科研之友
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摘要
Regional sediment budgets provide a useful method for quantifying erosion by large river systems over geologic time scales. The Colorado River (western United States) is well suited for such an analysis because the eroding source (Colorado Plateau) and sediment sinks in transtensional basins of the Salton Trough and northern Gulf of California are intact and well preserved. Using the distribution of Late Miocene basalt flows and new thermochronologic data, we calculate that similar to 3.4 +/- 1.2 x 10(5) km(3) of rock has been eroded from the Colorado Plateau since 10 Ma. Most of this erosion probably started ca. 5.5-6 Ma, when the river system became integrated and incision rates increased dramatically. We generate two estimates for the volume of Colorado River sediment that has accumulated in basinal sinks since ca. 5.3 Ma: (1) 2.8 +/- 0.6 x 10(5) km(3), assuming that crust between 5 and 10-12 km depth in the plate-boundary basins is young metasedimentary rock mixed with intrusions; and (2) 1.55 +/- 0.35 x 10(5) km(3), assuming that crust below 4-5 km is thinned pre-Cenozoic crystalline rock. The broad overlap of the first estimate with the calculated volume of rock eroded from the plateau provides new support for a model of lithospheric rupture and rapid sedimentation in the Salton Trough. Assuming an average density of 2.3-2.5 g/cm(3), and using the range of preferred volume estimates calculated here, the total mass transferred is similar to 5.1-11.5 x 10(14) t representing an average flux of similar to 156 +/- 60 Mt/yr since 5.3 Ma, the time when the Colorado River first arrived in the Salton Trough, or 172 +/- 66 Mt/yr if we assume that all sediment flux took place after 4.8 Ma. The calculated long-term flux is strikingly similar to historical pre-dam sediment discharge measured at Yuma (Arizona) in the early 1900s (172 +/- 64 Mt/yr). The similarity of flux estimates suggests that rates of erosion and sediment discharge in this system have been consistent, on average, over modern to geologic time scales. We suggest that ongoing positive feedback between late Cenozoic erosion and flexural uplift on the Colorado Plateau provides a mechanism that could sustain steady rates of regional erosion and sediment production for millions of years after integration of the Colorado River ca. 5.5-6 Ma.
- 出版日期2013-8
