HR: 1340h
AN: U33A-0027    [Abstracts]
TI: Bending or buckling of a Brasiliano belt: a paleomagnetic investigation of the curvature of the Paraguai belt, SW Amazon craton, central Brazil, with implications for Neoproterozoic/Cambrian tectonics of West Gondwana
AU: * Tohver, E
EM: etohver@usp.br
AF: Instituto de Geociencias, Universidade de Sao Paulo Rua do Lago, 562, Sao Paulo, SP 05508-080 Brazil
AU: Trindade, R I
EM: rtrindad@iag.usp.br
AF: Instituto de Astronomia e Geofisica, Universidade de Sao Paulo Rua do Matao, 1226, Sao Paulo, SP 05508-900 Brazil
AU: Font, E
EM: eric@iag.usp.br
AF: Instituto de Astronomia e Geofisica, Universidade de Sao Paulo Rua do Matao, 1226, Sao Paulo, SP 05508-900 Brazil
AB: The Paraguai belt (SW Brazil), which marks the Neoproterozoic limit of the SE Amazon craton, displays ca. 100 degrees of curvature along its ca. 1200 km extent. The origin of this arcuate shape could be primary, reflecting the original geometry of the post-Rodinia cratonic margin (reentrant); or secondary, resulting from regional-scale, crustal rotations (orocline) during late Neoproterozoic/Cambrian deformation. A preliminary paleomagnetic study of the Neoproterozoic Araras Fm, which lies in the middle section of the cap carbonate sequence, was undertaken along different strike domains in order to ascertain the primary or secondary origin of the curvature. Thermal demagnetization reveals a stable magnetic component with a moderately steep, downward direction, carried by pyrrhotite and/or magnetite. A negative result for a fold test, carried out on flat-lying rocks of the undeformed craton and steeply tilted beds of the adjacent fold-and-thrust belt, demonstrates that this magnetization is of a secondary origin. However, the covariance of the declination of this magnetic component with regional strike is noteworthy. Although this post-folding direction may have been acquired at different times by different portions of the belt during rotation of the Amazon craton as a whole, we favor a simpler explanation. We propose that the magnetization was acquired synchronously along the belt between two discrete episodes of deformation. In this scenario, the second deformational phase gave rise to the arcuate structure of the Paraguai belt, accompanied by the passive rotation of the observed paleomagnetic direction.. Thus, we suggest that the curvature of the Paraguai belt is the result of either: a) buckling of an initally straight belt during the accretionary history of the Panthalassan margin of W. Gondwana, or b) localized bending due to the indentation of the Paraguai belt during the assembly of W. Gondwana. In addition, paleomagnetic poles calculated from individual sites constrain the age of deformation in this portion of W. Gondwana to be younger than middle Cambrian (ca. 520 Ma).
DE: 9619 Precambrian
DE: 8102 Continental contractional orogenic belts
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1533 Remagnetization
SC: Union [U]
MN: 2004 AGU Fall Meeting