UW Assistant Professor Jay Chapman teaches a winter-term field course in southern
Arizona, during which students were able to investigate the Orocopia Schist in person.
Chapman is the author of a new paper that suggests the rock originated as sand and
mud subducted off the coast of California around 75 million years ago, then returned
to the Earth’s crust by rising up through the mantle as enormous diapirs. (UW Photo)
A University of Wyoming professor has used computer modeling to propose that sand
and mud subducted off the coast of California around 75 million years ago returned
to the Earth’s crust by rising up through the mantle as enormous diapirs, like blobs
in a lava lamp.
These blobs are now found at the surface of the Earth, far inland from the coast,
in places including the Mojave Desert and western Arizona.
“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.
“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.”
How the subducted sediments returned to the surface of the Earth and the distribution
of the sediments in the subsurface are some of the questions Chapman is trying to
answer with his research.
Numerical modeling of sediment subducted beneath North American crust suggests that
the sediment may buoyantly rise up through the mantle, like a lava lamp blob, and
become attached to the base of the crust.
“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 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.”
