California's varied landscape, once attributed mostly to plate tectonics, is better explained as a chain reaction that began when the Sierra Nevada mountain range cropped up some three million to five million years ago, according to new findings by a team of geologists at the University of Colorado at Boulder.
Dense rock underlying the Sierra Nevada 'dripped' into the earth's mantle at that time, causing the mountains to crop up, according to Craig Jones, associate professor of geology at CU-Boulder and fellow at the university's Cooperative Institute for Research in Environmental Sciences.
"This dripping event appears to be responsible for the majority of the landforms of California," Jones said, including the state's coastal mountain ranges and earthquake-prone, fault-bounded valleys.
Jones and CU-Boulder colleague G. Lang Farmer's paper titled "Tectonics of Pliocene removal of lithosphere of the Sierra Nevada, California," was published in the November/December issue of the Geological Society of America Bulletin. Jeff Unruh of consulting firm William Lettis and Associates also contributed to the report.
"As a consequence of the Sierra Nevada's uplift, the area just east of the modern Sierran crest began to spread out," Jones said. "To the east, this created the basin and range topography we see within about 30 miles to 50 miles of the eastern edge of the Sierra Nevada.
"Owens Valley, Panamint Valley, Eureka Valley, Long Valley, Mono Lake's basin, Washoe Valley and Carson Valley all came into existence between about three million and five million years ago," Jones said. "We think that area rose up and then spread out under its own weight."
To the west, the Sierra Nevada range began ramming into the Pacific tectonic plate and triggered the rise of the coastal mountain ranges. This theory of coastal range development differs significantly from previous ideas, according to Jones.
"The coastal mountains were previously thought to have been uplifted when the Pacific tectonic plate turned toward North America somewhat, about three million to five million years ago," he said, adding that recent studies have failed to show the Pacific plate shifted direction.
However, the earthquake-prone eastern California shear zone is the result of the Sierra Nevada affecting the collision between the North American and Pacific tectonic plates. "The area is a zone of geologic deformation, east of the Sierra, which is now thought to accommodate 20 percent of the motion between the tectonic plates," Jones said. "The area is home to the magnitude seven or greater earthquakes observed in 1872 at Lone Pine, 1992 at Landers and 1999 at Hector Mine."
Jones said a boat at a dock on the side of a river is a good analogy for the forces at work in the shear zone and can help explain why the San Andreas Fault may be slowing.
"The 'boat' is tied with rope and has a gangplank -- it doesn't move with the river. When the Sierra Nevada rose up and created normal fault zones, it was as if the boat's gangplank had been removed. The deeper parts of the Earth's lithosphere, below the crust, were weakened, as if the boat's ropes had been replaced with lines of taffy. The force that created the coastal ranges can be thought of as someone pushing the boat out into the river a little bit.
"Combine all these events, and the boat begins to move with the river relative to the coast. That motion of boat from dock is the eastern California shear zone. As it moves, the motion on the other side of the boat slows down -- thus the suggestion that the San Andreas Fault is slowing," Jones said.
Jones, Farmer and Unruh's conclusions are an expansion of a detailed study of the southern Sierra Nevada, co-authored by Jones and fellow CU-Boulder associate Professor Anne Sheehan that was published in the July issue of Science. Their work is part of a 16-year series of studies of California's diverse and active geology.