Collaborative Research: Formation of the Dry Valleys, Antarctica: Linking Thermochronometric (U-Th/He) and Cosmogenic Constraints on Landscape Development

California Institute Of Technology, Pasadena CA

Investigators

Abstract

9909435 Farley This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports an investigation of uplift and landscape evolution of the Dry Valleys segment of the Transantarctic Mountains. The time when mountains form as topographic features is a crucial factor for understanding their role as gravitational driving forces for crustal deformation, as well as for assessing their influence or dependence on climate variations. Yet, this aspect of orogenesis remains poorly documented because the geomorphologic expressions or sedimentation record associated with increasing surface elevations may be modified by later erosion. Similarly, geochemical tools used for tracking erosion and landform development (e.g., surface exposure dating) are seldom useful for this purpose because they usually provide information on only the most recent (<1 million year (Ma)) history. As a result, a large gap in our understanding of the temporal and physiographic/tectonic evolution of mountain belts remains. In the absence of direct constraints on the geomorphology of evolving mountain ranges, episodes of rapid cooling deduced from techniques like apatite fission track thermochronometry are often attributed to the topographic rise of mountain belts. This practice assumes that exhumation (movement of a rock upward with respect to the earth's surface) and bedrock uplift (movement of the rock upward with respect to the geoid) inferred from cooling data represent an equivalent increase in elevation at the earth's surface (and relief), even though workers have shown that such a correlation does not always hold. One way to address the problem of long-term landscape evolution is to combine low-temperature thermochronometry with surface exposure dating. The Dry Valleys region of the Transantarctic Mountains is an ideal place for such an exercise because many of the modern land surfaces in the region may be as old as 15 Ma, clearly requiring very low erosion rates since the mid-Miocene. Apatite fission tracks indicate that rocks in the Dry Valleys cooled through approximately 105 degrees C as recently as about 45 Ma, leaving a gap of about 30 Ma in our knowledge of the evolution of this mountain range. Application of the newly developed apatite (U-Th)/He thermochronometer (closure temperature of 70 degrees C) can further close this gap because the technique is sensitive to even lower temperatures and more importantly apatite helium ages can provide a uniquely sensitive record of the time at which topographic relief develops. This project is a tightly focused study designed to combine thermochronometric indications of river valley incision with cosmic-ray exposure ages to better constrain the formation and geomorphologic evolution of the Dry Valleys region. The apatite (U-Th)/He and exposure age data that will be collected will provide a much-needed link between the large amounts of data already available from apatite fission track studies on bedrock uplift and exhumation on the one hand, and landscape evolution on the other. In particular, by constraining the formation age of the Dry Valleys, this project will determine when the Transantarctic Mountains formed as a topographic feature. The results of this work should have general implications for the geodynamic evolution of the region, and will contribute to the debate regarding Cenozoic paleoclimate changes influencing the growth and stability of the East Antarctic ice sheet.