Conferencias y seminarios
Viernes 22 de Noviembre de 2019
Sala Ignacio Domeyko (Departamento Geología) (Plaza Ercilla 803, segundo piso. Santiago, Chile)
A toda la comunidad de Geología,
El Laboratorio de Paleomagnetismo del Departamento de Geología de la FCFM de la Universidad de Chile invita mañana viernes 22 de noviembre a las 10:00 horas al Coloquio de Paleomagnetismo y Tectónica en la Sala Domeyko del DGL.
En esta oportunidad expondrán los Dr. Ricardo Trindade de la Universidad de São Paulo, Brasil y el Dr. Pierrick Roperch de la Université de Rennes 1, Francia, en el siguiente programa:
The areal growth of the South Atlantic Geomagnetic Anomaly (SAGA) accompanies the fast decay of the Earth’s magnetic field recorded in the past two centuries, but its origin and longevity are still poorly understood given the scarcity of geomagnetic data in the Southern Hemisphere. We propose to fill this gap through the record of cave speleothems. These rocks can record the Earth's magnetic field with high fidelity but are usually neglected due to their very weak magnetic signal. In the talk we will present the challenges and perspectives on the magnetic study of speleothems, and show a study case on two speleothems from central Brazil, where a ∼1500-y record of unprecedented resolution was obtained close to the present-day minimum of the anomaly. This unique record reveals rapid variations in direction and intensity of the local geomagnetic field as a function of the location and magnitude of the anomaly. Synthetic secular variation models show this feature may result from westward migration, expansion, and intensification of reversed flux patches at the Core–Mantle boundary.
The growth of the South Atlantic geomagnetic anomaly is particularly well documented in Chile, where the intensity of the magnetic field has been halved over the past two centuries.
Paleomagnetic data obtained in Holocene volcanic flows show a strong field between 2000BC and 1700AD. A greater secular variation is observed at the end of the Pleistocene.
The India-Asia collision remains poorly understood. Today, specialists are torn between two collision models; on the one hand, a simple collision between India and Asia around 60-50 Ma, and on the other hand, a two-stage collision with a trans-Tethyan arc. As there are not many preserved rocks left in the India-Asia collision zone, researchers are looking to the sides of the zone, particularly in Myanmar's sedimentary basins and magmatic rocks of the late Cretaceous-Tertiary Wuntho-Popa arc. The paleomagnetic results, obtained on the Wuntho Cretaceous magmatic arc, allow to place the BT in equatorial position (~5°S) around 95Ma. This arc also records a clockwise tectonic rotation of about 60° since its formation. Paleomagnetic data in sedimentary rocks deposited about 40 Ma ago also show equatorial latitudes but no significant rotation. These results have several major implications. The equatorial position of the 95 Ma magmatic arc shows that this arc should not be correlated with the magmatism of the southern margin of the Lhasa microplate (Gangdese rocks) but supports the existence of a trans-tethyan arc probably connected to that of Kohistan, currently accreted south of the Pamir (see Figure). This reinforces the hypothesis of a first collision of India with a trans-tethyan arc around 60-50 Ma before a later collision with Asia. Article in Nature Geoscience 12, no 10.