MAGMATICALLY DRIVEN FLEXURAL UPLIFT OF THE CENTRAL CASCADES ARC, USA, ENCODED BY FLUVIAL INCISION OF THE COLUMBIA RIVER GORGE

Mountain building in arc settings occurs through an interplay between the eruption and intrusion of mantle-derived magma, superimposed on regional tectonics. Although challenging to deconvolve, arc topography records the local magmatic history as well as the structural and erosional response to volcanic perturbations. There is no better setting to study these processes than the Columbia River Gorge, where persistent incision by the Columbia has provided a time-evolving cross section of the Cascade Arc through a pulse of high volcanic output, accompanied by arc-scale uplift and faulting along the margins of the arc since 3.5 Ma. Regional plate kinematics are orthogonal to this deformation and so magmatic processes may play an outsized role in the generation of topography. We seek to understand how magmatic forcing is recorded in the landscape by applying tools of tectonic geomorphology to infer long-term patterns of magma flux.

Uplift across the gorge is recorded in Columbia sediments preserved in fluvial deposits that fill a channel abandoned by the river after 3.5 Ma. Deformation of this strain marker (up to ~900m), is mimicked in a set of fluvial knickpoints in 16 Columbia tributaries, suggesting a transient landscape response to the uplift pulse. Using both the geologic strain data and a bedrock river incision model, we model deformation as uplift of a thin elastic plate subject to basal forcing from intrusions. Data inversions show that uplift has been continuous since onset, and that rates of landscape adjustment are driven primarily by intrusive uplift (rather than surface deposition) allowing us to isolate the constructional component of topography from river profiles. We show that observed displacements across the gorge can be explained through magmatic processes alone with deep magma flux rates of ~10km^3/km/Myr and an intrusive:extrusive ratio of ~3:1. Best fit models from our flexural inversion imply concentrated stresses in the upper crust that could explain both arc marginal rifting and observed locations of off-axis volcanic vent fields. This coupled geophysical and geomorphic approach provides insights into magmatic forcing on arc topography and provides a self-consistent explanation for both the structural and morphological evolution of the Columbia River Gorge.