By rising up through the mantle as giant diapirs, similar to blobs in a lava lamp, sand, and mud subducted off the coast of California about 75 million years ago returned to the earth’s crust, according to a professor at the University of Wyoming made this hypothesis using computer modeling.
The Mojave Desert and western Arizona are just two locations where these blobs can currently be found at the Earth’s surface, thousands of miles inland from the coast.
“These rocks aren’t the prettiest to look at, but they went on an extraordinary journey and have an incredible story to tell,” says Jay Chapman, an assistant professor in UW’s Department of Geology and Geophysics who focuses on tectonics.
Chapman is the author of a new paper, titled “Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction,” which was published in the August issue of the journal Geology.
Geoscientists around the world are working to understand what gives the continental crust its unique composition, and subduction and reincorporation of sediment are a popular hypothesis. In addition, many researchers are now wondering whether fluids and elements released from the subducted sediments may have contributed to the concentration of economically important minerals and metals.
Jay Chapman
“The rocks started their lives as sediment eroded from the Sierra Nevada Mountains and carried by rivers and streams down to the ocean, where they ended up deposited in a subduction trench, similar to the modern-day Marianas trench,” Chapman says.
“Then, they were carried about 20 miles deep into the Earth by a subducting oceanic plate, where the sediments were metamorphosed into a rock called schist. That in and of itself is pretty amazing, but the truly special thing about these rocks is that they didn’t stay subducted, but somehow made their way back up to the surface, where you can go stand on them today.”
Some of the problems Chapman is attempting to address with his research include the distribution of the sediments in the subsurface and how the subducted sediments returned to the Earth’s surface.
“The prevailing theory is that the sediments were smeared against and plastered to the base of the North American tectonic plate, forming a sheet-like layer,” Chapman says. “However, the density of these sediments is much lower than rocks in the mantle or lower crust and, over millions of years, computer modeling predicts that the sediments will flow and buoyantly ascend, like hot wax in a lava lamp.”
The research has implications for understanding subduction zone processes and the distribution of natural resources.
“Geoscientists around the world are working to understand what gives the continental crust its unique composition, and subduction and reincorporation of sediment are a popular hypothesis,” Chapman says.
“In addition, many researchers are now wondering whether fluids and elements released from the subducted sediments may have contributed to the concentration of economically important minerals and metals.”
