All My Faults are Normal, But Not Really: Travels in Death Valley

Death Valley is the ultimate expression of the extensional forces that have ripped apart the crust of the western United States. The affected area reaches from northern Nevada and Oregon, east to central Utah, and south into Arizona. The broken up crust has resulted in the formation of countless fault basins and high mountain ranges (the entire region is called the Basin and Range Province). But few of those basins (really just one, the Owens Valley) approach the grandeur of Death Valley.




The valley (which is just part of the larger national park) is more than a hundred miles long, and it's deep. The vertical distance from the summit of Telescope Peak to the valley floor at Badwater is more than two miles (11,331 feet). Few places in America display greater relief. And the valley was not carved by water or any other erosional force: it is the result of faulting, the movement of the crust of the earth.

Most students of geology are taught early on that fault valleys are called grabens, and that they are formed by normal faulting. That begs the question of "what is normal?" (a concept I'm sure we all struggle with). Faults displaying vertical motion often have a sloping fault plane, and the fault block that "hangs" over the other is called the headwall (which therefore covers the footwall). When the crust is stretched, or extended, the headwall drops relative to the footwall, and that is what defines a "normal fault". If the crust is compressed, the headwall will move up relative to the footwall, forming an "abnormal fault"...no wait, that's my bad joke from the classroom. It's called a "reverse fault".

Death Valley is in an isolated lonely region, except for the main tourist area, which lies mostly along Highway 190 and Badwater Road which leads to...Badwater. But Badwater Road doesn't end there. It continues on to the south end of Death Valley and eventually over Jubilee Pass to the village of Shoshone. Few tourists ever venture this way. But there are things to see out there in the deep desert.

There is an odd little hill on the floor of Death Valley at the south end near the Ashford Mill (the remains of an old mine). It's a cinder cone, a small eroded pile of volcanic cinders and bombs that erupted tens of thousands of years ago. It's out on the valley floor in the midst of the alluvial fans, made up of the gravel and sands eroded from the surrounding mountains. The short climb from West Side Road provides a fine view of the graben of Death Valley. It's odd because it may be the only mountain you will ever climb whose summit is below sea level (-73 feet to be exact).
There are other reasons it is odd. Being in the middle of the valley, there seems no obvious way for lava to reach the surface of the valley. For another, it's in pieces. One half can be seen in the photo mosaic below.

From the main highway (below) it becomes apparent that the two pieces are offset from each other. It's been torn apart by faulting, but not by the kinds of faults we looked at above. The side are moving laterally. This kind with the lateral motion is caused by shearing and is called a strike-slip fault. The presence of the fault provides an explanation for the presence of the cinder cone (the magma was able to follow the fault fracture to the surface). But what are strike-slip faults doing in the Death Valley graben?

There are two kinds of strike-slip faults, right and left lateral. The type can be determined by looking at what the opposite block has done from the observers position: notice below that Pokey moved to Gumby's right. But from Pokey's point of view, Gumby has moved to Pokey's right. That's a right lateral fault.

One can therefore see that Cinder Hill in the Google Earth image below is offset in a right lateral manner, with the southwest portion moving northwest. That's a coincidence (not really) because the San Andreas fault, many miles away to the west, is also a strike-slip fault, and it is moving in the same direction. The two faults are roughly parallel. And that provides a clue about the nature of the faults in the Death Valley region.

There are other strike-slip faults in Death Valley, and they "step over" in such a way that a gap opens between the ends of the fault. In that area the crust is being stretched apart, forming a "pull-apart basin" (below). Death Valley National Park is being stretched apart to form grabens, but the overall motion is towards the northwest as the Sierra Nevada pulls away from the rest of the Basin and Range Province.

The clues to the broad forces affecting the crust of the planet show up in the way that they deform and fracture the rocks at the surface. Observations of an obscure little cinder cone at the south end of Death Valley reveals that the park is part of a much bigger process of continental motions that divide the North American plate from the Pacific plate. The faults might seem normal, but not all of them actually are.

An Iconic Bit of the Calaveras Fault in Hollister is Gone (But it will be back)

Corner of Locust and Central Avenues in 2016

I go on field studies trips year after year, and my camera is always around my neck, to the amusement of my students. They sometimes wonder why I would take pictures of things I already have dozens of pictures already. I sometimes wonder the same thing when I am trying to track down a specific shot from my archives of tens of thousands of pictures. But for a teaching geologist, there is a very good reason:

Things change.

Sometimes it is sudden, like a flood in Yosemite or on the Tuolumne River that actually alters the look of a landscape. It's nice to be able to catalog before-and-after views of a place. But in others, it is because of the incremental geologic changes. That's especially true with a couple of faults in Central California, the San Andreas and the Calaveras.

Corner of Locust and Central Avenues in 2013

Yesterday I updated the spot on Highway 25 near Pinnacles National Park where the San Andreas fault crosses the road (and I deeply appreciate the widespread response, especially on Twitter). The spot visibly changes every year. For decades, geologists have also been tracking the creeping of the Calaveras Fault in downtown Hollister. Generations of geology class field trips have walked several city blocks, tracking the fault as it offsets streets, sidewalks, curbs...and houses! I hope that all who do so remember to stay on sidewalks and not become nuisances to the residents. They have enough to contend with when you think about it.

The corner of Locust and Central Avenues in 2001

One curb has been iconic; it's been an illustration in any number of textbooks and PowerPoint presentations. The corner of Locust and Central Avenues is offset by the Calaveras Fault adjacent to the crosswalk, so it can be observed easily without bothering residents. It's one of the most vivid examples of right lateral offset imaginable, and if one knows the age of the sidewalk, the changes can be used to calculate the yearly rate of movement on the fault. The break is not a perfect measure because the deformation is spread out for several yards on both sides of the break (note how the sidewalk is curved in the pictures above).

So you can imagine my surprise on Saturday to find that the iconic curb disappeared sometime last year. It was for a good reason, as the city put in a wheelchair ramp, but it was still a shock. I was disappointed for a moment for my students until I realized that for the first time in 15 years, we have a brand new baseline of fault movement. Because as surely as the curb was offset before, it will continue into the future. We'll be watching for the first of the tensional cracks in the concrete, eventually to be followed by total rupture and offset curbs.

Corner of Locust and Central in 2017

Geology never stops.

It was the fault of no one, or maybe it's everyone's fault...the San Andreas and Calaveras faults in Central California

Central California has a lot of faults. One of them is a proclivity for bad geology puns. But it also has a great many exposures of active faults available for viewing in the space of an afternoon. The San Andreas, California's most famous fault cuts through the region, and the Calaveras fault lies a short distance to the east. It merges with the San Andreas north of Pinnacles National Park.

We became familiar with our faults today during our field trip between the town of Hollister and Pinnacles National Park in the California Coast Ranges. The faults are both actively creeping instead of building up ominous levels of stress. Earthquakes are a little bit less of a concern here, although some particular homeowners have problems with their foundations.

These are famous fault exposures. Photographs of these offset features have been published in textbooks all over the world. They are also easily accessible on public streets and parks. It is fascinating to watch the Earth in action.

The Calaveras fault is an offshoot of the San Andreas, extending north for about 70 miles from the vicinity of Pinnacles National Park. It has produced several magnitude 6+ earthquakes in the historical period, including a 1911 event in Morgan Hill with a magnitude of 6.5, and another quake in 1984 of magnitude 6.2.

A newly paved street at Dunne Park in Hollister

Dunne Park on Sixth Street in the town of Hollister is a good starting point to search out the fault zone. If you visit, please be respectful of the property lines. The people here see a lot of geology students, and while residents are almost always polite and even talkative, it would be sad to see "No Geologists Allowed" warnings springing up like "Neighborhood Watch" signs.

A short while later we headed south on Cienaga Road past the Hollister Hills Off-Road Vehicle park to an old winery. The warehouse was built directly on the creeping fault zone, and is slowly being ripped apart by the inexorable motion (two previous warehouses were apparently destroyed at this site as well). In the picture below, the cement wall was once connected to the wall with the blue tarp. They are offset by several feet. The dark plaque a bit to the left is a marker noting the establishment of the winery as a National Landmark.

The owners of the winery are friendly and have never refused our students the opportunity to look at the fault damage in the building, even when important functions were in progress. They are to be commended for maintaining access to this important site.

The drainage culvert on the south side of the building is probably one of the most photographed fault offsets anywhere.

Our last fault feature of the day before arriving at Pinnacles National Park was a spot on Highway 25 where the San Andreas fault crosses the pavement. It has been patched many times, and in the last year or two, the highway was completely repaved. On our previous visit the road showed no evidence of motion, but this year we could see modest cracks forming.

I would have felt really bad if we had set off a major quake by jumping on the fault zone, but then again it sure would have been neat to stand on the fault as a large earthquake commenced.

Back to the beaches in coming posts!