Transient and Steady-State Kinematic Response to Erosional Forcing in an Orogenic Wedge: Sandbox Perspective

The evolution of orogens is highly affected by surface processes that control mass distribution. Transportation and redistribution of mass at the Earth's surface modifies the gravitational load and alters the stress field and kinematics within orogens. We explore the role of asymmetric erosion, indenter dip angle, and flux steady/non-steady state in determining the patterns of deformation and exhumation in doubly-sided orogenic wedges. In our analogue model, shortening of the orogen is driven by rigid indenters, represented by Plexiglas wedged blocks (35 and 70 degrees) that deform a non-cohesive dry Coulomb material (walnut shells) representing crustal material. Three end-member erosional scenarios are considered. In the first case, erosion is not applied, and thus the doubly-sided orogenic wedge evolves without restraints (non-steady state). In the second case, erosion is concentrated solely on the indenters side of the orogen (retrowedge), and in the third case, erosion is focused on the flank opposite to the indenter side (prowedge). In the last two cases, steady-state conditions were present in the middle stages of shortening. Strain and exhumation were calculated using displacement fields from 2D particle image velocimetry (PIV analysis). 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 toward the indenter, and the shear bands evolve to steeply dipping structures. In the case of retrowedge erosion, the old fore-shears back rotate toward the indenter, and the shear bands evolve to shallowly dipping structures. In the case of prowedge erosion, old fore-shears rotate dramatically toward the indenter, and the shear bands become very steep and even overturned. Rate of rotation, number, initial dip angle, and lateral interval of formation of fore-shears are directly dependent on erosion, dip of the indenter, and flux steady state conditions. High exhumation rates are linked to places with high erosion rates. Exhumation rates in the retrowedge erosional cases are higher than those in the prowedge erosional case. Results indicate that spatially distributed erosion and steady-non-steady state conditions play a primary role in determining the patterns of deformation and exhumation in doubly-sided orogenic wedges. Results have important implications for P-T paths and for the interpretation of exhumation patterns from thermochronologic studies.