Boulder, Colo., USA: Yosemite Valley, nestled within California's towering Sierra Nevada range, is one of Earth's iconic landscapes, with imposing cliffs bounding a riverine valley floor. This topographic feature, part of the homeland of the Ahwahneechee people, played an outsized role in the history of modern environmental politics, fueled foundational debates in geomorphology, and is now visited by more than three million people per year as the centerpiece of Yosemite National Park.
Despite such broad interest, more than a century of geological investigation has failed to provide direct constraints on when Yosemite Valley formed. Previous studies exploring when it might have become a deep canyon rest on assumptions—that canyon incision coincided either with tectonics related to mountain building or with valley formation elsewhere in the range. Prior claims about canyon age range from two million to more than 50 million years.
In a new study published Wednesday in the Geological Society of America Bulletin, Kurt Cuffey and colleagues have obtained the first direct constraint on the age of canyon deepening by exploiting measurements of radiogenic Helium in apatite crystals extracted from granite bedrock samples taken from in and around the valley. The retention of Helium produced by radioactive decay of uranium and thorium in minerals like apatite depends on temperature, while the temperature history of a mineral now exposed at the surface reflects the history of erosion that exposed it. By comparing temperature histories of rocks taken from different locations, general patterns of the evolution of topography can be inferred. This was achieved in the present study by linking numerical simulations of crustal temperatures and statistical analyses to measurements of both the total accumulated Helium and its spatial distribution within crystals.
The analysis demonstrates that, after a period of quiescence lasting more than 40 million years, the topography of the Yosemite region was transformed by canyon incision in the late Cenozoic.
Tenaya Canyon, Yosemite's deep northeastern branch (named for the last chief of the indigenous inhabitants), most likely deepened by about one kilometer in the last five million years. Accounting for uncertainties, 40% to 90% of the canyon's current relief probably formed since 10 million years ago. The magnitude of inferred deepening depends on the geothermal gradient, with smaller gradients—such as those currently observed in the western Sierra—implying more deepening. Deepening likely occurred through a combination of fluvial and glacial erosion.
Canyon deepening identified in this study coincides with the documented end of subduction on the Pacific Plate boundary at this latitude, formation of the tall eastern escarpment of the Sierra Nevada, and rapid extension in the Basin and Range province to the east. Thus, uplift of the crest of the Sierra Nevada during the last five million years or so could have driven incision of the Yosemite region by increasing slopes of rivers and their eroding power.
This hypothesis was proposed by Francois E. Matthes of the U.S. Geological Survey in seminal work published in the 1930s but has remained controversial due to the lack of constraints on both canyon age and Sierra Nevada uplift. Such uplift must have been limited in magnitude because, according to several convincing studies, lands east of the current Sierra crest were already elevated.
Other explanations for Yosemite incision include reconfiguration of drainages following mantling by volcanic debris, and onset of Pleistocene glaciations. The topographic effects of glaciation vary spatially in this region. Although a deep glacial basin underlies the current sediment floor of the main Yosemite Valley, the upper reach of Tenaya Canyon maintains a fluvial cross-sectional profile.
In addition, this new study demonstrates that the Tuolumne Meadows region, northeast of Yosemite Valley, escaped deep erosion in the late Cenozoic despite episodic, extensive Pleistocene glaciation. Glaciers in this region were likely unable to erode due to the absence of a prior river canyon that focused ice flow, and due to the paucity of fractures that would make rock vulnerable to erosion.
Late Cenozoic Deepening of Yosemite Valley
Kurt Cuffey; Alka Tripathy-Lang; David Shuster; Greg Stock; Matthew Fox
