The Asunción Rift, Eastern Paraguay: a Preuminary Interpretation

The Asunci6n rift, eastem Paraguay, is a remarkable structure, first noticed by Degraft. It is a segmented, "Iazy Z", left-stepping rlft system, starting at Asunci6n, to the west, with a NW-8E direction. It then tums next into a E-W treneling middle saction, finally resuming, to the east, its inltlal NW-8E course. Its endings, at both extremes, are ill-defined. Total E-W length is about 150 Km, and width varies between 20 and 40 km. The E-W section represents the connecting, sheared neck of the stepped rift system, brought about by dextral movement of a pair of shear components with an E-W directlon. Subsidence within the central segment reached over 600 meters (field work by J.B.Presser) and Is not well established in the other sections. Alkaline occurrences (the ·Central alkaline province", eastem Paraguay) are found wlthln the rift system. Many of the rocks crystallized from mantle-derived magmas. The westem occurrences, in the Asunci6n segment, are seen mainly as isolated plugs and small bodies, with a sodic chemical tenelency. Ages seem to vary, from the Paleocene to the Oligocene (data by Bitschene). In the central sheared segment, alkaline rocks show K enrichment, and are emplaced as dike swarms and smaller plugs anel chimneys; some lava fields anel braccias are also found. In the central part of this segment, dikes show a general N30-50W strike, slightly curving over, to the north, into a northerly dlrectlon. Ages of these occurrences are around 130 Ma (Bltschene and others, confirmed by C.B. Gomes, V. Velazquez and coworkers), thus belonging to the 130 Ma "peak· of alkaline magmatism in these regions (Ulbrich et ai., 1991; BoI. IG-USP, Publ.Esp, 9:87-92) .