EFFECT OF THE ALTIPLANO-PUNA PLATEAU, SOUTH-AMERICA, ON THE REGIONAL INTRAPLATE STRESSES

A previous compilation of stress data for the sub-Andean region of northwest Argentina is complemented with eight additional earthquake focal mechanisms. Seven events (5.3-5.5 m(b)) had their mechanisms determined by inversion of teleseismic body waves (short-period P waves and long-period SH waves). All events had predominantly reverse faulting mechanisms: five had almost pure dip-slip motion and two had a component of oblique motion. The other mechanism was a composite fault plane solution of the Hualfin, Catamarca, series of July 1986 (maximum 4 m(b)), determined with data from a local temporary network and the permanent INPRES regional network. The local data shows that the Hualfin events occurred on a single fault plane oriented NW-SE with predominantly strike-slip motion. The stress data in the sub-Andean region suggests a radial pattern around the Altiplano-Puna plateau, with compressional stresses tending to be perpendicular to the eastern front of the plateau. This indicates that local spreading stresses due to lateral density variations (crustal thickening or lithospheric thinning of the plateau) may be comparable or even exceed the plate-wide regional stresses. At the southeastern border of the Puna plateau, where a stress rotation of about 30-degrees may be detected, preliminary order-of-magnitude calculations indicate that the local plateau-induced compression can be about 1-2 times greater than the regional horizontal deviatoric stress. The present predominance of the local stress over the regional stress is consistent with the decrease of the Nazca-South America convergence rate observed since about 10-20 Ma.