The importance of ignimbrite armor on mountain range evolution

Abstract:

In most tectonically-active mountain ranges, hillslope erosion and river incision continuously exhume bedrock and transport it out of the system. These surface processes thus act to balance rock uplift and maintain relatively steady elevations throughout the range over time. However, in volcanically-active mountain ranges such as the Andes, large eruptions can rapidly engulf the landscape, depositing thick, competent volcanic materials that perturb surface processes and drastically impact the rock uplift-erosion equilibrium.

The Andes have a protracted history of large explosive eruptions evidenced by thick and spatially expansive ignimbrite sequences. For example, in the Central Andes, the >1000 km³ Oxaya Formation ignimbrites have blanketed the western Andean flank since the early Miocene, demonstrating that volcanic deposits can affect the landscape for a prolonged period. Therefore, in a mountain range with relatively frequent, large-scale eruptions, volcanism is as important to the evolution of the system as tectonics, climate, or surface processes.

In this study, we investigate landscape response to the deposition of volcanic materials on mountainous landscapes. Using a numerical landscape evolution model, we test the relative importance of regional background influences such as tectonics (i.e. rock uplift), climate (i.e. rainfall), and surface processes (i.e. river incision), as well as mechanical parameters intrinsic to volcanic materials (i.e. erodibility), to understand how volcanic eruptions have affected the evolution of volcanic mountain ranges. We address questions such as: How long does it take to remove a competent ignimbrite from the surface? What is the expected pace and pattern of erosion? How important is the initial topography or the geometry of the ignimbrite surface? How do the intrinsic mechanical properties of the volcanic deposits affect how the mountain range responds?