Trenches and terranes mark the Lode

Editor’s note: This is the third part of an ongoing series about the geology of the Mother Lode.

“The Melones fault is the Mother Lode,” John Kramer said in an interview.

A lanky, veteran geologist with Condor Earth Technologies of Columbia in Tuolumne County, Kramer specializes in hydrogeology. He lives in nearby Vallecito, once a thriving Gold Rush town in Calaveras County but now a quiet rural residential community.

Kramer was describing a long-inactive earthquake fault zone that research has traced from north of Downieville, on the North Fork of the Yuba River in Sierra County, all the way south to Mariposa, more than 250 miles in all. A mile or more wide and flanked by other fault zones to the east and west, it passes under old gold camps, through Nevada City and Coloma, where James Marshall first found gold in January 1848, through Placerville, Jackson, Mokelumne Hill, Angels Camp, Carson Hill and Jamestown, among other towns. Other fault zones might have had gold-bearing quartz in them, but none as abundant or as close to the surface as the Melones.

If you drive the full length of Highway 49 you are traveling almost all the way on top of the Melones Fault Zone. This particular zone is notable for two things. One is the gold-bearing quartz it exposes on and near the surface. The other is that long before it was a fault zone it marked the location of a deep ocean trench that stretched northwest to southeast and far to the west. Called by some the Sierran trench, it was a submerged canyon that first formed in an ancient sea more than 200 million years ago. And it was likely the place where parts of the Farallon Plate went to their final rest.

This trench wasn’t the only ocean-bed valley worthy of the title Sierran trench. Over millions of years there were others, said Jeff Tolhurst, a geology professor at Columbia College near Sonora.

“There have likely been multiple subduction zones existent during the geologic history of the development of the rocks in the Sierra Nevada,” he said.

Tolhurst said that Rich Schweickert, an emeritus professor of geological sciences at the University of Nevada, Reno, has done extensive research in the Sierra. Schweickert grew up in Sonora and went to Stanford University for a bachelor’s degree and a PhD in geology and spent his career mapping in the Sierra.

Tolhurst said that Schweickert suggests that there are three tectonic models for interpreting the Jurassic paleogeography. The first shows both eastward and westward dipping subduction zones, while the second and third show eastward dipping subduction zones that turn on and off through time. In the process they scrape off material that geologists call “terranes.”

This process occurs where part of the Earth’s outer layer called a tectonic plate submerges beneath another tectonic plate. There are seven or eight major plates around the world – visualize an egg with cracks in its shell – and many smaller ones that abut each other, override each other, sink below another one, or do all of these things on different margins. They are solid, heavy rock sliding on top of the earth’s mantle, and the continents and islands, made of lighter material, ride on top of them.

The lighter materials, like seafloor sediments, islands, and whole archipelagos that ride atop the heavy submerging plate don’t descend with it. These “island arc terranes” pile up against the continents, get stacked up against each other – accrete – and in our case expand the landmass of North America. California is made of them.

Evidence of these exotic terranes, another term geologists use for island arc terranes, is found often partly digested right where they were placed many millions of years ago. They have names like the Calaveras Complex, the Shoo Fly Complex, the Smartville Block, the Foothills Terrane and others. Each is distinct in the nature of the rocks it contains.

The marble that forms today’s Mother Lode tourist attractions – California, Mercer and Moaning caverns – arrived here on the back of a subducting plate. It also provided the raw material for the enormous Blue Mountain marble quarry next to the Stanislaus River north of Columbia.

Mariposa slate, which began as an ocean sediment, is found throughout the foothills of the Mother Lode, sometimes thousands of feet thick. It forms the walls of many road cuts, including a prime example on Highway 49 just north of the Archie Stevenot Bridge that connects Tuolumne and Calaveras counties. It also is exhibited as the tombstone formation rocks that poke up like grave markers, vertically, from hills and pastures in the region.

Subduction and heat also had other geological consequences. Through cracks in the earth, like the Melones Fault Zone, hot mineralized liquid rose toward the surface above the descending plate. The liquid was superheated and greatly compressed, more like a dense, heavy, molten fluid than what we think of as steaming water. It came from the moisture boiling out of the plate that was being subducted and from surrounding rocks. As that fluid rose it captured minerals, chemicals like silicates, silver and copper, and, of course, gold. It created the quartz veins.

Just to the west of the Mother Lode is the Foothill Copper-Zinc Belt, formed in the same way, full of ancient greenstone, and west of that are the clays, lignite and quartz sands of the Ione Formation, the remains of shallow cays that bordered the shoreline in ancient times.

And to complicate it even more, volcanic ash and lava flows have blanketed areas of the Mother Lode with such names as the Valley Springs Formation and the Mehrten Formation.

Contact Buzz Eggleston at buzz.calent@gmail.com.

Next installment: The Gold Belt.

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