THE REGIONAL INTRAPLATE STRESS-FIELD IN SOUTH-AMERICA

A compilation of lithospheric stress directions for continental South America and the inferred major patterns of the regional intraplate stress field are presented. Stress orientations are based primarily on earthquake focal mechanisms and Quaternary fault slip inversion published in the literature. Four new focal mechanisms based on short-period P wave modeling, and selected centroid-moment-tensor solutions (published by U.S. Geological Survey) consistent with P wave first motions at South American and other World-Wide Standard Seismograph Network stations are also included in the data base. The observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the South American plate. In the Andean plateau (altitudes greater than 3000 m), N-S extensional stresses predominate. E-W compressional stresses are observed in the sub-Andean and platform regions extending up to about 1000 km east of the Andes. The maximum horizontal stress (SH(max)) is uniformly oriented in the E-W direction throughout western South America. Averages of the SH(max) direction at grid points spaced 2.5-degrees were taken as representing the "regional" field. The E-W direction of this regional field is not affected by the change of strike of the Andean chain nor by the contact of a flat subducted slab underneath. The regional SH(max) direction is oriented 15-degrees more clockwise than thc direction of the Nazca plate convergence. The difference between the regional SH(max) orientations and the absolute plate motion may be only about 6-degrees. This may indicate that contact forces with the Nazca plate may not be the only major contributor to the intraplate stresses in western South America. The eastern limit of this Andean stress province seems to coincide with aseismic regions in the Upper Amazon basin and in the Parana basin. In the central Amazonian region, seismicity and stress data suggest a seismic province with N-S compressional stresses. The origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle Amazon basin. In northeastern Brazil, seismicity is characterized by upper crustal strike-slip earthquakes bordering the Potiguar marginal basin. A model is proposed for this region in which the stress field is the result of a superposition of regional E-W compressional stresses and local extensional stresses (oriented perpendicular to the continental margin) possibly related to density contrasts and sediment loading in the continental shelf.