DEFORMATION AND EROSION OF DOUBLY-SIDED OROGENIC WEDGES: SANDBOX PERSPECTIVE

Surface processes play an important role in orogenic evolution. Mass can be transported and redistributed at the earth surface, which modifies the gravitational load and alter the stress field and the kinematics within orogens. Using sandbox analog modeling, we explore the role of spatially distributed erosion in determining patterns of deformation and exhumation in doubly-sided orogenic wedges under steady state flux conditions. Orogenic shortening is driven by a rigid indenter, represented by a Plexiglas wedged block that deforms a non-cohesive dry Coulomb material representing crustal material. Shortening occurs over a crustal decollement represented by a horizontal Plexiglas ramp. Three end-member erosional scenarios are considered. In the first case, erosion is not applied, thus the doubly-sided orogenic wedge evolves without restraints. In the second case, erosion is concentrated solely on the indenters's side of the orogen (retrowedge), and in the third case, erosion is focused on the flank opposite to the indenter's side (prowedge). In the three cases the model deforms as a combination of lateral compaction and localization of strain in shear bands. In the early stages of deformation, a “pop-up” structure develops, bounded by a fore-shear on the front and a back-shear toward the indenter. As deformation continues, a new fore-shear develops, and the previous one remains inactive and is passively pushed up the wedge. In the case of no erosion, the old fore-shears rotate slightly clockwise toward the indenter, and the shear bands evolve to steeply dipping structures. In the case of retrowedge erosion, the old fore-shears rotate counter-clockwise toward the indenter, and the shear bands evolve to shallowly dipping structures. In the case of prowedge erosion, old fore-shears rotate dramatically clockwise toward the indenter, and the shear bands become very steep and even overturned. Results indicate that spatially distributed erosion plays and important role in determining patterns of deformation and exhumation in doubly-sided orogenic wedges. Quantification of individual particle trajectories within the orogen using 2D particle image velocimetry (PIV analysis) will have important implications for P-T paths and thermochronologic studies and for the interpretation of exhumation patterns.