Studies of Plateau Uplift using (U-Th)/He Apatite Thermochronology and 13C-18O Carbonate Paleothermometry

California Institute Of Technology, Pasadena CA

Investigators

Abstract

The Colorado Plateau illustrates the paradox of continental plateau uplift particularly well. Like much of the North American continental interior, it is a Paleozoic-Mesozoic platform that slowly subsided 4000 m from Cambrian to Cretaceous time while sedimentation maintained its elevation near sea level. But unlike the interior, at some time after 80 million years ago, net uplift of both rocks and the topographic surface resulted in a plateau with its current mean elevation of 1900 m. The focus of this project is to provide new constraints on how and why this uplift occurred, and the relationships between topographic uplift, rock uplift and erosional unroofing. The research team is using recent innovations in helium thermochronometry and carbon-oxygen isotope thermometry to investigate both the unroofing and surface uplift histories of the southeastern portion of the plateau. Because of their long residence below the annealing temperature of fission tracks (110 deg C), plateau apatites show considerable variation in helium diffusivity caused by radiation damage. This in turn places strong constraints on the timing and rate of low-temperature cooling events down to near-surface temperatures. This project will also continue use of the 13C-18O carbonate clumped isotope paleothermometer to place constraints on paleoelevation using lacustrine carbonates. Thus far, the team has demonstrated episodic, southwest to northeast erosion beginning in Late Cretaceous time along the margin of the Plateau and culminating in rapid unroofing of the plateau interior in the last 10 million years. A surprising result is that apatites from the bottom of the Upper Granite Gorge of the Grand Canyon have indistinguishable cooling histories from apatites on the surrounding plateau. These histories indicate that the canyon was formed in Late Cretaceous time, and that the erosion level lowered itself as an equilibrium landscape onto the present topography, primarily in Late Oligocene time. Clumped isotope results to date have included studies of modern lakes, which show a clear temperature-elevation trend. For Miocene lake carbonates, the temperature change as a function of elevation is the same as the modern trend, but the temperature at any given elevation is nearly 8 degrees C hotter. This result suggests that little relative elevation change has occurred between the plateau and surrounding lowlands since ca. 16 Ma, and quantifies the amount of climatic cooling. The geologic evolution of the Grand Canyon region has been a focal point for educating the general public about the geological sciences, and science in general, for over a century. The region is also among the best natural laboratories in the world for addressing the relationship between uplift of the continental crust and the erosional forces acting upon it. This project uses new techniques rooted in the chemistry and physics of the earth?s naturally occurring isotopes of helium, carbon and oxygen to address two fundamental questions that might occur to anyone who has ever peered over the edge of the Grand Canyon: When and why did the high plateau, whose surface is made of limestone deposited in a shallow ocean, rise to its current height of over 2000 m above sea level? And when did the carving of the Grand Canyon take place?at the same time uplift occurred, or at a much later time? These questions were first posed by John Wesley Powell and associates in the late 19th century, and were the centerpieces of growing awareness in Europe at that time that American science (and scientists) were a force to be reckoned with. Even today, these questions are surprisingly controversial. Results from this research team's previous work suggest that the canyon was originally cut in the Cretaceous Period, by a major river that flowed from the southwest to northeast, opposite the modern direction of flow of the Colorado River. Further, data suggest that in the Upper Granite Gorge region, the canyon was cut in strata much younger than those exposed today. In a pulse of erosion about 20 million years ago, the erosion surface was lowered to a point very close to its modern position. These results, which will be further tested by this project, indicate that Grand Canyon was not carved by the Colorado River, which has only been in existence over the last six million years.