DRAINAGE BASIN DYNAMICS AND TRANSIENT LANDSCAPE EVOLUTION IN DECAY-PHASE OROGENS

Landscape evolution in decay-phase orogens is traditionally thought of as a slow and steady process of progressive landscape beveling to flat. The conceptual appeal of this paradigm has allowed it to persist for more than a century; however, nearly all decay phase orogens show evidence of unsteady landscape evolution in their stratigraphic and geomorphic records long after tectonic activity ceases. Most researchers favor external drivers, such as uplift due to dynamic mantle processes or late Cenozoic climate change, as a means to explain the unexpected dynamics of ancient mountain landscapes, yet conclusive supporting evidence is usually lacking. Here, I use digital elevation models and numerical modeling to demonstrate that dynamic landscape evolution is characteristic of decay-phase orogens due to incision through rocks of variable resistance to erosion. Using the southeastern U.S. as a natural laboratory, I show that erodibility associated with different rock types varies by more than a factor of five within the Upper Tennessee drainage basin. A series of knickpoints are observed in the Upper Tennessee basin and inverse modeling of fluvial topography shows that the knickpoints represent the propagating front of incision resulting from ~150 m of base level fall at the river mouth at 9 ± 3 Ma. A single rapid base level fall event is consistent with capture of the Upper Tennessee Basin by the Lower Tennessee River basin; an inference supported by Late Miocene changes in sediment dispersal patterns to the Gulf of Mexico, aquatic species distributions and calibrated phylogenetic histories of fishes and salamanders. Modeling of theoretical steady-state river network elevations using a reconstructed pre-capture paleo-drainage network of the southeastern U.S. shows that incision through a resistant sub-horizontal capstone into underlying more erodible shales in the ancient Alleghanian foreland basin facilitated expansion of the Lower Tennessee basin and ultimately capture of the Upper Tennessee basin. The results of this study suggest that lithologically driven changes in river network topology gave rise to the model Tennessee River system and emphasize that landscape evolution in decay-phase orogens is inherently dynamic, even in the absence of even in the absence of external drivers.