Saturday, March 28, 2015

Driving Through the Most Dangerous Plate Boundary in the World: Reconnaissance

We're headed on a blog adventure through the most dangerous kind of plate boundary in the world. To make things clear, the boundary we are exploring is not currently the most dangerous in the world, although it is certainly very hazardous. As described in my introduction yesterday, most subduction zones are not easy to explore. Most parts lie underwater or deep in the crust. We are instead traveling through the fossil subduction zone in California that was active from about 200 million years to about 29 million years ago.
An active subduction zone, like those that lie offshore of Indonesia, Peru/Chile, Japan, or the Philippines, is capable of producing monstrous earthquakes ranging as high as magnitude 9.5. A quake of that size can rupture the sea floor over distances of 800-900 miles (1,300-1,500 kilometers), with offsets in the range of 50 or 60 feet (15-20 meters). Such quakes, happening five or six times in a century around the world, have killed hundreds of thousands of people. The worst volcanic disasters of recent history (Krakatoa in 1883, Tambora in 1815, Pinatubo in 1991, and Mount Pelée in 1902) wiped out several hundred thousand lives as well, and have even altered world climate.
The San Andreas fault on the San Francisco Peninsula. It runs along the linear valley containing Crystal Springs and San Andreas Reservoir (yes, that's where the fault got its name). The fault runs out to sea near Pacifica, but emerges on land again at Point Reyes National Seashore to the north.
California once was this kind of geologic environment. It's unimaginable the number of disastrous earthquakes and eruptions that took place over nearly 200 million years that were never witnessed or felt by any human being. The rocks produced by this intense geologic activity underlie most of California, and the rocks are a complicated mess. In Central California, though, there is a certain organization that still exists. Looking at the geologic map above one can see three broad strips of rock or sediment trending roughly north-west, the mostly green Coast Ranges, the yellow Great Valley, and the red, blue and green of the Sierra Nevada. These three belts correspond roughly to the accretionary wedge, forearc basin, and magmatic arc of the now inactive subduction zone.
The Golden Gate Bridge and the entrance to San Francisco Bay. The Marin Headlands are on the left, and San Francisco is on the right. The San Andreas would cross the scene just below the bottom margin of the photograph, underwater.
The California we know of today is dominated by a different kind of plate boundary, a transform. The name San Andreas is known to most, a fault zone famous for the San Francisco earthquake of 1906, but the state is split by dozens of other active faults. They mostly trend to the northwest, and are causing the movement of Baja California, Los Angeles, and Monterey as a large landmass towards Alaska at the stunning rate of about 2 inches (5 cm) per year (be thankful it isn't faster). The largest earthquakes expected on this type of boundary fall within the range of magnitude 7.8-8.0. Such earthquakes are deadly, capable of killing thousands of people, but an 8.0 releases only about 1/30 of the energy of a magnitude 9.0 earthquake. Another way to understand the difference is to realize that the one quake in Japan in 2011 (magnitude 9.0) released more energy than all of California's earthquakes over the last 150 years combined.
The Sacramento-San Joaquin River Delta is one of the most complex regions in the state. The rivers split into multiple channels, forming nearly three dozen islands. The rich farmlands are protected from flooding by poorly built levees and dikes. The area is vulnerable to liquefaction damage in the event of earthquakes.
Everyone who has started out on a major journey wishes they had an accurate map, and I'll wager that the pioneers who set out for California in the Gold Rush days wished they could have had an aerial view of their route (if they had seen their route, they probably would have stayed home back east). And that's what we are doing today. Getting across the rocks of the subduction zone is not far as the crow flies, perhaps a hundred miles, but the varied kinds of rock are problematic. Rugged topography complicates road-building in both the Coast Ranges and the Sierra Nevada. Rivers and floodplains complicate road-building in the Great Valley. For a long time it wasn't easy getting around at all, and in some places it is still difficult traveling.
The East Bay hills, the Diablo Range, and the Carquinez Strait, where the Sacramento River flows into the bay. These hills are underlain by the accretionary wedge deposits of the Franciscan Complex.
The Coast Ranges are the most complicated part of our journey. A major river system, the Sacramento-San Joaquin drains the waters of the Sierra Nevada and Great Valley at the delta and the Carquinez Straits. The accretionary wedge deposits, called the Franciscan Complex, contain many kinds of rock, and each rock produces a different kind of topography. Compare the pictures above of the different corners of San Francisco Bay.
The eastern margin of the Diablo Range in the Coast Ranges near Patterson. The parallel stripes of rock are the sedimentary rocks of the Great Valley Sequence, which were deposited in a forearc basin
The eastern part of the Coast Ranges show more organization. The linear strips seen in the picture above are the tilted sediments of the Great Valley Sequence. These are the rocks that underlie the floor of the Great Valley, sometimes to depths of five miles (8 kilometers). They were deposited in a forearc basin setting.
The San Joaquin portion of the Great Valley near Patterson and Modesto. The floodplain of the San Joaquin River is the gray streak across the middle of the photograph.
The floor of the Great Valley is one of the most altered landscapes on planet Earth. The soils that developed in the semiarid climate are rich with nutrients, and more than 500 kinds of crops, fruits and nuts are grown there. 95% of the landscape has been co-opted by agriculture. Irrigation makes it possible, so the rivers have been altered as well. Some no longer flow to the sea (although efforts are being made to change that). Vast amounts of water, equivalent to a large natural river, are pumped and carried south through the canals of the California Water Project. Much of it ends up in the Los Angeles basin.
The town of Colusa in the northern Sacramento Valley. The Sacramento River winds through the area.

We then reach the foothills of the Sierra Nevada. The Sierra is a huge westward tilted block of granite and metamorphic rock 400 miles long, and 50-60 miles wide. The mountains start gently enough, an almost imperceptible change of slope, but the bedrock is close to surface so groundwater is not available for irrigation purposes. The prairie is mostly used as cattle range, and remains much as it did hundreds of years ago, aside from barbed wire fences and exotic European grasses that have crowded out the native bunchgrasses. Thousands of acres have recently been converted to almond groves with uncertain water sources.
The Sierra Nevada foothills near Sonora and Oakdale. Highway 108 crosses the middle of the photograph. The rocks are mostly composed of volcanic lahars (mudflows) of the Mehrten formation, dating to around 10 million years.

We finally reach the alpine landscape of the high Sierra Nevada. The rocks exposed here are the granitic plutons that once fed the volcanoes of the magmatic arc of the subduction zone. More than a hundred individual intrusions have been mapped, ranging in age from about 200 to 80 million years. Each of the intrusions probably fed a volcanic field miles above, but erosion has stripped away those miles of overlying rock. The remnants of the ancient subduction zone volcanoes can be seen as cobbles in the rocks of the Great Valley Sequence.
The high country of the Sierra Nevada at the headwaters of the San Joaquin River.
As has been mentioned before, the subduction zone is still active in Northern California. The active volcanic centers at Mt. Shasta and Lassen Peak still threaten the small towns in the region. Eruptions took place at Lassen in 1914-17, and at Shasta in 1786.
Mt. Shasta and the hummocky topography of a gigantic debris avalanche that spread northward from an ancient cone of Shasta about 350,000 years ago. Several active glaciers can be seen around the summit region of the 14,180 foot (4,,322 meters) high mountain.
We've now seen the aerial view of the main elements of our coming blog journey. I don't know yet the precise route we will be following, but I suspect it will begin in the Marin Headlands and cross the Golden Gate Bridge onto the San Francisco Peninsula. From the South Bay, we'll cross the interior Coast Ranges at Mt. Hamilton and Del Puerto Canyon. We'll cross the San Joaquin Valley, with stops on the floodplain of the San Joaquin River, and then make our way through the Sierra Nevada foothills and to the crest of the range. It's a drive that could be done in a day, but it's a route I would prefer to savor over several days.
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