Archive for the ‘The Hayward fault’ Category

When did everything Franciscan begin?

25 June 2018

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.

The Hillside School and the Hayward fault

20 November 2017

It was a most enjoyable hike that I led on Saturday for the Berkeley Path Wanderers Association, wandering for 3-plus miles in the city’s wonderful rock parks and along the Hayward fault. As usual, visiting the fault has its troubling side, and here I couldn’t ignore the implications of this splendid building, the old Hillside Elementary School on Le Roy Avenue.

This is the second Hillside School, built after the first one burned in the 1923 Berkeley Hills fire. For its time it was well designed, well made and well appointed, but after 50 years scientists confirmed that an active trace of the Hayward fault runs under it. The next major earthquake on this fault will rupture and ruin the structure. Besides that, the site is on a deep-seated active landslide.

The clear and present danger to children and teachers led the authorities to abandon the building in 1980.

In 2011 the German International School of Silicon Valley bought the site and pledged to do right by the building. In 2016 they fixed the roof with new, historically authentic materials, but at year’s end they moved out after deciding they couldn’t afford any more of the needed work.

There are two facts on the ground here. It’s a geological fact that this building is doomed and dangerous. But it’s an emotional and political fact that this building is precious.

The city declared the school a historic landmark in 1980, and it’s on the national register too. People love it and have strengthened the structure twice since 1925. But after the next major quake — not to mention a repeat of the 1923 fire — the school will be kindling, or ashes. Spending good money in a lost cause is an example of escalation of commitment, or the fallacy of sunk costs.

Without a building here, the site would be an excellent resource throughout the disaster period, and in between disasters an excellent little park.

During the walk, I made the modest proposal that we remove the building, and give it a nice funeral, before disaster strikes. Because now is the time that one day we’ll remember as “before.”

By holding a funeral for a building I mean, for starters, a respectful demolition. Make a virtual-reality model of the structure, recording the rooms and their beautiful wood beams and floors. Collect stories from the people who taught or attended school there. Hold a farewell concert in the old auditorium (with everyone signing waivers). Salvage the good materials. Build a memorial and have ceremonies.

All that stuff and more would befit the facts on the ground. I think it would raise the community’s consciousness of disaster preparedness, and at the same time mark the fact that in this case our ancestors won their bet against geology.

South Dunsmuir Ridge

29 May 2017

I finally got to a sweet corner of town last week, the sunny side of Dunsmuir Ridge, this lovely hill in the Google Maps 3D view.

The view is to the north-northwest, such that the Hayward fault runs straight up about a thumb’s width from the left edge. The maps below start with the 1915 topo map, in which the ridge’s top is the lobed outline of the 625-foot contour.

That straight creek valley along the hill’s south side — the gorge in the foreground of the top image — keeps catching my eye, but it seems to be inaccessible, which might make it Oakland’s wildest piece of land. The watershed map below may help in visualizing the hill and its surroundings. The two black dots are where the fire trail I took starts and ends.

Dunsmuir Ridge is city land, rescued from development after several aborted attempts to put high-end estates on this broad hilltop overlooking (in both senses) the deadly Hayward fault. The fire trail starts at the end of Cranford Way and winds up the ridge to join the fire road from the other side, which I’ve featured here before.

The walk is very scenic. To the north, downtown rises against Mount Tam.

Or if you prefer, there’s the new profile of San Francisco.

Higher up, the view opens out. Here San Leandro Creek is made visible as a line of trees coming out of the canyon toward its mouth near the airport.

But the main attraction is to the south. This is the best place to take portraits of Fairmont Ridge and its quarry scar. Unlike most places, this trail sets off the hill with a foreground of wild, forested land.

The prominent cleared space midway up the trail — a staging pad for firefighters — has regular visitors who find the spot special.

Interestingly, this spot is mapped as a patch of the peculiar Irvington-aged gravel that first brought me to Dunsmuir Ridge in 2009. However, I didn’t notice much of it, if any. See it on the geologic map — the white dots mark the ends of the fire trail.

There are rocks to be seen too. The soil is thin in most places. This little cut displays a profile of the soil and the decaying bedrock — saprolite — just beneath it.

The bedrock varies, and it doesn’t match the geologic map very closely. I would say nearly all of the lower part is not Leona volcanics (Jsv) but San Leandro Gabbro (gb). It has the gabbro’s pepper-and-salt appearance but is stained orange instead of the pristine rock’s bluish gray (as I saw earlier that day in San Leandro). You’ll see it well exposed in the trail itself, where this winter’s heavy rains carved fresh runnels.

If the city fills them before you get there (which it should before they become gullies), there are still roadside exposures that display the rock well, and it’s unmistakably gabbro where the map says volcanics. The top of the hill, though, is unquestionably Leona volcanics.

My long-term plan is to revisit every bit of bedrock in Oakland and log it. Besides sheer nerdery and the chance to improve the map, my motive is to come back to views like this one over and over again.

The old quarry is still for sale. Developers have tried to put houses there, but they keep getting shot down. Better, I say, for the Regional Parks District to acquire the land and develop it for quiet recreation.

The Hayward fault at Warm Springs

17 April 2017

Every extension of BART opens up a new region accessible to geologizers using public transit. So the other week I paid a visit to the far end of the Hayward fault, less than a mile from the new Warm Springs station in south Fremont. The station has nice views of the San Mateo Peninsula mountains to the west and Mission Peak to the east.

It appears, too, that the Irvington Gravels site to the north is accessible for determined walkers who bring provisions — that is, hikers.

To get to the fault just walk east on South Grimmer Boulevard toward the place marked “Weibel” on Google Maps.

Here’s the same area in Jim Lienkaemper’s detailed 1992 map of the fault. The map has a key to all the annotations. Note that both images are tilted to make the fault vertical; north is at about 1:30.

The fault runs through the “D” in “Blvd.”

Look back at the Google Maps image. See the line of green along the fault trace? That’s because of the 1972 Alquist-Priolo Act, which forbids new construction within 50 feet of an active fault. The area in the middle must have been built up before the act took effect. That’s where I went.

South Grimmer reveals the offset from fault creep well. This view is looking east toward Mission Peak. On the fault map, the locality (just below the horizontal dashed line) is circled and labeled “C1,rc,rf” signifying “strongly pronounced” evidence of creep in the form of right-offset curbs and a right-offset fence line.

And this is the other side of the road, looking west. Notice that the sidewalk is offset as well as the curb.

There’s another, much smaller offset higher up the slope that I didn’t get a good picture of. Repeated measurements show that together these offsets add up to about 6 millimeters per year. The slope itself is a sign of the fault, too.

To the north across little Arroyo Agua Caliente Park on Gardenia Way, this nice set of echelon cracks marks the fault trace. That’s what the “ec” in the circle labeled “C1,ec,rc,cc” stands for.

The fault nips the corner of Gardenia and Ivy Way, bending this curb (the “rc” in the label).

The city or the homeowner copes with the sidewalk by patching it as needed. You’ll see stuff like this everywhere on the Hayward fault.

Walking north through the park to Parkmeadow Drive on its north edge, you can look west down the street and see both an offset curb and the change in slope that marks the fault.

You can do this yourself all along the fault. The map has all the evidence (and the USGS has an updated version as of 2008).

A week later I hosted two French journalists — a writer and a photographer — for an afternoon, showing them fault offset features like these up in Hayward and Oakland. The writer went and spoke to a resident whose home was on the fault, and his fatalistic response took her aback a bit. She said “we don’t have attitudes like this in France.” I told her we Californians have been this way since the Gold Rush.