--> --> The Volcanic Margin of the Pelotas Basin: Evolution of Rifting, SDR Formation and the Breakup of Gondwana

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The Volcanic Margin of the Pelotas Basin: Evolution of Rifting, SDR Formation and the Breakup of Gondwana

Abstract

The Pelotas Basin, localized in the southern margin of Brazil, is the classical example of a volcanic passive margin displaying large wedges of seaward-dipping reflectors (SDR). The main objective of this work is to contribute to the understanding of the role of this magmatism in the evolution of rifting until the breakup and formation of the South Atlantic. In this new interpretation, the SDR fill entirely the rift throughout the basins, characterizing the abundant syn-rift magmatism (133–113 Ma). The Paranà-Etendeka Large Igneous Province (LIP), adjacent to west, constituted the pre-rift magmatism (134–132 Ma). The interpretation of ultra-deep seismic lines showed a very different geology from the adjacent Santos, Campos and Espìrito Santo basins, which constitute examples of magma-poor passive margins. Besides displaying rifts totally filled by volcanic rocks, diverse continental crustal domains were defined in the Pelotas Basin, such as an outer domain, probably constituted by highly stretched and permeated continental igneous crust, and a highly reflective lower crust probably indicating underplating. The analysis of rifting in this portion of the South Atlantic is based on seismic interpretation and on the distribution of regional linear magnetic anomalies. The lateral accretion of SDR to the east towards the future site of the breakup and the temporal relationship between their rift and sag geometries allows the reconstitution of the evolution of rifting in the basin. Rifting also propagated from west to east, in the direction of the final breakup, showing the migration of tectonic and magmatic activity towards the future sea-floor spreading center. Breakup propagated from south to north in three stages (130–127.5; 127.5–125; 125–113 Ma) physically separated by oceanic fracture zones (OFZ). The width of the stretched, thinned and heavily intruded continental crust also shows a three-stage increase in the same direction and at the same OFZ. Consequently, the Continental-Oceanic Boundary (COB) exhibits three marked shifts, from west to east, from south to north, resulting in margin segmentation. The importance of the Paranà-Etendeka LIP upon the overall history of rupturing and breakup of Western Gondwanaland seems to have been fundamental in time and in space only to the Pelotas Basin. In the other basins it merely occurred as a post-rift volcanism in southern basins and as a pre-rift volcanism in northern basins.