Topographic evolution of Yosemite Valley from Low Temperature Thermochronology

Abstract:

In this contribution, we interrogate the timing of km-scale topography development in the region around Yosemite Valley, California. Our goal is to determine when this spectacular glacial valley was carved, and how this might help address controversy surrounding the topographic evolution of the Sierra Nevada. At the scale of the range, two rival hypotheses are each supported by different datasets. Low-temperature thermochronology supports the idea that the range has been high-standing since the Cretaceous, whereas geomorphic evidence suggests that much of the elevation of the Sierra Nevada was attained during the Pliocene. Recent work by McPhillips and Brandon (2012) suggests instead that both ideas are valid, with the range losing much elevation during the Cenozoic, but regaining it during Miocene surface uplift.

At the local scale, the classic study of Matthes (1930) determined that most of Yosemite Valley was excavated by the Sherwin-age glaciation that ended ~1 Ma. The consensus view is in agreement, although some argue that nearby comparable valleys comparable were carved long ago (e.g., House et al., 1998). If the Quaternary and younger glaciations were responsible for the bulk of the valley’s >1 km depth, we might expect apatite (U-Th)/He ages at the valley floor to be <2 Ma. Instead, samples collected from bedrock outcrop at the floor of Tenaya Canyon, yield ages ranging from 35-40 Ma, while a sample from El Portal yields an age of ~74 Ma. Valley rim samples yield ages of ca. 60 Ma.

To further constrain the timing of valley carving, we have conducted apatite ⁴He/³He thermochronometry from samples along both the valley floor and rim. By restricting the permissible thermal histories at these locations, these data constrain patterns of valley topography development through time. We also supplement these data with zircon ⁴He/³He thermochronometry, which is a newly developed method that provides information on continuous cooling paths through ~120-220 °C. We will present both the apatite and zircon ⁴He/³He data and, in conjunction with thermo-kinematic modeling, discuss the ability and limitations of these data to test models of Sierra Nevada topography development through time.

Matthes (1930) USGS Professional Paper

House et al. (1998) Nature

McPhillips and Brandon (2012) American Journal of Science