Earth scientists untangle the curious landscape of China’s Tarim Basin
Sometimes the answers to our most persistent questions are found in the most unexpected places. Just ask Douglas Burbank.
|In China's Tarim Basin, the entire top of a fold has been beveled from bottom of the image up to the water gap |
where the river cuts through [Credit: Digiglobe Image Landsat/Google]
Despite lying in arid desert and being the site of rapidly growing, elongated folds of stratified rock called anticlines, the Tarim Basin region features huge flat surfaces that have been beveled across the tops of those folds. The folds are caused by the ongoing convergence between India and Asia. Some anticlines have risen 3 miles in less than 2 million years -- the blink of an eye in geologic time. And despite the desert climate and rapid uplift, huge expanses -- hundreds of square miles -- have been beveled by rivers. Today these rivers lie in narrow gorges cut across these growing folds. So how did they erode such huge areas in the recent past, geologically speaking?
To answer that question, graduate student and lead author Aaron Bufe turned to St. Anthony Falls Laboratory (SAFL) in Minneapolis. There, with SAFL director and co-author Chris Paola, Bufe built a 160-square-foot stream table that could progressively "grow" a mechanically driven fold across which small "rivers" flowed, causing erosion. The experiment simulated conditions believed to be the cause of the beveling -- the most recent episode of which occurred during the period of deglaciation following the last Ice Age, about 18,000 years ago. The researchers' findings appear in the journal Nature Geoscience.
160-square-foot stream table was designed to progressively “grow” a
mechanically driven fold across |
which small “rivers” flowed, causing erosion [Credit: University of California - Santa Barbara]
This erosion was so effective due to a process called channel switching. When the fluctuating flow of heavy sediment gets trapped in the channel and the river floods again, the water is diverted by the trapped sediment, so it moves laterally into another area.
"This sort of channel switching had to happen at a prolonged, rapid and spatially large scale for it to jump across and be able to erode such broad beveled zones," Burbank said. "We hypothesized that when there was more sediment, the channel switched positions more often and therefore a larger fraction of the fold was beveled off."
sketch of the experimental setup shows how the stream table simulated
conditions that caused massive |
beveling of folds in the Tarim Basin [Credit: University of California - Santa Barbara]
"Our theory posited that the degree of fold beveling -- rather than being simply incised vertically by the 'river' -- would be determined by the amount of channel mobility, how rapidly the channels switched back and forth across the stream table," Burbank said. "Moreover, we thought that this mobility was likely to be a function of the sediment-to-water ratio: More sediment would cause more rapid channel shifting. And in fact, this is what the experiments showed."
Author: Julie Cohen | Source: University of California - Santa Barbara [August 24, 2016]