Abstract: Topographic and Structural Expressions of the South American Plate Ongoing Compression

Lima, Claudio and Lima Neto, Francisco - Petrobras/Cenpes

The South American plate is now in horizontal compression and shortening, as shown by stress data compilation, intraplate stress field numerical models and space-based geodetic results. Thrust regimes prevail in Southeastern Brazilian margin, Central Brazil and in the Amazon region; strike-slip regimes prevail in Northeastern Brazilian margin. All the available base-lines from the French DORIS system that cross the Andes towards the midplate are shortening 13-19mm/yr, and midplate shortening reaches 7mm/yr over the Brazilian craton. Geodetic results have also confirmed that the Andean belt is still moving eastwards thrusting the Brazilian craton. The observed compression/shortening is probably mostly due to the South American plate convergence with Nazca and the divergence with Africa. Analysis of composite images of plate topography and bathymetry, geology, gravity, and midplate seismicity have shown a number of observations related to the ongoing compression/shortening. (1) As stated elsewhere, the Euler equator of the Tertiary convergence between South America/Nazca symmetrically bisects the Andean salient topography and the underlying slab. We have observed that this equator is also roughly coincident with a chain of gravity isostatic highs crossing all the midplate, from the salient up to Northeastern Brazil. That chain concentrates seismicity, is the major plate divide and separates the plate in two completely different geotectonic, gravity and topography domains. Implication is that strong mechanical links exist between the convergence of South America and Nazca, the Andean deformation and the intraplate deformation. Moreover, it is implicit that the plate wide deformation is strongly dependent on older Precambrian major structures. (2) The western edge of the Brazilian craton in front of the Andean salient has been uplifted along a line that mimics the structural lines of the chain (Fig.1) This uplift is probably due to a flexural bulge induced by vertical and horizontal loads associated with the dynamics of the chain. In response to the uplift, denudation of a Tertiary and older sedimentary cover has been taking place, producing sedimentary scarps that retreat towards the Brazilian craton, exhuming the Precambrian shield and producing large areas of neogenic sedimentation like the great Pantanal basin. (3) All continental basins, including those with a Tertiary cover, are actually remnants of basins, whose erosional edges are associated with positive isostatic anomalies, and frequently with seismicity. These edges have been raised, forming now retreating scarps, where Cretaceous and Devonian marine sediments may be found several hundreds of meters high. As a result of the retreating process, Precambrian crystalline rocks are exhumed adjacent to basins, outcropping at lower topographic levels, being locally associated with neogenic sedimentation. Conversely, centers of these basins are usually depressed and associated with neogenic sedimentation. In this scenario, continental basins are now expected to be in a state of incipient inversion. Analysis of leakoff and hydraulic fracture data in the Potiguar basin (Fig. 2) has confirmed this inference, as reverse and strike-slip regimes prevail, except for areas close to the shelf break. As stated elsewhere, a strong correlation between flood anisotropy and the maximum stress orientation (SHMAX) was found in 80 fields throughout the world. General implication is that ongoing compression is potentially able to modify previous structures and present-day petroleum migration paths, which is a key information for basins with modern peak source rock generation.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil