HR: 11:35h
AN: GP42A-06    [Abstracts]
TI: Oroclinal bending in the Bolivian Andes: New evidence for post 10 Ma rotations linked to shortening gradients in the fold-and-thrust belt
AU: Barke, R
EM: richba@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AU: * Mac Niocaill, C
EM: conallm@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AU: Lamb, S H
EM: simon.lamb@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parks Road, Oxford, OX1 3PR United Kingdom
AB: The pronounced bend in the Andean mountain chain is, perhaps, the best-known example of an orocline. More than 25 years of palaeomagnetic investigations have revealed vertical-axis rotations of crustal blocks synchronous with the development of the mountain chain. The predominant pattern is of clockwise rotations to the south of the bend and anticlockwise rotations to the north. There is, however, still no consensus as to the magnitude and timing of these rotations, or what the driving mechanism is, with some models arguing that they represent only local vertical-axis block rotations, whereas others suggest that they represent large scale bending of the mountain chain. Similarly some authors suggest that bending occurred progressively throughout the Cenozoic, whereas other suggest that the rotations occured in discrete phases. We present new results from a palaeomagnetic study comprising two extensive regions (grouped together as localities) one either side of the oroclinal axis, in the Bolivian Eastern Cordillera. Palaeomagnetic analyses of samples from 31 sites in the 13-2 Ma Los Frailes ignimbrite complex, south of the oroclinal axis, yield a characteristic magnetization with a mean direction (D=$007\deg$; I=$-43\deg$; k=25; a95=$7\deg$; N=19 flows) that has been rotated by 9 $\pm$ $8\deg$ clockwise (at the 95% confidence level) since ca. 7 Ma with respect to South American reference poles. This is one of the youngest statistically significant rotations derived for the Central Andes. Samples from 12 sites in the 13-5 Ma Morococala and Eucalytpus volcanic complexes, to the north of the oroclinal axis, yield a mean direction (D=$356\deg$; I=$-34\deg$; k=9; a95=$19\deg$; N=9 flows) that suggests an anticlockwise rotation of $2\deg$, although this is not statistically significant. We combine our results from the southern limb of the orocline with all previous palaeomagnetic studies of Miocene to recent rotations for the Eastern Cordillera and Sub-Andes, and compare these rotations with the shortening gradients along the fold-and-thrust belt of the Sub-Andes (where the majority of post 15 Ma Andean crustal shortening has occurred). Through a joint inversion, we show that the observed rotations are consistent with the shortening gradients in the Sub-Andes, and therefore demonstrate that the Bolivian orocline has become more pronounced in the last c. 10 Ma.
DE: 8110 Continental tectonics--general (0905)
DE: 8150 Plate boundary--general (3040)
DE: 7218 Lithosphere and upper mantle
DE: 1206 Crustal movements--interplate (8155)
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting