TECTONIC EVOLUTION OF THE ASUNCIÓN RIFT, EASTERN PARAGUAY
CLAUDIO RICCOMINI, VICTOR F. VELÁZQUEZ, CELSO B. GOMES, ANDERSON MILAN AND ALETHÉA E. M. SALLUN
Instituto de Geociências, Universidade de São Paulo, 05508-900 São Paulo, SP, Brazil.
The Asunción Rift is an important tectonic feature of Mesozoic-Cenozoic age in eastern Paraguay. With a width between 25 and 40 km, this structure consists of three segments: the well-defined western segment with a NW-SE strike and extending over 90 km between Benjamin Aceval and Paraguarí; the central E-W segment of about 70 km in extent linking the cities of Paraguarí and Villarrica; and the less-defined eastern segment, 40 km-long, with a NW-SE strike, between Villarrica and the Cordillera del Ybytyruzú.
Tectonic studies in the region revealed a first phase of faulting during the Early Cretaceous associated with tholeiitic magmatism in the eastern segment of the rift and followed by expressive alkaline (potassic) magmatism mainly in the central segment of the rift. Structural analysis of diabase and alkaline dyke swarms indicated the action of a paleostress field with s1, NW-SE oriented /horizontal, s2 vertical, and s3 NE-SW/horizontal, related with an E-W oriented, right-lateral strike-slip binary.
During the Paleocene, the western segment of the rift was filled by fanglomeratic, aeolian and volcaniclastic deposits of the Patiño Fm. Deep NW-trending lithospheric faults served as conduits for ultra-alkaline rocks, of nephelinitic composition, bearing spinel lherzolite mantle xenoliths. These rocks intruded the still unlithified sediments of the Patiño Fm. causing synsedimentary hydrothermal silicification. This fact and the presence of volcanic fragments (bombs and lapilli) indicates that the Patiño Fm. represents the sedimentary record associated with tectonic and magmatic episodes that occurred in the Asunción Rift during the Paleogene. Structural analysis of nephelinitic plugs, necks and dikes indicated a paleostress field with s1, NW-SE/horizontal, s2 vertical, and s3 NE-SW/horizontal, also related with an E-W oriented right-lateral strike-slip binary.
Quaternary faulting, recorded in the western segment of the rift, shows a stress field with s3 horizontal along the E-W direction, probably responsible for the installation of the Ypacaraí Graben and the morphological compartmentalization of the region. (May 24, 2002).
* E-mail: riccomin@usp.br
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
