Studies of Plateau Uplift using 4He/3He Apatite Thermocronometer and 13C-18O Carbonate Paleothermometer

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The Colorado Plateau, like most of the North American craton, is a Paleozoic-Mesozoic platform that on average subsided slowly for nearly 500 million years while sedimentation maintained its elevation near sea level; but unlike most of the craton, at some time after 80 million years ago, via net uplift of both rocks and the topographic surface, the plateau attained its current mean elevation of about 1900 meters with little internal strain of the upper part of the crust. Post-Cretaceous erosional unroofing of the plateau ranges from zero in preserved early Tertiary basins on the perimeter of the plateau to as much as 4500 meters in the Upper Granite Gorge of the Grand Canyon. How and when did the lithosphere acquire so much buoyancy, and what is the relationship between topographic uplift, rock uplift and erosional unroofing? This project 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. The helium apatite thermochronometer is being used to constrain the timing and rate of low-temperature cooling events down to temperatures significantly lower than those accessible by conventional whole crystal (U-Th)/He apatite ages. The new C-O carbonate paleothermometer that constrains both the carbonate growth temperature and the oxygen isotopic composition of the waters from which it was derived is also being used. This method requires fewer assumptions regarding climate change and seasonality than conventional two-phase carbonate-water oxygen isotope thermometry, and permits more effective isolation of the altitudinal contribution to the isotopic record. With this overall approach, significant new constraints are being placed on the timing of erosion of Mesozoic units off of the Permian Kaibab Limestone to form the remarkably flat southwestern surface of the plateau and the timing of incision of the Grand Canyon. In addition, analysis of a variety of Cenozoic carbonate units both on and directly adjacent to the plateau will better constrain the elevation history and its relationship to Mesozoic and Cenozoic tectonic events that surround it.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. 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 meters 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, even today, are surprisingly controversial.