U-series dating and stable isotope analysis of Quaternary travertines with implications for incision rates in western Rio Grande rift, Carrizo Arroyo, New Mexico

—Travertine deposits associated with fluvial terraces in Carrizo Arroyo, central New Mexico indicate that Plio-Pleistocene incision rates in this area have fluctuated since 3.7 Ma. Carrizo Arroyo is an east-west trending intermittent tributary to the Rio Puerco on the west side of the Rio Grande rift. It has incised deeply into the Carrizo Mesa basalt (3.7 ± 0.4 Ma) and underlying Paleozoic and Mesozoic strata exposed by the Lucero uplift. Based on the age of the Carrizo Mesa basalt and its height above the modern streambed (~ 183 m), Carrizo Arroyo incised at a minimum long-term average of 50 m/my over the history of the arroyo. Since the deposition of the basalt, terraces of abandoned river deposits were deposited at different elevations above the modern stream in Carrizo Arroyo. These terraces contain travertine as both flowstone and gravel-coating cement. We used the travertine in these terrace remnants to calculate late Pliocene and Quaternary incision rates since 3.7 Ma using the U-series radiogenic method. The terraces with travertine deposits located at 2, 18, and 130 m above the present stream give U-series and model ages of 3 ka ±0.015 ka, 170-180 ka ±1 ka, and 1180 ka ±440 ka, respectively. This indicates that the drainage incised at an average rate of ~19 m/my from 3.7 Ma to 1180 ka, but subsequent incision increased dramati - cally to ~150 m/my. The nearby lower Rio Puerco shows a similar incision history, with increased incision rates around 1 Ma. This regional trend may reflect: (1) climatic changes, which resulted in more erosive conditions during the late Pleistocene, (2) surface uplift due to Quaternary volcanism and mantle driven uplift associated with the Jemez lineament, and/or (3) integration of the Rio Grande system through the Albuquerque Basin.