When did everything Franciscan begin?

A paper I read last week led me a long way in an interesting direction that started in El Cerrito, just up the ridgetop from Oakland, at the property fondly known as the Mira Vista Golf Course (restored in 2011 to its original glory and rechristened with its original name, the Berkeley Country Club).

I first visited Mira Vista in 1999 to see the Hayward fault, which runs through the fairways and helps give the land its picturesque form. Trenching studies there have added to our knowledge of this threatening feature.

But what brought Mira Vista to mind last week was a paper in the journal Tectonics titled “Early Onset of Franciscan Subduction.” A handful of rock harvested here gave the authors a new answer to an old and vexing question.

California is known to geologists around the world as the type example of a subduction zone. Its rocks preserve a record — a messy one — of a long period of geologic time when a tectonic plate consisting of ocean crust was moving toward and diving (subducting) underneath North America, which consists of continental crust. That went on for some 150 million years, interrupted when the San Andreas fault system first formed about 30 million years ago and turned the plate boundary into the sideways-moving setup we have today.

During the subduction period, North America scraped off parts of the top of the oceanic plate. That collection of stuff, analogous to a pile of dirt on the blade of a bulldozer, is a mixed-up lithological scrapple called the Franciscan Complex.

One of the most basic questions we have about the Franciscan is, how old is it? That is, when did subduction begin?

The authors of the paper, led by Sean Mulcahy of the University of Washington, looked for special samples from “high-grade blocks,” lumps of rock that have survived being stirred by the bulldozer blade deep below the crust and returned intact. Most promising of these are the highest-grade rock type I’ve found in Oakland: blueschist. The authors studied just two rock samples, one from the high-grade blueschists of the Tiburon Peninsula and the other from Mira Vista, where high-grade blocks crop out of a matrix of serpentinite along with other Franciscan “knockers.”

The high-grade blocks are part of the golf course as well as the surrounding terrain.

The course is right next to The Arlington. It was easy to get there on the 7 bus line. Getting off near Madera Circle, I spotted telltale boulders at the roadside: Blueschist.

Mulcahy got his team’s sample from this knocker behind the fire station. Their International Geo Sample register says it was collected by “hammer and chisel.”

Close up, a fresh exposure of the rock — maybe the actual collection site — glistened with blue and green crystals: the high-pressure metamorphic minerals glaucophane and omphacite.

The real work began under the petrographic microscope, where Mulcahy had to untangle the complex set of high-pressure events that affected this rock from the geometry of these thin sections.

Briefly, the big garnet grains preserved grains of very old material inside them, protecting it from later reactions. Those grains were about 176 million years old; the same minerals outside the garnets were about 160. Their ages had been re-set during a later episode of high pressure.

Moreover, the earlier episode had a much higher pressure than the later one, high enough that the rock had been not blueschist, but eclogite (ECK-la-jite). It had been carried at least 50 kilometers deep, a depth only attainable by subduction, and just a few tiny shreds protected inside garnet grains, measured in microns, preserved the evidence.

That’s pretty cool. This finding sets a new record back in deep time for the beginning of Franciscan subduction, in the early Jurassic.

It also offers a telling clue about another California geology mystery: What’s the exact relationship between the Franciscan Complex and the Coast Range ophiolite? In brief, there are three main hypotheses, and this evidence weighs against two of them and favors the simplest one. That will change the book I’m in the middle of writing.

By the way, stairs and footpaths lead from here down through Motorcycle Hill to the El Cerrito del Norte BART station. Take that hike some time.

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One Response to “When did everything Franciscan begin?”

  1. Manuel Garcia, Jr. Says:

    Very interesting post. I’ll be curious to see your book, when published. I enjoy your blog, so I’m sure the book will be fascinating.

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