GeoSur2013 25-27 November 2013 – Viña del Mar (Chile)

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Secular variation of the Earth magnetic field and application 2-30 to paleomagnetic dating of historical lava flows in Chile

Pierrick Roperch*1, Annick Chauvin1, Luis E Lara2 and Hugo Moreno3

(1) Geosciences Rennes UMR 6118, CNRS & University of Rennes 1, 35042 Rennes , France. (2) Servicio Nacional de Geología y Minería, Santiago, Chile. (3) Servicio Nacional de Geología y Minería, OVDAS, Temuco, Chile. * Presenting Author’s email: [email protected]

The 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 (-40° at 40°S) and low intensities of the geomagnetic field (-25.7µT at 40°S). We report new paleomagnetic results (directions and paleointensities) from several sites in two well dated lava flows in Chile linked to the 1835AD eruption of the Osorno volcano and the 1751AD eruption of the Llaima volcano. In addition, paleointensities 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. The GUFM model is now included in the more recent CALS3k.4 model of Korte and Constable (2011). The amplitude of the secular variation in inclination (~20°) and intensity (~25µT) during the last three centuries makes paleomagnetism one of the most reliable methods to date historical lava flows in South America. We report new paleomagnetic data from several sites in historical lava flows from the Antuco, Llaima and Villarrica lava flows that permit to refine the ages of the major historical fusiveef volcanic events. At Villarrica, new results from several sites in lava flows dated at 500±30BP (calibrated age 1440AD±30) provide paleomagnetic directions that are different from the CALS3k.4 model indicating that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of global geomagnetic

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Fig. 1 - Paleomagnetic results in dated volcanic units from Chile (red dots). All data relocated to Villarrica location. The blue curve is the calculated magnetic field values at Villarrica location from model CALS10k.1b (Korteet al., 2011).

models prior to 1700AD. We sampled dated lava flows and pyroclastic flows from several Chilean volcanoes (Llaima, Solipulli, Villarrica, Mocho-Choshuenco, Osorno, Calbuco). The steepest inclination of the geomagnetic field (-71.6°) and the highest intensity (70µT±5) are found in the time range 850-900AD. This observation is made from paleomagnetic results from a pyroclastic flow from the Osorno volcano (2 uncalibrated 14C ages of 1250±40 BP and 1180±30 BP giving a calibrated age range of 782-966AD at a 95% confidence level with a median age of 890 AD). The steep inclination is also observed in dated lava flows of the same age range to the north of the Llaima volcano (Rio Captren, 14C ages of 1160±70BP and 1280±60BP; calibrated median age of 856AD with a range at 95% from 720 to 980AD. The corresponding high inclination and high intensity indicates a significant dipole wobble at that time. Paleointensities of about 60µT determined in samples from the pyroclastic flows of the Mocho-Choshuenco and the Pucón ignimbrite at Villarrica 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 before 10,000BC. The new paleomagnetic results from Chile indicate that the large and rapid secular variation of the geomagnetic field during the last three centuries is an anomalous feature of the Earth’s magnetic field during the Holocene, which permits very precise paleomagnetic dating for this time interval.

REFERENCES • Jackson, A. et al. (2000). Four centuries of geomagnetic secular variation from historical records, Phil. Trans. Roy. Soc. A (2000) Vol. 358, pp957-99) • Korte, M. & Constable, C.G., (2011). Improving geomagnetic field reconstructions for 0-3ka, Phys. Earth planet. Inter., doi: 10.1016/j.pepi.2011.06.017, 188(3-4), 247-259 • Korte, M, et al., (2011). Reconstructing the Holocene geomagnetic field. Earth and Planetary Science Letters. 312:497-505. 10.1016/j.epsl.2011.10.031

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