A team of scientists from Ohio State University and the University of Hawaii is constructing a dense GPS network over the entire eastern flank of the Andean Plateau within Bolivia and northernmost Argentina in order to address a variety of questions about orogenic wedge processes and Andean plateaus margin kinematics. The project will to study contemporary patterns of deformation in the Andean Plateau, and focus on its eastern flank, which is comprise of the Eastern Cordillera, the Interandean Zone, and the Subandes. To better understand orogenic wedge processes they research team will: (1) determine whether a strong eastern flank velocity gradient is a signal of a slipping-locked transition zone on the basal decollement or of out-of-sequence thrusting; (2) measure differences in the width of the active wedge north and south of the Santa Cruz bend and determine its relationship to the differences in rainfall (as a proxy for erosion) on either side of the bend; (3) assess mechanisms by which active shortening is transferred from the deeper, inboard portions of a plateau to the shallower outboard wedges. Andean plateau kinematics will be elucidated by: (1) determination of the kinematic framework of the east flank of the Andean plateau; (2) measurement of the variation of active shortening along-strike; (3) assessment of the influence of paleogeography and foreland basin stratigraphy on crustal displacement and foreland shortening; (4) examination of the relationship between the absence of Paleozoic sediments in the foreland basin of the Chapare region and the Cochabamba Shear Zone; (5) exploration for evidence of extrusion tectonics in the main physiographic transition or topographic ramp above the northern Subandes.
Orogenic plateaus have formed throughout much of geological history, and because they are produced by massive crustal thickening accompanied by large scale crustal melting, their formation plays an important role in the evolution of continental crust. The best prospect for understanding the mechanisms responsible for plateau orogenesis is the study of active continental plateaus, such as the plateau in the central Andes. Current theories for the development of this high plateau suggest either a slow two-stage uplift process or a rapid uplift scenario. This study will provide data that will contribute to the resolution of this controversy as well as improve understanding of the active processes along the plateau's eastern margin. This project should lead to a deeper understanding of the way that mountain belts and topographic plateaus deform. The research should improve understanding of the generation of earthquakes in the highly populated regions flanking high mountains.
In order to better understand the processes by which mountain belts like the Andes form and grow we built a dense GPS network over the entire East Flank of the Andean Plateau. Surface motions recorded by our GPS network suggest that the subhorizontal fault underlying this area is locked up to 100 km from the fault tip. Analysis of the fault scarps formed where this fault breaks the surface indicates that the fault has repeatedly slipped in large earthquakes. We suggest that the rupture of the entire locked section of the fault could generate an earthquake of magnitude 8.7–8.9 - an unexpected finding that suggest assessments for the seismic hazard for this area need reexamining. In order to lay the ground work for a comprehensive evaluation of the seismic hazard for this portion of the central Andes we provide the first detailed constraints on the distribution and timing of deformation at the fault front. We use field observations and geophysical survey data to analyse the onset and evolution of faulting. Our observations indicate that, in southern Bolivia, stresses generated by the plates converging to the west of the Andes are released incrementally by earthquakes that primarily rupture a narrow section near the eastern limit of fault zone. The faults and folds comprising this zone pose a major source of regional seismic hazard. Numerical modeling was performed to explore how the faults that underlie mountain systems like the Andes evolve over time in different climatic conditions. We find that there are distinctive evolutionary patterns for the fault systems depending on the climate, which controls the erosional efficiency of the growing mountains. For mountains in arid conditions, and thus little erosion, faulting is distributed throughout a wide zone. In a stable wetter climate, with fixed erosional efficiency, faulting focuses towards the distal tip of the system. In conditions where a change to a wet climate leads to increasing erosion, an initial period of diffuse fault activity is observed followed by a stable focusing of faulting towards the tip, generating a long-lived deformation front. This last scenario compares well to the structural evolution identified for our study area, suggesting that the southern Bolivian Subandes are not in a steady state but are responding to an increase in erosion starting ~5 million years ago.
