Yosemite Valley Age Determined by Radiogenic Helium
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
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
Contact: firstname.lastname@example.org, University of California, Geography, and
Earth and Planetary Science, Berkeley, California
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