Published Saturday, December 22, 2001

Where the fault lines met, a mountain was born

By Andrea Widener CONTRA COSTA TIMES

Though it is 3,849 feet tall, Mount Diablo is just a baby.

The mountain that lends its name to everything from roads to races is less than 5 million years old, a mere blip in a geologic lifetime.

It began as barely a bump on the floor of a large, shallow bay whose waters licked at the Livermore Valley's edges. And Mount Diablo is still growing, thanks to both an occasional jarring push from the area's earthquakes and a newly discovered fault hidden deep under the mountain.

"Geologically, Mount Diablo is a very, very interesting place," said Jeff Unruh, a geologist with the Walnut Creek firm of Lettis and Associates who has become an expert on the East Bay's preeminent landmark.

The rocks that dominate Mount Diablo's crest were created on the deep Pacific Ocean floor 140 million years ago, hundreds of miles from where the mountain now stands and perhaps as far away as the equator.

As the continental plates moved and collided over the next 80 million years, these hard rocks from what geologists call the Franciscan era were pushed underneath rocks from the North American plate. This left two layers of hard rock lying in a flat expanse stretching from the Berkeley hills across the Central Valley to the young Sierra Nevada mountains. This valley occasionally filled with water, creating a bay similar to Mexico's Gulf of California but very shallow, said U.S. Geological Survey's Russell Graymer, a research geologist who has studied Mount Diablo.

All the while, the area that would become Mount Diablo was lying dormant, collecting deposits from rivers and the bay.

A trip along this ebbing and flowing shore 10 to 12 million years ago would show you the same fossils of clams, oysters and sand dollars that you can see at Shell Ridge in Walnut Creek or Black Diamond Park on the mountain's northeast side. You can still see the bluish sandstone filled with volcanic rock that rivers carried from the Sierra Nevada to the East Bay, said Jim Walker, a USGS researcher and master's degree candidate at San Jose State.

"Mount Diablo was not a big mountain. It might not have been even a hill," Walker said.

Finally, some clues

Until a few years ago, geologists didn't really understand what caused Mount Diablo to rise from this valley, much less whether it was still growing and changing. Unsatisfying theories about the mountain's origin ranged from a dormant volcano to an unexplained spike in the earth's crust.

What they did know was that 5 million years ago, something changed in the Bay Area that started it shaking with large earthquakes that continue today.

Unruh, working with Reno colleague Tom Sawyer and others, first formally proposed that Mount Diablo was created by a bend connecting two earthquake faults. Pressure from the Greenville fault jumping west to the Concord-Green Valley fault scrunched the land in between to create Mount Diablo, he said.

In the five years since, Unruh and Sawyer have suggested that an unseen fault hidden under the mountain itself adds enough power to keep Mount Diablo growing. Unlike other Bay Area faults, which tend to move side to side with the continental plates, the Mount Diablo fault pushes one side of the fault up over the other. Each earthquake pushes the north part of the mountain higher, gradually building the mountain.

"All of a sudden, instead of just sitting there having stuff deposited on it, (Mount Diablo) started to get pushed up," Graymer said. "It broke right in the middle."

Over the years, this movement has flipped up the ancient layers of rock for all to see. The deep ocean Franciscan rocks are now at the top of Mount Diablo. The Tassajara hills south of Mount Diablo are part of the same folding that is creating the mountain itself, but this area hasn't been turned upside down by pressure from the two faults.

If you could see it, the fault inside the mountain would come to the surface in the San Ramon Valley, but it is buried by this flipping and folding, Unruh explained. The fault starts near the surface, then slopes deep into the earth as it nears the San Joaquin Valley.

Since this so-called "blind thrust fault" is hard to study through traditional methods, Unruh and Sawyer looked at rock patterns and old oil-drilling maps to find evidence of the fault.

Though Unruh and his colleagues haven't convinced the entire geology community, the Mount Diablo fault was given enough weight to be included two years ago in a Bay Area earthquake risk assessment. For definitive proof, scientists will have to wait for another earthquake underneath the mountain.

"Clearly the faults are still active; things are still being pushed up," Graymer said. "Whether or not it will geologically speaking continue to get taller ... it is hard to predict."

article found at: http://dailynews.yahoo.com/h/krcontracosta/20011222/lo/where_the_fault_lines_met_a_mountain_was_born_1.html