EVIDENCE FROM THREE ISOTOPIC PROXIES FOR THE ESTABLISHMENT OF HIGH RELIEF BEFORE NEOGENE IN THE UPPER STREAM DRAINAGE OF THE COLORADO RIVER

Recent geologic and geophysical observations bring opposing hypotheses regarding the creation of high relief in the upper stream drainage of the Colorado River in NW Colorado. Extensive preservation of basalt flows of ~10 Ma old at elevations of 3.0–3.4 km and intense late Miocene-recent river incision both suggest the presence of a broad, low-relief surface prior to ~10 Ma, which imply that the region experienced broad uplift before 10 Ma and relief was created by river incision after 10 Ma. However, a negative correlation between crustal thickness and high topography suggest differential uplift of mountain ranges relative to basin floors by mantle processes. Here we use three proxies, carbonate clumped isotope and oxygen isotope and hydrated volcanic glass hydrogen isotope geochemistry to constrain the timing of creation of high relief in order to test and refine the tectonic models.

We collected Neogene fluvial carbonate cements, lacustrine carbonates and volcanic ashes from basin floors of ~1700 m high and mountain ranges of ~3500 m high. X-ray diffraction analysis and cathodoluminescence microscope observation show that the carbonates are mostly in calcite, with a few samples contain a small component of aragonite, and the carbonate did not experience extensive deep and late digenesis. Clumped isotope temperatures, which represent growth temperatures of the carbonates, changed from ~15-30 ̊C in the latest Oligocene to ~5- 9 ̊C in the latest Miocene, and the temperature difference between the high and low elevation sites persisted during the Neogene. The calculated surface water δ¹⁸O values and δD values vary between -17 and -13 ‰, and -125 and -94 ‰, respectively, and the isotopic differences between the high and low elevation sites persisted during the Neogene. The three sets of isotopic proxies show that the high relief in the upper drainage of the Colorado River was created before the Neogene, possibly by differential uplift induced by mantle processes.