Gravitational tectonics evidences at RGS (Brazil) Coastal Plain using Ground Penetrating radar

Abstract

This paper presents and discusses the gravitational tectonic influence on the geological evolution of the Coastal Plain of Rio Grande do Sul State (Brazil). The gravitational tectonic was identified from a series of normal faults recorded in GPR surveys, aerial photographs, high resolution imageries, and in reinterpreted geological sections based on drill-holes. The RGS Coastal Plain is the immersed part of the Pelotas Basin, which was developed from Barremian/Aptian on, after Gondwana rupture. The actual geomorphic and stratigraphic configuration of the RGS Coastal Plain is due to a deltaic alluvial plains and four lagoons/barrier systems developed as consequence of sea level changes from Middle Pleistocene to Holocene. Two long topographic scarps, arcuate in map view, coincide with normal faults identified in GPR surveys. GPR survey sections revealed that a holocenic peat layer equivalent radar-facie underwent normal fault displacement (synthetic and antithetic geometry) and arcuate close to faults (drag folds). The Lagoa do Peixe Fault is oriented N35E, while the Rio Grande Fault is regionally arcuate from N30E (southern segment) to N60E (northern segment). These both huge faults intercept each other in the Bujuru District area. Regionally, the peat layer radar-facie dips toward the continent. The peat layer dipping and fault geometry indicate gliding rotational process associated with faults. The fault gliding process developed associated depression (lagoons) and elevated fault block segment near the shoreline (structural barrier). The elevated bulkhead and structural barrier controlled the erosion and sedimentation in the area under influence of these gravitational structures, which corresponds to previously defined Lagoon-Barrier system IV (5.6 ka).