HR: 14:30h
AN: GP33A-05 INVITED     [Abstracts]
TI: Mountain Building in the Central Andes: Paleomagnetic Constraints on Temporal and Spatial Crustal Rotations.
AU: * Arriagada, C
EM: cearriag@cec.uchile.cl
AF: Departamento de Geologia, Universidad de Chile, Santiago, Chile
AU: Roperch, P
EM: pierrick.roperch@ird.fr
AF: IRD UR154, Geosciences Rennes, Rennes, 35042 France
AU: Cobbold, P
EM: peter.cobbold@univ-rennes1.fr
AF: UMR 6118 Geosciences-Rennes, Campus de Beaulieu, Rennes, 35042 France
AU: Mpodozis, C
EM: cmpodozis@sipetrol.cl
AF: Sipetrol, Vitacura, Santiago, Chile
AB: More than 25 years of paleomagnetic investigations have revealed vertical-axis rotations in the Central Andes synchronous with the development of the mountain chain. The large paleomagnetic database now available for the Central Andes enables a better description of the magnitude, timing and driving mechanism of these rotations. The forearc of northern Chile between ~23-29°S records large (up to 50°) paleomagnetically detected clockwise rotations. Such rotations are observed in Mesozoic and Paleocene to Eocene rocks. The few Neogene paleomagnetic data do not show evidence of rotation. Eocene red beds sediments covering the forearc of southern Peru record a gradient of counterclockwise rotations increasing northward from 0 near Arica at 18-19°S up to -50° at 15°S. Oligocene sediments record about half of the rotations recorded by Eocene sediments. Although secular variations are not fully averaged, available paleomagnetic results in Miocene ignimbrites do not show rotation. Large counterclockwise rotations are observed in pre-Neogene rocks of the northern Altiplano and the Eastern Cordillera of Peru and northern Bolivia while no more than 10° of counterclockwise rotations have occurred during the last 10My. Clockwise rotations are observed within the southern Bolivian Andes and the Puna, especially within dextral transfer zones indicating that rotations are mainly associated with shortening gradient during the Neogene. Clockwise rotations along the Chilean margin correspond to a period of Eocene dextral oblique convergence between the Farallon and the South American plate. Northward displacements along the Chilean forearc were likely accommodated by counterclockwise rotations of the Peruvian margin and shortening in the Abancay-Cuzco deflection. The breakup of the Farallon plate resulted in a more E-W normal relative convergence direction between the Nazca and South American plates during the Oligocene. The increase in rate of convergence, about twice the present-day rate, is associated with an eastward shift of the front of the Andean deformation to the Eastern Cordillera of Bolivia and the Puna. Large late Oligocene-early Miocene shortening within the Eastern Cordillera achieved the bending of the Andean forearc. During the late Miocene-Pliocene, the 10° counterclockwise rotations within the northern Altiplano and the clockwise rotations within the southern subandean belt of Bolivia are related to the propagation of the curved Andean front toward the east. Changes in relative plate convergence and the major role of inherited structures in the propagation of Andean deformation explain the pattern and timing of rotations within the Central Andes. The paleomagnetic data indicate that the Bolivian orocline was formed during a long period from the Eocene to middle Miocene. This observation contradicts recent claims that deformation within the Andes was increasing with decreasing relative convergence during the last 10My.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 8102 Continental contractional orogenic belts
DE: 8150 Plate boundary--general (3040)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly