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Abstract

This paper compares and contrasts salt tectonics on two different types of continental margins in Brazil.

Narrow margins, such as the Jequitinhonha and Camamu basins, are 50–100 km wide and have a steep, (up to 5°) seaward-dipping base of salt seismic horizon. These margins are sediment starved due to their steepness. Consequently, sediment has bypassed the salt basin and been deposited on the abyssal plain. Pronounced contractional folding of the salt overburden is present on these margins. This commenced at the oceanward pinch-out of the salt and propagated back up the continental slope into areas, which are expected to be in extension due to gravity sliding. This is the opposite sense of fold and thrust propagation compared to ‘normal’ mountain fold and thrust belts. The bathymetric highs above pre-existing diapirs and fold anticline crests were rapidly eroded on narrow margins, which allowed the folds to grow more easily to large amplitudes (1.5 km) at the top salt seismic horizon. Folds continued to unroof until the salt reached the seabed and produced a duck-head shaped diapir due to downslope flow of a salt glacier. This was followed by collapse of the salt structure producing an unconformity-bounded graben.

Wide margins, such as the Campos and Santos basins, are >100 km and <650 km wide and have a subhorizontal to landward-dipping base of salt seismic horizon. Salt basins on wide margins are effective sediment traps; e.g., sediment loading in the Santos basin has produced a 2° landward-dipping base salt seismic horizon across the outer portion of the basin. Landward dip at the base salt seismic horizon has promoted development of counter-regional faults and enhanced later folding, which appear to develop approximately simultaneously across the whole basin. The folds are limited in amplitude to a maximum of 1 km, as little or no erosion has taken place over the crests and the thick sediment lid above the salt structures produces more competent rocks, causing a high vertical confining pressure, which inhibits fold growth. This compression is probably due to both downslope gravity sliding, and regional tectonic compression is due to ridge push or Andean collisional events.

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