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 valuable constraints on the geometry and state of stress of the subducting Nazca plate. These earthquakes occurred northeast of the Bolivian orocline, which is bracketed between two regions of a "flattened" Wadati-Benioff zone. Furthermore, these events occurred in an area that experienced no seismicity in at least the past 30 years. North and south of this zone, the deep seismicity is abundant and indicates down-dip compression. Collectively, the seismicity defines the geometry of the deep Nazca slab, which is continuous and bent about a sharp arc. This bending 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 orocline, the South American crust has shortened less; the differential shortening occurs in conjunction with the flattened and bent Wadati-Benioff zone. The upper Nazca slab has undergone a tremendous amount of horizontal deformation that is controlled by the convergence direction relative 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.