Laccoliths as a probe for bedrock landscape transient dynamics: Numerical modeling and application to erosion history of the Colorado Plateau

Abstract:

The response of bedrock landscapes to localized perturbations that uplift the land surface remains an important and unsolved problem in geomorphology. Laccoliths, shallow plutonic structures relatively common in magmatic environments that dome overlying crustal rocks, represent a simple and near-axisymmetric perturbation on the landscape. The structures are commonly smaller than typical drainage basins, allowing their erosional response to be used as a proxy for relative uplift rates and position in pre-existing drainage networks. Using a landscape evolution model, we analyze the coupling between pre-intrusion drainage conditions and laccolith uplift. We study the effects of laccolith uplift rate, geometry, and basin position on drainage development on both the laccolith and surrounding landscape. We use a Monte Carlo scheme to sample the large parameter space and develop a series of metrics based on channel network and landform geometries to characterize the results. In particular, we develop two dimensionless numbers to quantify the importance of uplift versus erosion, and initial condition (position of laccolith in pre-existing landscape).

We then apply our results to Oligocene-age laccoliths on the Colorado Plateau, USA. The age of the Plateau’s present drainage network is debated, with some arguing for a long-lived drainage network active since 60-70 Ma, and others arguing for a much younger pulse of incision at <6 Ma to integrate the modern drainage system. The laccoliths (i.e. the Henry, La Sal, and Ute Mountains, and Navajo Mountain), emplaced at 25-28 Ma, are potentially well suited to inform this debate. Laccolith erosion patterns on the Colorado Plateau are highly asymmetric, with long-axis orientations paralleling present directions of the Colorado River and its tributaries. Thus, the current drainage pattern is strongly imprinted on the laccoliths, suggesting we can characterize controls on laccolith erosion to invert for timing and rates of erosion.