BENDING THE ANDES: OROCLINE KINEMATICS AND 3-D DEFORMATION

Efforts to document the geodynamic evolution of the Altiplano plateau located at the central Andean orocline have produced a wealth of data that constrain the kinematics of the retro-arc fold-thrust belt from 13 – 22°S. Developing an accurate model of orocline deformation requires integrating vertical axis rotations, fault displacements, and internal strain into a reconstruction capable of resolving the 3-D displacement field. We present a new high-resolution, map-view reconstruction of the Bolivian Andes that incorporates shortening estimates and deformational timing constraints from five published transects; an extensive regional paleomagnetic database; and recently acquired strain data from the orocline axis. Reconstruction results indicate that orocline limb rotation documented by paleomagnetic and GPS data is accommodated at the orocline core by translational displacements parallel to the orogenic structural trend. Field evidence supports the presence of translational motion on specific foreland-directed thrust faults, but the magnitude of displacement is unconstrained. From the reconstruction, we show that the magnitude of translation is dependent on the magnitude of limb rotation. The largest permissible limb rotations based on paleomagnetic data and extrapolated GPS data (±14°) require ~85 km of N-S, orogen parallel translation in the southern limb while the minimum tested rotations (±6°) require ~150 km of translation. Additionally, the reconstructions suggest the possibility of 20–30 km (minimum/maximum rotation scenarios) of left lateral slip on the Cochabamba Fault, which separates the northern limb from the orocline core. Differential shortening between transects where shortening estimates are available is insufficient (< 3°) to account for limb rotation, suggesting that curved slip paths or actual orocline bending may have contributed to total limb rotation. The combination of limb rotation and orogen parallel translation focuses more crustal material at the orocline core. This could potentially enhance crustal thickening below the Altiplano beyond thicknesses postulated from 2-D shortening estimates alone. Recent seismicity at the orocline core (consistent with translational faulting) as well as regional GPS data suggests that oroclinal bending may be active today.