EVALUATING OROGEN PARALLEL TRANSPORT OF THE UPPER CRUST WITHIN THE CENTRAL ANDEAN OROCLINE

The geometry of the Central Andean orocline presents a compex, three dimensional, kinematic system that is not well understood. Previous work in the region has defined a shortening gradient that increases towards the orocline axis and a paleomagnetic rotational pattern that argues for convergence of upper crustal material towards the bend. This study evaluates the structural capacity for orogen parallel transport of material through a fault block model constrained geometrically by new and existing geologic map data while displacement magnitudes are varied spatially in accordance with the observed shortening gradient. To more accurately evaluate the structural systems potentially accommodating crustal convergence in the central Andes, we developed this new high-resolution fault block model to palinspastically restore block displacements using shortening data from balanced cross sections to the north and south of the bend. Using existing shortening data, the model indicates areas with block overlaps or gaps where the fault framework and displacement vectors either provide a viable restoration or violate the kinematic constraints of the convergent margin. Areas that produce kinematically untenable restorations indicate regions requiring further study to determine if orogen parallel displacement will result in a permissible restoration. At this stage, the resulting plan view displacement indicated that little, if any orogen parallel transport is required in the northern orocline limb to accommodate the previously published value of ~280km shortening. Near the orocline axis, highly focused transpressional displacement is required within and to the south of the Cochabamba fault zone to account for ~300km shortening documented for the area. Conversely, to the south of the orocline axis less than ~100km of transpressional displacement, distributed within the Interandean zone, is required to restore the documented ~330km in shortening for the area. This difference in deformational styles across the orocline is spatially coincidental with the increase in width of the sub-Andean foreland, implying that the structural boundary between the Interandean and sub-Andean zones strongly constrains transpressional displacement magnitudes.