QUATERNARY STATE OF STRESS IN THE NORTHERN ANDES AND THE RESTRAINING BEND MODEL FOR THE ECUADORIAN ANDES

Inversion of shallow focal mechanisms in the Northern Andes, together with a neotectonic analysis of the Ecuadorian Andes, shows that the st ate of stress is homogeneous in most of the Northern Andes (E-W-trendi ng sigma(1)). However, north of 5 degrees N, sigma(1) is roughly NW-SE trending. This difference in state of stress is due to the force bala nce between the Nazca, Caribbean and South American plates, South of 5 degrees N, the state of stress appears to be mainly controlled by the Nazca-South American plate interaction, while, north of 5 degrees N i t appears to be mainly controlled by the Caribbean-South American plat e interaction. This state of stress difference from south to north is consistent with dextral motions along faults trending parallel to the chain south of 5 degrees N and sinistral motions along faults trending parallel to the chain north of 5 degrees N, This inversion of shallow focal mechanisms also illustrates the incomplete strain partitioning of the oblique convergence between the Nazca and South American plates along the Ecuadorian-southern Colombian trench. The convergence obliq uity that increases northward controls the active transcurrent fault s lip rate in the upper plate. Finally, this convergence obliquity is re sponsible for the location of the seismicity in the upper plate. In ad dition, the Andean Block is considered in this paper as a large triple zone under constriction, with one of its borders (Caribbean) behaving partly as a free border.