APATITE 4HE/3HE THERMOCHRONOLOGY AND THE TOPOGRAPHIC DEVELOPMENT OF YOSEMITE VALLEY, CALIFORNIA

The spectacular landscape of Yosemite Valley, with its immense granite walls towering above the valley floor, has fascinated scientists since before John Muir explored the valley in the late 1800s. Muir argued for a wholly glacial origin that he supported with vivid descriptions of glacial striations and moraines as he traced ancient glacial pathways through the valley. Other researchers suggested a combination of fluvial and glacial processes, which culminated in the rigorous work of Matthes (1930), who measured the topography of upland tributaries and concluded that the majority of Yosemite Valley was fluvial in origin, with only ~500 m of glacial erosion, at a maximum, shaping the landscape we see today.

The debate of western Sierran canyon origin today focuses on the timing of incision. Were these canyons carved in the Late Cretaceous, as House et al. (1998) suggest, or are they largely Miocene to Pliocene features (e.g. Wakabayshi, 2013)? In this contribution, we use single-crystal apatite ⁴He/³He thermochronology to explore the timing of incision of Yosemite Valley, arguably the most famous of these canyons, and whether our data can disentangle the fluvial versus glacial components.

We collected samples from 11 bedrock sites located in and around Yosemite Valley, and resultant apatite (U-Th)/He ages vary. In the depths of Yosemite Valley, ages are ~40 Ma, whereas downstream, in rocks with no evidence of past glaciation, ages range from ~70-80 Ma. Samples from the upland surfaces north of Yosemite are >60 Ma.

Apatite ⁴He/³He data allow us to search time-temperature space for probable thermal histories. Samples within the glaciated landscape of the valley bottom support late Cenozoic rapid cooling. Samples located outside the regions of glaciation (e.g. downstream samples and northern upland surface samples) instead show an early Cenozoic cooling signal, likely related to exhumation after pluton emplacement. We will discuss this thermal modeling in detail while probing various complexities, such as non-uniform U-Th concentration distribution and radiation damage. Finally, we will address how well these data elucidate the degree to which the topography formed by fluvial versus glacial processes.

Matthes (1930) USGS Prof. Paper

Wakabayshi (2013) Geosphere

House et al. (1998) Nature