GEOMETRY AND STATE OF STRESS OF THE NAZCA PLATE BENEATH BOLIVIA AND ITS IMPLICATION FOR THE EVOLUTION OF THE BOLIVIAN OROCLINE

A series of deep earthquakes beneath. Bolivia in 1994 provides valuabl e constraints on the geometry and state of stress of the subducting Na zca plate, These earthquakes occurred northeast of the Bolivian orocli ne, which is bracketed between two regions of a ''flattened'' Wadati-B enioff zone, Furthermore, these events occurred in an area that experi enced no seismicity in at least the past 30 years, North and south of this zone, the deep seismicity is abundant and indicates down-dip comp ression, Collectively, the seismicity defines the geometry of the deep Nazca slab, which is continuous and bent about a sharp are. This bend ing is a function of the dynamic evolution of the South America-Nazca plate system, Since the Tertiary, South America has incurred a maximum shortening along the Bolivian orocline, North and south of the orocli ne, the South American crust has shortened less; the differential shor tening occurs in conjunction with the flattened and bent Wadati-Beniof f zone, The upper Nazca slab has undergone a tremendous amount of hori zontal deformation that is controlled by the convergence direction rel ative to the trend of the trench. In response, the lower slab has been slowly bent about an axis parallel to the dip orientation of the deep Nazca plate.