversal of the Earth's magnetic field by oceanic crustal rocks had enabled the quantification of seafloor spreading. During the last three decades of the 20th century, paleomagnetism also made major contributions to the quantification of mountain deformation by measuring tectonic rotations and latitudinal drifts. The work carried out in Rennes on the formation of the Iberian-Armorican arc is a good example (Perroud and Bonhommet, 1981). At that time, it was common to see entire sessions devoted to paleomagnetism and tectonics at international conferences (AGU, IAGA, etc.). However, interest in these topics has waned and is now almost extinct in France. Using our recent study in Myanmar as an example (Westerweel et al., 2020, 2019), we will show that it is still possible to obtain significant results that can fundamentally change the way other geological data are interpreted, even for a recent period such as the Cenozoic, when one might think that everything is known. Obviously, our precise knowledge of geodynamic conditions is still much less for more ancient times. We won't be able to understand the evolution of our planet and have reliable plate reconstructions without active paleomagnetic research. Perroud, H., Bonhommet, N., 1981. Palaeomagnetism of the Ibero-Armorican Arc and the Hercynian orogeny in Western Europe. Nature 292, 445–448. Westerweel, J., Licht, A., Cogné, N., Roperch, P., Dupont‐Nivet, G., Kay Thi, M., Swe, H.H., Huang, H., Win, Z., Wa Aung, D., 2020. Burma Terrane Collision and Northward Indentation in the Eastern Himalayas Recorded in the Eocene‐Miocene Chindwin Basin (Myanmar). Tectonics 39. https://doi.org/10.1029/2020TC006413 Westerweel, J., Roperch, P., Licht, A., Dupont-Nivet, G., Win, Z., Poblete, F., Ruffet, G., Swe, H.H., Thi, M.K., Aung, D.W., 2019. Burma Terrane part of the Trans-Tethyan arc during collision with India according to palaeomagnetic data. Nat. Geosci. 12, 863–868. https://doi.org/10.1038/s41561-019-0443-2
Paleomagnetism and Tectonics
No full textNational audienceRock magnetism was one of the essential building blocks in the construction of plate tectonics, the major revolution in Earth science. In fact, continental drift was quantified by apparent polar wander path of virtual geomagnetic poles, and records of the reversal of the Earth's magnetic field by oceanic crustal rocks had enabled the quantification of seafloor spreading. During the last three decades of the 20th century, paleomagnetism also made major contributions to the quantification of mountain deformation by measuring tectonic rotations and latitudinal drifts. The work carried out in Rennes on the formation of the Iberian-Armorican arc is a good example (Perroud and Bonhommet, 1981). At that time, it was common to see entire sessions devoted to paleomagnetism and tectonics at international conferences (AGU, IAGA, etc.). However, interest in these topics has waned and is now almost extinct in France. Using our recent study in Myanmar as an example (Westerweel et al., 2020, 2019), we will show that it is still possible to obtain significant results that can fundamentally change the way other geological data are interpreted, even for a recent period such as the Cenozoic, when one might think that everything is known. Obviously, our precise knowledge of geodynamic conditions is still much less for more ancient times. We won't be able to understand the evolution of our planet and have reliable plate reconstructions without active paleomagnetic research. Perroud, H., Bonhommet, N., 1981. Palaeomagnetism of the Ibero-Armorican Arc and the Hercynian orogeny in Western Europe. Nature 292, 445–448. Westerweel, J., Licht, A., Cogné, N., Roperch, P., Dupont‐Nivet, G., Kay Thi, M., Swe, H.H., Huang, H., Win, Z., Wa Aung, D., 2020. Burma Terrane Collision and Northward Indentation in the Eastern Himalayas Recorded in the Eocene‐Miocene Chindwin Basin (Myanmar). Tectonics 39. https://doi.org/10.1029/2020TC006413 Westerweel, J., Roperch, P., Licht, A., Dupont-Nivet, G., Win, Z., Poblete, F., Ruffet, G., Swe, H.H., Thi, M.K., Aung, D.W., 2019. Burma Terrane part of the Trans-Tethyan arc during collision with India according to palaeomagnetic data. Nat. Geosci. 12, 863–868. https://doi.org/10.1038/s41561-019-0443-2
Block rotations within the northern Peruvian Altiplano
Get PDFCounterclockwise tectonic rotations in the northern central Andes and clockwise rotations in the southern Central Andes have been systematically reported (see Roperch et al. 2006 and Arriagada et al., 2006 for a recent summary) and interpreted to be mostly driven by oroclinal bending associated with shortening in the Eastern Cordillera and in the subandean belt. Large counterclockwise rotations have been found in the Eastern Cordillera of Southern Peru (Gilder et al. 2003). While these rotations were initially attributed to a Cretaceous event of deformation, Gilder et al. (2003) interpreted these rotations to be coeval with the rotations found along the forearc (Roperch et al., 2006). Rotations along the forearc from Arequipa to Caravelli are larger than 40° and occurred mainly during the late Eocene - Oligocene. However, the lack of data within the Peruvian Altiplano precludes a good description of the spatial and temporal evolution as well as a clear understanding of the different tectonic processes leading to rotations. Here we report results from a paleomagnetic study from Nazca to Cusco (Figure 1). This transect corresponds to the location of the northern end of the Altiplano and a transition with the central Peruvian Andes. Near Cusco, the Eastern Cordillera is also strongly deflected toward the east with a complex deformation as shown by the curved fold and thrust system associated with the Manu Indenter
Using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism: A case study from southern Peru
Get PDFInternational audience[1] We report a combined study of anisotropy of low field magnetic susceptibility (AMS) and paleo-magnetism from 16 sites in a sedimentary sequence of Eocene–early Oligocene red beds in southern Peru. Incipient tectonic strain is recorded during the early stages of deformation. Nonhorizontal magnetic linea-tion in geographic coordinate suggests either non-cylindrical folding and/or interference of two phases of compressive deformation and tectonic rotation. Applying the classic tilt correction results in significant dispersion in paleomagnetic declinations and apparent clockwise and counterclockwise relative tectonic rota-tions. A dispersion in the orientation of the magnetic lineation also arises from a simple classic tilt correction inducing apparent local rotation in paleostress determi-nation. The magnetic lineation is a good proxy to detect a complex history of folding when the finite strain is not large enough to reset the magnetic fabric acquired during the early stages of deformation and when detailed geo-logical field mapping is not available or not possible. In the present study, a double correction rotating first the lineation to the horizontal reduces significantly the dis-persion of the paleomagnetic data with respect to con-ventional tilt correction (Fisher parameter k increases from 14 to 35). The interest of this double correction must obviously be evaluated for each study according to the complexity of the folding and the intensity of the deformation. Assuming a mean age of 40 Ma for the sedimentary sequence, no significant rotation (−4.5° ± 8.4) is observed in this area of the Peruvian Andes. Citation: Roperch, P., V. Carlotto, and A. Chauvin (2010), Using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism: A case study from southern Peru, Tectonics, 29, TC6005
Secular variation of the Earth magnetic field recorded in Holocene lava flows from Chile
No full textInternational audienceThe recent secular variation of the Earth's magnetic field is mainly characterized by the large growth of the South Atlantic Magnetic Anomaly during the last three centuries, first documented in the geomagnetic field model GUFM (Jackson et al., 2000). This present-day magnetic anomaly is characterized in Chile by low magnetic inclinations and low intensities of the geomagnetic field (-40 and 25.7 T at 40 S). In order to better describe the secular variation during the Holocene, we sampled 21 dated lava flows or pyroclastic flows from several Chilean volcanoes (Lonquimay, Llaima, Solipulli, Villarrica, Mocho-Choshuenco, Osorno, Calbuco). Juvenile clasts from basaltic-andesitic pyroclastic flow deposits provide reliable paleomagnetic results (Roperch et al, 2014). We also sampled 56 sites in Holocene lava flows with only relative ages with respect of the dated units. Paleomagnetic results were obtained from several sites in two well-dated historic lava flows; 9 sites and 11 paleointensity results (PI) from the 1835AD eruption of the Osorno volcano and 8 sites and 23 PIs from the 1751AD eruption of the Llaima volcano. In addition, 14 PIs were obtained in bricks from shelters built along the main path across the Andes from Santiago (Chile) to Mendoza (Argentina) in 1768AD. These results confirm the high reliability of the global geomagnetic model GUFM for the last three centuries. At Villarrica, results from 10 sites in lava flows (calibrated age 1440AD 30) provide paleomagnetic directions that are different from the CALS3k.4 model (Korte et al., 2011) indicating that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of global geomagnetic models prior to 1700AD. The steepest inclination of the geomagnetic field (-71.6 ) and the highest intensity (70 T 5) are found in the time range 850-900AD. This observation is made from paleomagnetic results from a pyroclastic flow from the Osorno volcano (calibrated age range of 782-966AD). The steep inclination is also observed in dated lava flows of the same age range to the north of the Llaima volcano (calibrated age range from 720 to 980AD). The VGP associated with the steep inclination is not much different from the VGP recorded at European sites suggesting a significant dipole wobble at that time. Mean PIs of 63, 60.3 and 57.5 T obtained in three dated units in the time range 0-2000BC confirm the high geomagnetic dipole moment of the Earth's magnetic field for the two millennia BC. In contrast, paleointensity results from the Lican ignimbrite at Villarrica and the Curacautin ignimbrite at Llaima volcano show that the magnetic field strength was low just prior to the Holocene (-14000 -15000 BC). The available paleomagnetic results from Chile indicate little geomagnetic secular variation in direction during the Holocene. Thus the large and rapid secular variation during the last three centuries appears to be a recent anomalous feature of the Earth's magnetic field. Jackson, A. et al. (2000). Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. A, 358, 957-99. Korte, M, et al., (2011). Reconstructing the Holocene geomagnetic field. Earth Planet. Sci. Lett. 312, 497-505. Roperch et al. (2014). Paleomagnetic study of juvenile basaltic-andesite clasts from Andean pyroclastic density current deposits. Phys. Earth Planet. Int., 227, 20-29
Comportement du champ magnetique terrestre au cours de transitions de polarite
No full textSIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc
A quick look at the MAGIC database
No full textNational audienceIn order to publish an article in most scientific journals, it is now often required that access be given to all paleomagnetic data, especially demagnetization data, rather than interpretations (FAIR principles). However, this requirement remains largely unfulfilled despite the existence of a dedicated database. Indeed, the MAGIC database (https://www2.earthref.org/MagIC/about) provides the possibility of a fairly comprehensive archive of the various data and interpretations obtained during most paleomagnetic studies. As of June 2023, about 250 of the more than 4400 contributions listed contain raw data. Most of the archived data correspond to studies published in the last ten years. MAGIC is based on a collection of open source Python programs for analyzing and uploading data but can be accessed without relying on the tools provided (Pmagpy). The complexity of the database archiving process is often cited as a reason for not publishing data. However, this complexity is partly the price to pay for the diverse information that can be stored. Despite the small number of contributions to the MAGIC website, we can already identify a number of problems, such as the failure of some users to follow basic rules such as sample orientation. There are also inconsistencies between the interpretations published in the articles and the actual data. Some data, such as low field susceptibility, which provides important information about magnetic carriers, is rarely archived. From my own experience with data archiving in MAGIC to reviewing a number of contributions, I will try to convince you of the usefulness of this archiving, which in my opinion should also be accompanied by a process of data evaluation. During an article evaluation process, reviewers should be able to evaluate the data, and the archiving condition should be automatic upon acceptance of the article, and not remain a promise of future publication by the authors. Such a measure would probably strengthen the MAGIC initiative that our American colleagues have been pursuing for almost 20 years. We should not forget that it is not the fanciful interpretations published in an article that are important, but the data
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