Supra-Salt Extensional Fault Evolution in the Santos Basin (Brazil)

Tvedt, Anette B.; Jackson, Christopher A.; Rotevatn, Atle; Gawthorpe, Robert; Fossen, Haakon

The structural style and evolution of normal fault arrays related to salt-mobilization is less well understood than those in rift basins. In this study we use three-dimensional seismic data from the Santos Basin to characterize supra-salt extensional fault arrays and assess the influence of mobile evaporites on their evolution. To evaluate the structural style and growth history of the fault arrays we apply the following methods: 1) 3-D mapping of faults and age-constrained growth strata, 2) geometric and kinematic classification of fault segments, and 3) mapping of three-dimensional displacement distributions on fault surfaces. The supra-salt fault arrays are localized either along the edge of salt stocks, or at the center along the top of the flat-topped salt stocks. Most of the surrounding supra-salt rock volume is undeformed. The fault arrays strike N-S to NE-SW and consist of a series of relatively straight individual fault segments, defining a series of grabens. The majority of the faults appear planar on the seismic data, with a slight listric component towards the lower tip line. The faults tip out downward into large salt stocks and display relatively laterally long, hard-linked segments, up to 12.5 km long with maximum displacement up to 180 ms TWTT, indicating that they are "under-displaced". Salt rollers are present in the footwall of the faults and increase in amplitude with increasing displacement. Throw analyses suggest a staged evolution comprising an initial stage of rapid fault growth including propagation, linkage and surface-breaching, followed by a stage characterized by limited tip propagation. We hypothesize that the localization and evolution of the supra-salt fault arrays result from one or more of the following: (i) pre-existing irregularities in the top of the salt stocks, (ii) salt stock geometry and flank steepness, and (iii) internal collapse within the salt stock related to salt heterogeneity. Based on general mapping and three dimensional displacement analyses we conclude that accommodation of strain by the salt focuses the deformation and allows the localization of the long, "under-displaced" faults, leaving the surrounding cover relatively undeformed.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013