TESTING KEN PIERCE’S CONCEPTUAL MODEL OF TRANSIENT LANDSCAPE EVOLUTION AROUND THE HOTSPOT – RESEARCH ALONG FLUVIAL TRANSECTS OFF THE YELLOWSTONE PLATEAU

Ken Pierce, through his diverse research and the Pierce and Morgan review papers, synthesized a picture of the Yellowstone region as actively deforming due to a hotspot impacting the continental lithosphere and progressing across southern Idaho over time. Regional topography forms an arching swell termed the Yellowstone Crescent of High Terrain (YCHT) at the leading edge of the youngest calderas, hypothetically generated by dynamic and thermal uplift. Our work is attempting to quantify the timing and rates of erosion and topographic-relief generation in the region across the arrival of the hotspot to test this conceptual model.

We are investigating fluvial-basin records traversing off the hotspot, inspired by Ken and others who estimated incision rates and differential uplift along rivers draining the leading edge of the YCHT. A goal is to test whether incision and rock uplift indeed increase upstream toward the Yellowstone plateau using multiple geo- and thermochronometers, starting along the Snake River to the south and now focused along the Gallatin River to the north.

A suite of fluvial terraces was mapped, dated by luminescence, and analyzed along the Hoback-Upper Snake River in work published in Tuzlak et al., 2021, GSA Bulletin. The Upper Snake in Alpine Canyon is incising at roughly 0.3 m/k.y. (300 m/m.y.), which is sim­ilar to estimates from the lead­ing margin of the Yellowstone sys­tem. The pattern of incision, however, is not consistent with YCHT uplift in the headwaters. Instead, it matches incision driven by downstream baselevel fall toward the Snake River Plain and knickzones arranged upstream of active, local normal faults.

The Gallatin River study transect, oriented north off the YCHT, has relatively moderate Quaternary glaciation of headwaters and fewer potentially active faults. Yet, Initial findings again suggest a long-profile pattern of terrace deposits that is more in line with downstream baselevel fall than of headwater uplift along the YCHT. Thermochronology and paleo-landscape reconstruction addressing the longer-term evolution of erosion will put this tentative finding into context, as we hope to resolve the patterns of landscape evolution that Ken Pierce introduced.