Unusual radial topography results from dynamic removal of dense eclogite root in the Wallowa mountains, (Oregon, USA)

The Mesozoic Wallowa batholith stands high, encircled by a nearly-symmetric topographic ring. The batholith is topped by Miocene Columbia River Flood Basalt (CRB), vertically offset over 2 kilometers since its deposition. The radial bulge encircling the batholith is draped with the same CRB units, anticlinally flexed since their deposition. The range sits close to a geodetic pole, suggesting that it is under relatively little horizontal tectonic stress, and thus the development of the modern topography must be driven by recent changes in its underlying density structure. Prior studies have suggested that the batholith uplift resulted from post-Laramide loss of a dense eclogite root, but do not address the origin of the bullseye topography.

We suggest that the axial topography may result from syn-foundering lower crustal flow in a radially symmetric geometry. To test this idea, we implemented a 2D cylindrical finite element model of visco-elasto-plastic Stokes flow using the TerraFERMA/FEniCS modeling system. Preliminary results show a pronounced "pinching-out" of viscous lower crust adjacent to the batholith during root removal. We expect that the combination of this proximal pinching out, combined with elastic flexure of the upper crust can reproduce the observed axial topographic signature.