Paleomagnetism and super(40)Ar/ super(39)Ar Chronology of Lavas from Meseta del Lago Buenos Aires, Patagonia

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 5; no. 1; p. np
• Main Authors: Brown, Laurie L, Singer, Brad S, Gorring, Matthew L
• Format: Journal Article
• Language: English
• Published: 01.01.2004
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2003GC000526

As part of a larger project investigating paleosecular variation in South America, 36 paleomagnetic and geochronology sites were sampled on Meseta del Lago Buenos Aires (47 degree S, 289 degree E), 200 km east of the active plate boundary in southern Patagonia. Basaltic lavas sampled range from late Miocene to late Pleistocene yet most of the individual lavas are younger than 3.3 Ma. Thirty-eight isochron ages determined via super(40)Ar/ super(39)Ar incremental-heating experiments frame a detailed stratigraphy. The isochron ages range from 66.9 plus or minus 4.1 ka for scoria cones and a youthful lava flow in the Rio Pinturas valley to 10.12 plus or minus 1.35 Ma for thick plateau-forming lavas exposed along the SE edge of the Meseta. The last 3.3 myr is characterized by seven episodes of volcanism at ca. 3.2-3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430-330 ka, and <110 ka. The bulk of lavas forming the surface of the Meseta erupted in the last 1.7 my. All sites have stable magnetization, and after step demagnetization using either thermal or alternating field techniques, yield characteristic directions held by magnetite and/or titanomagnetite. Ten sites have distinct transitional directions (defined by pole latitudes <55 degree ), and associated ages indicate possible connections to known reversals within the Matuyama Chron, including the onset of the Jaramillo subchron (1.016 plus or minus 0.01 Ma), the Cobb Mountain subchron (1.25 plus or minus 0.03 Ma), the Ontong-Java 1 event (1.37 plus or minus 0.03 Ma), and the termination of the Olduvai subchron (1.72 plus or minus 0.02 Ma). Remaining sites are divided into normal (14 sites) and reversed polarity (12 sites). At 95% confidence mean normal and reversed directions overlap. The mean direction for 26 sites is I = -63.0 degree , D = 3.4 degree , [alpha] sub(95) = 5.4 degree , which is indistinguishable from the expected geocentric axial dipole. Paleosecular variation, measured by the dispersion of virtual geomagnetic poles about the rotation axis, is higher than expected at 20.0 degree . Model G, using data for the past 5 Myr, predicts a dispersion of 17 degree . This discrepancy may be due to true dispersion of the field in southern South America or it may be an artifact of inadequate sampling.