Lithologic controls on knickpoint formation in Sierra Nevada bedrock channels.

摘要

In the Sierra Nevada Mountains, the distribution of knickpoints in bedrock channels is often used as evidence to argue that the mountain range has been recently uplifted. The assumption that bedrock channel knickpoints are transient features responding to a recent pulse of uplift contributes to the ongoing debate about the range's geomorphic and tectonic evolution. Determining how knickpoints form in heterogeneous granitic landscapes remains fundamental in understanding the lithologic, tectonic, and climatic controls on bedrock incision. Defining the controls on knickpoint formation may limit how knickpoints are used to infer the tectonic history of the Sierra Nevada. In this study, tributaries of the North Fork Feather and Merced Rivers are examined because they are underlain by a variety of rock types, thereby providing an opportunity to investigate the relationship between lithologic diversity and knickpoint formation. In these tributaries, bedrock above knickpoints is often finer grained with a higher percentage of quartz than bedrock below the knickpoint. The differences in bedrock grain size and quartz content are found to be statistically significant. This study argues that variations in rock hardness predispose lithologically heterogeneous bedrock channels to express transitions in a channel's resistance to erosion as knickpoints. Thus, knickpoints in Sierra Nevada bedrock channels may have formed by differential erosion rather than uplift-driven incision, and using Sierra Nevada bedrock channel knickpoints as a tool to interpret tectonic activity is arguably inappropriate.